Thermal and mass implications of magmatic evolution in the Lassen volcanic region, California, and minimum constraints on basalt influx to the lower crust

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B2, PAGES 3003-3013, FEBRUARY 10, 1996

Thermal and mass implications of magmatic evolution in the Lassen volcanic region, California, and minimum constraints on basalt influx to the lower crust

Marianne Guffanti

U.S. Geological Survey, Reston, Virginia

Michael A. Clynne and L. J. Patrick Muffler U.S. Geological Survey, Menlo Park, California

Abstract. We have analyzed the heat and mass demands of a petrologic model of basalt- driven magmatic evolution in which variously fractionated mafic magmas mix with silicic partial melts of the lower crust. We have formulated steady state heat budgets for two volcanically distinct areas in the Lassen region: the large, late Quaternary, intermediate to silicic Lassen volcanic center and the nearby, coeval, less evolved Caribou volcanic field. At Caribou volcanic field, heat provided by cooling and fractional crystallization of 52 km 3 of basalt is more than sufficient to produce 10 km ø of rhyolitic melt by partial melting of lower crust. Net heat added by basalt intrusion at Caribou volcanic field is equivalent to an increase in lower crustal heat flow of -7 mW m -2, indicating that the field is not a major crustal thermal anomaly. Addition of cumulates from fractionation is offset by removal of erupted partial melts. A minimum basalt influx of 0.3 km 3 (km 2 Ma) -• is needed to supply Caribou volcanic field. Our methodology does not fully account for an influx of basalt that remains in the crust as derivative intrusives. On the basis of

comparison to deep heat flow, the input of basalt could be -3 to 7 times the amount we calculate. At Lassen volcanic center, at least 203 km 3 of mantle-derived basalt is needed to produce 141 km 3 of partial melt and drive the volcanic system. Partial melting mobilizes lower crustal material, augmenting the magmatic volume available for eruption at Lassen volcanic center; thus the erupted volume of 215 km 3 exceeds the calculated basalt input of 203 km 3. The minimum basalt input of 1.6 km 3 (km 2 Ma) -• is >5 times the minimum influx to the Caribou volcanic field. Basalt influx high enough to sustain considerable partial melting, coupled with locally high extension rate, is a crucial factor in development of Lassen volcanic center; in contrast, Caribou volcanic field has failed to develop into a large silicic center primarily because basalt supply there has been insufficient.

Introduction

Mass and heat added to the continental crust by ascent of basaltic magma from the mantle are critical agents in creation of new crust by intrusion and underplating and in modification of the regional thermal regime of the crust. Owing to its low viscosity, high melting temperature, and large latent heat, basaltic magma is an efficient heat transfer fluid and can melt and mobi- lize silicic material from the crust [Lachenbruch and Sass, 1978]. As was emphasized by Hildreth [1981], the thermal basis of almost all continental magrnatism is influx of mantle-derived basalt.

We investigate some of the thermal and mass implications of the petrologic model of Clynne [1990, 1993] and Bullen and Clynne [1990] for the Lassen region of the Cascade Range. In this model, basalt-driven magmatic evolution is dominated by mixing of fractionated mafic magmas with silicic partial melts of the lower crust. We evaluate two volcanically distinct areas within the Lassen region. At the Lassen volcanic center, a major late Quaternary volcanic focus that consists of an andesitic stratovolcano flanked by a younger silicic dome field,

This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union.

Paper number 95JB03463.

large volumes of rhyolitic magmas are generated by partial melting of the lower crust rather than by fractional crystallization of mafic to intermediate magma bodies in the middle to upper crust. At the Caribou volcanic field, a nearby coeval group of cones and shields of mostly basaltic andesite, much smaller volumes of partial melting are involved. For both volcanic areas we use eruptive volumes determined from geologic mapping and magmatic volumes inferred from petrologic modeling in order to formulate steady state heat budgets that compare heat provided by cooling and crystallization of basalt to heat needed for crustal partial melting.

Our analysis constrains the rate of basalt influx from the mantle to the lower crust. Because the calculations are based

on erupted volumes, however, we cannot directly estimate the total influx of basalt that may crystallize in the lower crust or evolve to form derivative magmas that crystallize in the crust, in neither case leaving eruptive evidence. Our treatment therefore is not a comprehensive analysis of crustal growth, and our estimates of basalt influx are minimum values.

Geologic Setting The Lassen volcanic region of NE California comprises the

southernmost segment of the Cascade Range [Guffanti and

3003

3004 GUFFANTI ET AL.' LASSEN MAGMATIC EVOLUTION AND BASALT INFLUX

Juan de Fuca Plate

Mendocino F

Canada

U.S.

Washington

ß

• ß Oregon __..ß

ß ß Basin and

• Range

Nevada

114 ø 51 ø

47=

42 ø

Pacific 17 = Plate

---50 km 3 of andesite; a later stage erupted ---20 km 3 of silicic andesite. A more silicic phase of activity at Lassen volcanic center, centered north of Brokeoff volcano, began at 400 ka with a caldera-forming eruption of ---75 km 3 of rhyolitic tephra, domes, and flows termed the Rockland sequence by Clynne [1990]. Subsequently, a dome field formed by episodic eruptions of ---50 km 3 of dacite, mostly in the time intervals 250- 200 ka and 100-0 ka. East of the dome field, ---t0 km 3 of mostly andesites, termed "hybrid" by Clynne [1990] because of strong petrographic evidence of mixing of mafic and silicic magmas, erupted at 325-250 ka and 100-0 ka. (See Clynne [1990] and Guffanti et al. [1990, page 19,455] for references to geochronologic data.)

Caribou volcanic field (Figure 2) exemplifies the regional suite. Roughly 50 km 3 erupted from ---tOO small cones and shields at 700-20 ka [Cl3;nne and Muffler, 1990; G. B. Dal- rymple, unpublished K-Ar data, 1991-1994]. About half of this volume consists of olivine-pyroxene basaltic andesite and andesite; olivine basalt and pyroxene andesite occur in smaller volumes. Lavas having >63% SiO 2 are sparse.

Pacific Ocean

0 200

' I•m ' 32

ø • 114 ø 131 126 ø 120 ø

Figure 1. Plate tectonic setting of the Lassen volcanic region (enclosed by small rectangle labeled L).

Weaver, 1988] and occurs above the subducting Gorda plate, part of the Juan de Fuca plate system (Figure 1). A Wadati- Benioff zone extends from north of the Mendocino triple junction to a depth of about 90 km beneath the Lassen region [Oppenheimer et al., 1993; Walter, 1986; Cockerham, 1984]. Crustal thickness beneath the Lassen region is 38 _+ 4 km [Mooney and Weaver, 1989].

The Lassen region also lies along the NW margin of the Basin and Range province, within a broad zone of distributed extension in the lithosphere east and southeast of the southern Cascade arc [Pezzopane and Weldon, 1993]. ENE crustal extension throughout the Lassen region is deduced from abundant normal faults and volcanic-vent alignments that strike NNW [Guffanti et al., 1990].

Abundant volcanism in the Lassen segment began at ---7 Ma and occurs on two scales [Clynne, 1990]: (1) regional volcanism that comprises hundreds of small, short-lived, mafic to intermediate volcanoes, and (2) focused volcanism that consists of larger, longer-lived, intermediate to silicic volcanic centers su- perimposed on the broad platform built by regional volcanism. Rock types range from sparsely phyric olivine basalt to high- silica rhyolite. Mineralogically simple olivine-pyroxene basaltic andesites and pyroxene andesites dominate the regional suite, whereas porphyritic, mineralogically complex, pyroxene andesites and hornblende dacites dominate at the large centers.

Lassen volcanic center (Figure 2), the youngest of at least four large focused centers in the Lassen segment, began with construction of Brokeoff volcano, a stratocone of---80 km 3 active from 650 to 400 Ma [Clynne, 1990]. An early stage of Brokeoff volcano erupted •-10 km 3 of basaltic andesite and

Petrogenetic Model Magmatic Evolution

Two geochemical features of calc-alkalin½ magmas in the Lasscn region are fundamental to the interpretation of their origin [Clynne, 1990, 1993; Bullen and Clynne, 1990]. First, the coherence of the geochemical trends indicates that all calc- alkaline magmas in the Lasscn region are related by broadly similar, albeit complex, magmatic processes. Second, the geochemical diversity of magmas in the Lasscn region is greatest at the mafic end of the compositional spectrum and decreases with increasing SiO:. This feature requires that mixing be the dominant control of the geochemical evolution. All calc- alkaline magmas in the Lasscn region derive from primitive calc-alkalin½ basalt from the mantle and rhyolitic partial melt from mafic lower crust through a series of fractional- crystallization and magma-mixing events.

Another mantle-derived primitive magma, low-potassium olivin½ thol½iitic basalt, also is present in the Lasscn region [Clynne, 1993] and appears to be associated with basin-range style lithospheric extension [Hart et al., 1984; Gulfanti et al., 1990]. This magma is not an important component of the calc-alkalin½ magmatic systems in the Lasscn region [Clynne, 1993] and is not further considered here.

The Caribou suite is best modeled by fractional crystallization of basalt of 50% SiO: and 8-10% MgO with simultaneous addition of high-silica (up to 75% SiO:) rhyolit½ melt. The liquidus assemblage spin½l-olivin½-clinopyroxcn½ of primitive regional basalts suggests that fractionation takes place in the lower crust at pressures of at least 8-10 kbar (1 kbar = 10: MPa). At Lasscn volcanic center, multiple phcnocryst popula- tions and a variety of textural and chemical disequilibrium features clearly attest to the dominant role of magma mixing [Clynne, 1990]. Magmatic evolution of Brokeoff volcano also was controlled by crystal fractionation and by simultaneous addition of silicic melt. Importance of the silicic melt component gradually increased during the lifetime of Brokeoff volcano and dominated magmatic evolution of Lasscn volcanic center after 400 ka. In the dome field dacit½s, undercooled magmatic inclusions formed when small amounts of mafic magma were mixed into silicic magma batches; subsequent disaggregation of the inclusions resulted in porphyritic dacit½

GUFFANTI ET AL.' LASSEN MAGMATIC EVOLUTION AND BASALT INFLUX 3005

41øN 122øW

40ø15'N, 122øW

o ooo

oo o

o o

ß LSNO

L. ALMANOR o

o

41øN, 120ø30'W

20 km I

40ø15'N, 120ø30'W

Figure 2. Vent map of the Lassen volcanic region. Irregular lines outline perimeters of vent fields of Lassen volcanic center (LVC) and Caribou volcanic field (CVF). Small open circles depict Quaternary volcanic vents throughout the Lassen region. BV marks the eroded center of Brokeoff volcano; R is the schematic center of Rockland eruption. Vents for dome-field dacites are shown with plus symbols; vents for hybrid andesites are shown with open squares. Large stars indicate centers of other major volcanic loci active in the Quaternary: D, Dittmar; M, Maidu; and S, Snow Mountain. The solid square is location of heat-flow hole LSNO of Mase et al. [1982]. The boundary of Lassen Volcanic National Park (LVNP) also is shown.

[Clynne, 1989, 1993]. Mixing between subequal amounts of porphyritic dacite and mafic magma produced homogeneous hybrid andesire.

Temperatures and Degree of Melting

In our analysis the heat content of primitive basalt near its liquidus temperature causes partial melting of gabbroic crust at lower temperature. Quantification of the heat budget requires estimates of the temperatures of the initial magmatic components.

A variety of evidence indicates that the initial temperature of the basalt component was ---1300øC. Equilibrium temperatures for Ca in early-crystallizing olivine in primitive Lassen basalts, calculated using the method of Jurewicz and Watson [1988], indicate that these basalts begin crystallizing at 1275 ø- 1300øC [Clynne, 1993]. The calculated liquidus temperature for a typical Lassen primitive basalt is 1320øC at 10 kbar and oxygen fugacity of QFM-NNO with 0.4% H20 , using the program of Ghiorso [1985]. Tatsumi et al. [1983] suggested ---1320øC for primary basaltic magmas from the NE Japan arc. Johnston and Draper [1992] obtained an anhydrous liquidus temperature of 1325øC at 12 kbar for a primitive arc basalt similar in composition to primitive Lassen basalts. The presence of water would lower that liquidus temperature approximately 25øC per wt. % H20 [Basaltic Volcanism Study Project, 1981]. The complete absence of hydrous phases in Lassen

basalts and andesites indicates relatively low water contents of ---1 wt. %.

The most primitive rhyolite at Lassen volcanic center con- tains sparse phenocrysts of sodic plagioclase, amphibole, orthopyroxene, and apatite, is weakly peraluminous, and has up to 75 wt. % SiO 2. This rhyolite is assumed to approximate the primary silicic component involved in magma mixing. Major- element, trace-element, and radiogenic-isotope data suggest that the source rocks for rhyolitic magma are calc-alkaline mafic rocks in the lower crust [Bullen and Clynne, 1990; Clynne, 1993], probably mafic magmas that underplated the lower crust during the long history of Mesozoic subduction beneath western North America. If the primitive rhyolite is generated by melting of crystallized basalt (amphibole-pyroxene gabbro and diorite) similar to regional Lassen mafic lavas, enrichment factors of the most incompatible trace elements constrain the degree of melting to a maximum of 20%.

Partial melting in the lower crust takes place under fluid- absent conditions, and the resulting magmas are water undersaturated [Clemens and Vielzeuf, 1987]. Melts are generated by amphibole dehydration reactions that take place over a narrow temperature interval. The amount of magma produced is a function of the mineralogy and water content of the source rock, but it is constrained (for rhyolite) to less than about 15-20%. Higher degrees of melting produce liquids that have lower silica contents and plagioclase-free anhydrous residua.

3006 GUFFANTI ET AL.: LASSEN MAGMATIC EVOLUTION AND BASALT INFLUX

The crystal-poor nature and sparse hydrous mineralogy of the primitive rhyolites suggests that they were relatively hot and dry. Fe-Ti oxide geothermometry of typical Lassen dacite suggests that it erupted at -900øC [Heiken and Eichelberger, 1980]. Borg [1995] calculated temperatures of -900øC for primitive silicic lavas from the Lassen region using the expression of Harrison and Watson [1984] for the distribution of phosphorous in apatite.

Helz [1976] investigated melting of basaltic to mafic-andesite compositions at water-saturated to undersaturated conditions and high pressures P. Although not exactly applicable to the Lassen situation, her experimental results indicate that partial melting of basaltic to mafic-andesite compositions at 800- 900øC, 5-10 kbar, and PH20 •< PTOT will produce dacitic to rhyolitic liquids coexisting with amphibole-pyroxene-plagioclase-magnetite.

Beard and Lofgren [1991] reported on dehydration experi- ments of hydrous basaltic compositions. At 850ø-900øC, 6.9 kbar, and water-undersaturated conditions, they generated 6-12% weakly peraluminous, high-silica rhyolite melt in equilibrium with a pyroxene, amphibole, apatite, and sodic- plagioclase residuum. These high-silica melts are similar in composition to primitive rhyolite of Lassen volcanic center. Water-saturated melts at lower temperature are more alumi- nous than rhyolite of Lassen volcanic center and contain calcic plagioclase or do not coexist with plagioclase. Furthermore, their chemistry is unlike that of rhyolite from Lassen volcanic center.

On the basis of these and other experimental results, Pea- cock et al. [1994] constructed a generalized petrogenetic grid for fluid-absent melting of basaltic compositions at the base of arc crust. Melting ranges from about 5% at 800øC to about 15% at 925øC, at which point amphibole is exhausted.

On the basis of the preceding discussion, we choose the following parameters: initial temperature of basalt, 1300øC; partial-melting temperature to produce rhyolite, 900øC; partial melting of lower crust, 15%.

Methodology We have adopted a conceptual model in which basalt from

the mantle intrudes the lower crust to cool and fractionate at

depths greater than 25 km, producing mafic to intermediate magmas and providing heat for partial melting of surrounding lower crustal rock similar in composition to calc-alkaline basalt. Partial melting of 15% at -900øC produces rhyolitic melt. Some rhyolitic magma erupts with little chemical or thermal modification, and some mixes with evolved mafic magmas to yield erupted lavas of andesitic to dacitic composition. Small amounts of upper crustal assimilation and fractional crystallization are ignored. For the initial conditions we assume that the average background temperature of the crust at a depth of 25 km is -800øC, in accord with an initial regional conductive heat flow at the surface of-95 mW m-2 and a reduced heat flow from the mantle into the base of the crust of -75 mW

m -2 [Lachenbruch and Sass, 1978]. The final temperature of the lower crustal is assumed to be -900øC on average. The temperature-depth relationships assumed in our model are consonant with those of DeBari and Coleman [1989] for fractional crystallization of basaltic magma in the exhumed lower crustal roots of a mid-Jurassic arc in Alaska.

We disregard transient thermal effects of the sort quantified by Huppert and Sparks [1988], who analyzed the fluid dynam-

ical and heat transfer processes involved in melting wall rock at the roof of a single basalt sill emplaced into the upper crust. The record of abundant Quaternary volcanism in the Lassen region confirms the occurrence of hundreds of intrusive events and justifies a more generalized heat budget method.

We formulate steady state heat budgets that allow comparison of the amount of heat produced by cooling and fractionation of basalt to the amount of heat needed by partial melting. We work backward from extrusive magma volumes and do not presuppose what fraction of a given magma batch has erupted. Rather, we are interested in what can be inferred indepen- dently from the observed surface volcanic products of a magmatic system. We recognize that basalt fractionation and crustal partial melting are processes closely related in space and time [e.g., Huppert and Sparks, 1988; Bergantz, 1989]. For the purpose of organizing our calculations, however, we break out fractionation and partial melting as separate steps for Car- ibou volcanic field and for each of the six major eruptive groups at Lassen volcanic center. Table 1 summarizes heat and mass budgets.

The first step is to calculate the volume of basalt input from the mantle, V•, that is required by the eruptive volume of lavas and by the modeled percentages of crystal fractionation and addition of rhyolite melt. The input-basalt volume V• is related to the eruptive volume V• by:

v,- x•v, + x,•v, = v• ( • )

The percent crystals subtracted from 50% SiO2 basalt during fractional crystallization (Xc, from Table 2) multiplied by V• gives the volume Vx• of crystals subtracted during fractionation. The percent rhyolite added (X•, from Table 2) multiplied by V• gives the volume Vv3• of silicic melt added.

Using the above volume information, we next calculate the amount of heat Q v•hc provided by fractional crystallization of basalt. Q v•Ac is the sum of three components: (1) Q c, heat provided by conductive cooling of basaltic magma from its initial liquidus temperature T• to the temperature at which the last crystals form Tx•; (2) QF, heat provided by crystal formation during fractionation; and (3) Qx•, heat provided by conductive cooling of the crystals from Tx• to the final lower crustal temperature of-900øC.

QFR^C: Qr' + QF + Qx,, (2)

The heat content of the evolved mafic magma resulting from basalt fractionation is not counted in this heat budget because that magma is involved in mixing and moves quickly out of the lower crustal system, erupting to form the observed volcanic rocks.

Using values of parameters in Table 3, the three heat components are calculated as

Qc = (r,.- rx,•)c,,p,,l/, (3)

QF: Amlpx,.Vx,• (4)

Qx,• = (Hx,.- H,x.)px,•Vx,. (5)

where cn and Pn are the specific heat capacity and density of basaltic magma, respectively. AH I. is latent heat of fusion (or crystallization) of the crystal assemblage formed, and Pxi. is the density of the crystal assemblage. Hxz• and Hz• c are the heat contents of the crystal assemblage at Txi" and at the final lower crustal temperature of 900øC. Values of these variables are calculated from mineral percentages in Table 2.

GUFFANTI ET AL.: LASSEN MAGMATIC EVOLUTION AND BASALT INFLUX 3007

Table 1. Summary of Heat and Mass Budgets for Caribou Volcanic Field and Lassen Volcanic Center Lassen Volcanic Center

Brokeoff Volcano

Caribou

Volcanic Basaltic Silicic Rockland

Definition Field Andesite Andesite Andesite Sequence Hybrid

Andesites

Silicic

Dome

Field

Total

Lassen

Volcanic

Center

Vv: erupted volume, km 3 50 10 50 20 75 X c crystal fraction, % 23 21 41 54 XR rhyolite melt added, % 19 20 30 33 V• volume input basalt, km 3 52 10 55 25 l/x•. volume crystals, km 3 12 2 23 14 l/•.,w volume melt, km -• 10 2 17 8 75 Qc heat of cooling magma, 10 •8 J 14 3 25 13 Qr latent heat, 10 •8 J 23 4 40 24 Qx• heat of cooling crystals, 10 is J 15 3 33 18 QFRAC crystal fractionation heat, 10 • J 52 9 89 50 QLc heat of crustal temperature rise, 22 5 38 18 167

10 •8 J

QrR heat of fusion of rhyolite, 1018 J 7 1 11 5 51 Q•,xt partial melt heat, 1018 J 29 6 49 23 218 AQ Q•,xt -- QFRAC, 1018 J Vx additional basalt required, km 3 Vx + V• total basalt required, km 3

10

7

0.4

3.4

1.2

0.9

0.7

3

8

2

10

50

15 1

36

3

2

2

6

8O

24

104

215

112

40

141

157 316

94

410 253

>91'

>203*

*Minimum value, assuming full crystallization to gabbro in lower crust.

The heat required for partial melting of mafic lower crust Qt,•u is the sum of (1) Qt.c, the heat required to raise the temperature of the some portion of the lower crust from its background temperature to the partial-melting temperature and (2) QFR, heat for rhyolite melting.

QpM = QLc + QrR (6)

Qt.c is calculated as

= - HLc) pLcX[•tVt,w Qrc (H•,• -• (7)

Ht,•u and Ht.c are heat contents of mafic lower crust at the partial-melting temperature and at the initial background temperature of 800øC. Xt,•u is the fraction of lower crust melted and equals 15%, and Vt,•u is the volume of rhyolitic partial melt. For 15% partial melting of mafic lower crust, the most fusible one seventh of any given volume is melted (assuming uniform composition); thus a volume of lower crust 7 times as large as the volume of partial melt must be heated to 900øC. We assume that melting occurs efficiently and that no additional volume of lower crust need be heated to allow for non-

ideal partial-melting conditions.

Q FR is calculated as

QrR = AHf•p•V?M (8)

AH•, the latent heat of fusion of rhyolite, and p•, the density of rhyolite, are calculated using a representative observed composition of Rockland rhyolite (see Table 3) rather than using fictive proportions of minerals inferred to be melted out of lower crustal rocks.

Equations (1)-(8) were calculated separately for Caribou volcanic field and for Brokeoff basaltic andesite, andesite, and

silicic andesite (Table 1), using different percentages of fractional crystallization and silicic melt addition and different thermodynamic values as indicated in Tables 2 and 3. The Rockland sequence was modeled as an essentially pure partial melt, and accordingly, only equations (6)-(8) were used to calculate Q

Mass balance modeling by crystal fractionation of the dome field dacites and hybrid andesites proved unsuccessful, and evidence for magma mixing is abundant [Clynne, 1990]. There- fore we treated these volcanic units as mixtures of 52% SiO 2 basalt and 68% SiO2 dacite. The 52% SiO 2 basalt was assumed

Table 2. Summary of Results of Petrologic Modeling for Brokeoff and Caribou Volcanic Units

SiO 2 Rhyolite Interval Total Melt

Average for Crystals Added Erupted Modeling, Fe-Ti Subtracted (X•),

Volcanic Group SiO2, % % Ol Cpx Opx Plag Oxide (Xc), % %

Basalt 52 50-52 32 59 0 0 9 6 6 Brokeoff basaltic 56 50-56 26 21 0 45 8 21 20

andesite

Brokeoff andesite 60 50-60 13 25 9 46 7 41 30 Brokeoff silicic 63 50-63 10 23 12 49 6 54 33

andesite

Caribou 56 50 -56 23 35 0 32 9 23 19

Silica interval indicates starting and ending SiO 2 contents used in modeling, with ending silica content equal to average silica content of erupted units. Mineral proportions (O1, olivine; Cpx, clinopyroxene; Opx, orthopyroxene; Plag, plagioclase) are percentages of crystal phases subtracted during fractionation (normalized to 100%) and are used to calculate some of the thermodynamic values listed in Table 3.


Table 3. Input Parameters for Heat Budget Calculations

Temperature Heat TxL at Content

Which Last Hx•, at Crystals Tx• ,, J Heat Content

Composition Formed, øC g-• H, J g-•

Heat of

Fusion AH, Density J g-• p, g cm -3

Basaltic magma from mantle*

Crystal fractionates 6% fractionate formed during

evolution of calc-alkaline

basalt

21% fractionate formed during evolution of Brokeoff basaltic andesite

41% fractionate formed during evolution of Brokeoff

andesite

54% fractionate formed during evolution of Brokeoff silicic andesite

23% fractionate formed during evolution of Caribou basaltic andesite

Rhyolite partial melt

Mafic lower crust

10% Fo, 11% Diop, 11% En, 33% An, 26% Ab, 4% Or, 5% FeTiOx

N/A N/A N/A 4 7 0 ( AHœ,• ) 2.9 (p,•)

32% Fo, 59% Diop, 1250 1360 930 (HLc at 900 ø) 650 (AHf) 3.3 (PXL) 9% FeTiOx

26% Fo, 21% Diop, 1225 1330 930 (HLc at 900 ø) 580 (AHœ) 3.2 (PXL) 45% Plag, 8% FeTiOx

13% Fo, 25% Diop, 9 1175 1260 925 (HLc at 900 ø) 560 (AHf) 3.1 (PxL) % En, 46% Plag, 7% FeTiOx

10% Fo, 23% Diop, 1150 1230 925 (HLc at 900 ø) 560 (AHf) 3.1 (PXL) 12% En, 49% Plag, 6% FeTiOx

23% Fo, 35% Diop, 1225 1320 925 (HLc at 900 ø) 590 (AHr) 3.2 (PXL) 32% Plag, 9% FeTiOx

20% Or, 10% An, 32% N/A N/A N/A 250? (AHfR) 2.7 (pR) Ab, 33% Qtz, 4% En, 1% FeTiOx

5% Fo, 15% Diop, N/A N/A 930 (H•,M at 900 ø) N/A 2.9 (PLC) 10% En, 44% An, 24% Ab, 2% FeTiOx 815 (H• c at 800 ø)

Using mineral proportions (from Table 2) and temperatures based on crystal-fractionation modeling, heat contents and densities are calculated from data of Robie et al. [1979], and latent heats of fusion are calculated from data of Lange and Carmichael [1990]. Temperatures at which the last crystals formed in evolved magmas are estimated using runs of the SILMIN computer program of Ghiorso [1985]. Abbreviations are Fo, forsterite; Diop, diopside; En, enstatite; An, anorthite; Ab, albite; Or, orthoclase; Plag, plagioclase; Qtz, quartz; N/A, not applicable.

*Tz• = 1300øC, cB = 1.23 J g-løC. ?Based on representative observed composition of Rockland rhyolite.

to result from 6% crystal fractionation of 50% SiO 2 basalt with 6% of 72% SiO 2 partial melt added (see Table 2). The 68% SiO 2 dacite was assumed to be 72% SiO 2 melt contaminated with 52% SiO 2 basalt. Two linear mixing equations based on SiO 2 content were used to derive V• and Vpa4 of the end- member compositions from the intermediate compositions.

Our methodology is highly model-dependent and involves many uncertainties and assumptions that we do not attempt to quantify comprehensively. Some of the most significant uncertainties are embedded in three of the parameters used to calculate Q•,•, (heat needed to raise the temperature of the lower crust from the background to the partial-melting temperature) and hence in Q•,M: (1) If the eruptive volume of Rockland were as high as 100 km 3 (rather than 75 km-•), in- corporating that value for Vp,u into equations (6)-(8) would increase Q•,• for the Rockland sequence by --•80% and Qva4 for Lassen volcanic center by --•45%. (2) Decrease in the percent of partial melting increases the volume of lower crust from which a given amount of rhyolitic partial melt can be produced. If 10% partial melting rather than 15% is used in (10), more heat is needed to raise a larger volume to the partial-melting temperature, and Qm4 increases by --•40% for both Lassen volcanic center and Caribou volcanic field. (3) The basic calculations summarized in Table 1 are based on a temperature difference of 100øC. Changing that difference to 150øC by low- ering the background temperature from 800øC to 750øC would

increase Qva4 by 43% at Lassen volcanic center and 45% at Caribou volcanic field.

Discussion

Thermal and Mass Implications of Magmatic Evolution: Caribou Volcanic Field

At Caribou volcanic field, the heat Q FR^c (52 X 10 •8 J; see Table 1) provided by cooling and fractional crystallization of 52 km 3 of basalt (V•) exceeds by a factor of nearly 2 the heat Qv2u (29 x 10 TM J) needed to produce the 10 km 3 of rhyolitic melt Vpa4 required by our petrologic model. Thus the calculated basalt input is more than sufficient to drive the Caribou volcanic system.

Basalt intrusion transfers heat across the base of the lower

crust. For Caribou volcanic field, the net amount of heat available to warm the lower crust is the amount of heat provided by basalt crystallization minus the amount of heat in the rhyolitic melt involved in mixing.

QFRAC- (Qt"R 4- XpMQLc) = 52 x 10 Is J- (7 x I0 Is J

+ 0.15 x 22 x 10 Is J) = 42 x 10 •8 J (9)

The heat content in erupted mafic lavas, which does not con- tribute significantly to warming of the crust, already has been excluded from Q FR^C' The 42 X 10 •8 J of available heat


averaged over the area and life span of the volcanic field is equivalent to an increase in deep heat flow of 7 mW m -2, which indicates that Caribou volcanic field is not a major crustal thermal anomaly. The associated temperature increase at 25 km, ignoring transient effects, is --•75øC [from Lachen- bruch and Sass, 1978, equation 3] which is in good agreement with our model condition of warming to --•900øC from an initial lower crustal temperature of--•800øC.

Little net addition of mass to the lower crust is required to support the Caribou volcanic field. The petrologic modeling results in Table 2 indicate that for each cubic kilometer of

basalt intruded into the lower crust, 0.23 km 3 of crystal cumulates form during fractional crystallization and 0.19 km • of silicic partial melt is extracted from the lower crust. In volu- metric terms (Table 1), of the 52 km • of input basalt calculated for Caribou volcanic field, 12 km 3 (23%) remains in the lower crust, whereas approximately 40 km 3 (76%) of complementary evolved mafic magmas are involved in mixing to produce erupted lavas and thus do not remain in the crust. The addition of 12 km 3 of cumulate material to the lower crust is offset by the extraction of 10 km 3 of silicic partial melt from the lower crust for incorporation into erupted lavas, for a small net addition of 2 km • of material to the lower crust. Thus there is

no significant space deficiency ("room problem") in the lower crust with respect to basalt intrusion; the overall addition to the crust of 52 km 3 of material from the mantle is accommo-

dated mostly at the surface. The lower crust beneath Caribou volcanic field does un-

dergo two types of compositional change as a result of our model of magmatic evolution: (1) ultramafic (dunite- pyroxenite) cumulates are produced by fractional crystallization of the intruded basalt, and (2) granulite residues are produced by extraction of rhyolitic partial melt from mafic lower crust; for 10 km 3 of rhyolitic magma extracted by 15% partial melting, 67 km 3 of crust (10 km 3 divided by 0.15) is involved in melting and approximately 57 km 3 (67 - 10 km 3) of granulite residue remains in place. Both the cumulates and the partial-melting residues are observed in exhumed arc ter- ranes, providing geologic support for our results. Ultramafic- mafic cumulate bodies containing olivine, clinopyroxene, and plagioclase are found in the roots of deeply exhumed arcs [Lapierre et al., 1992; Burns, 1985; DeBari and Coleman, 1989]. Xenoliths entrained in alkali-olivine basalts from the Arabian

Peninsula are interpreted to be cumulates of fractionated arc basalts and restites of partial melting after extraction of a felsic liquid [McGuire and Stern, 1993].

Estimate of Basalt Influx Rate: Caribou Volcanic Field

The rate of basalt influx to the lower crust is defined in this

study as an input volume of mantle-derived basalt normalized over the area and life span of a volcanic system and is reported in units of km 3 (km 2 Ma) -•, which is equivalent to the some- times-used unit of kilometers per million years. The area used (Figure 2) encompasses vents related to each other by age, compositional, and structural criteria. Normalizing basalt input over a time period calculated from the onset of volcanism is not strictly correct. After formation in the lower crust, magmas would have required some additional time prior to eruption to rise through the crust; however, because this additional time is short relative to the life span of the system, we use the measured eruptive interval. We also do not account for whatever time lag existed between basalt intrusion and heating of the surrounding crust to the point of partial melting.

At Caribou volcanic field, 52 km 3 (Table 1) averaged over an area of 270 km 2 and a life span of 700 ka yields an influx rate of 0.3 km 3 (km 2 Ma)-•, which falls within the range of 0.1-1 km m.y. -• determined by Lachenbruch and Sass [1978] as characteristic of Basin and Range conditions. The volume of volcanic rocks extruded at the surface (50 km 3) is nearly iden- tical to the calculated volume of basalt input to the lower crust (52 km3). Consequently, our rate of basalt influx and the overall volcanic extrusion rate for the field are essentially the same.

Basalt influx of 0.3 km 3 (km 2 Ma)-• supports the volume and variety of erupted lavas at Caribou volcanic field and satisfies the thermal and mass demands of our petrologic model. The value is undoubtedly a minimum because our methodology cannot account for an influx of basaltic magma that may crystallize in the lower crust or evolve to form derivative magmas and partial melts that crystallize throughout the crustal column, with no eruptive component. Significant unaccounted volumes of intrusive rock could be lodged in the crust as gabbros or diorites below Caribou volcanic field.

Comparison With Heat Flow Analysis: Caribou Volcanic Field

In order to assess the degree to which unaccounted basalt contributes to the total influx at Caribou volcanic field, we use the heat flow analysis of Lachenbruch [1978]. He models the relations among deep heat flow, extensional strain rate, and basalt supply in the Basin and Range province, based on the premise that the ultimate source of hig h heat flow in extensional regimes is convective transport of basaltic magma into the lithosphere by intrusion and/or underplating. Our ap- proach is to estimate the deep convective component of heat flow due to magmatic intrusion into the base of the crust from Lachenbruch [1978] and compare that value to our calculation of heat from basalt crystallization minus heat convected out of the lower crust by volcanism (= 42 x 10 •8 J or 7 mW m-2; see equation (9) and following text).

In order to estimate the deep magmatic component of heat flow from Lachenbruch [1978], we need to choose a rate s of crustal extension and a reduced heat flow qr (regional surface heat flow minus the radioactive heat contribution from the

upper crust). The Caribou region displays extensional faulting and is on

the western margin of the Basin and Range province. Esti- mates of extension rate s of the Basin and Range as a whole generally range from 0.5 to 1.0% m.y.- • [Lachenbruch, 1978, p. 41]. The existence of young volcanism from vents along extensional faults suggests that s in the Caribou volcanic field may exceed the Basin and Range average, perhaps reaching 2% per million years.

For the Caribou region, the nearest good surface heat flow determination q is 93 mW m-2 in the drill hole LSNO of Mase et al. [1982] at the boundary of the Lassen region and the Sierra Nevada province (Figure 2). The regional surface heat flow of the Caribou volcanic field is almost certainly higher than this value in Sierra Nevada granodiorite, perhaps reaching 100-110 mW m -2. Assuming that surface heat production Ao = 2.1 x 10 -6 W m -3 and characteristic depth D = 10 km, the reduced heat flow (qr = q - A oD ) in the Caribou

--2 volcanic field is -80-90 mW m

Reduced heat flow consists of a conductive component due to heat conducted from the asthenosphere and a convective component due to magmatic transport across the mantle-crust interface. If the conductive component of reduced heat flow


Table 4. Conductive Heat Flow q, From the Mantle to the Base of the Crust as a Function of Reduced Heat Flow and Crustal Extension Rate at Caribou Volcanic Field, Interpolated From the Analysis of Lachenbruch [1978]

qn, mW m -2

Surface Reduced 0.75% m.y. -• 1.00% m.y. -• 1.50% m.y. -• 2.00% m.y. -• Heat Flow, Heat Flow, Aq, mW mW m -2 mW m -2 UPL INT UPL INT UPL INT UPL INT m -2

79 59 40 40 33 32 20 16 8 1 92 71 54 53 48 46 37 33 25 19

100 79* 64 62 58 56 47 43 36 31'

111 90? 76 74 72? 71 61 59 50 47 121 100 .... 66 -- 55 125 105 .... 73 -- 61

48

18

Values of qn are given for four extension rates and two modes of magmatic transport (UPL, underplating mode; INT, intrusion mode). Maximum and minimum values of Aq due to transport of basalt across the mantle-crust interface are calculated by subtracting lowest and highest values of qn from lower and higher values of reduced heat flow expected in Caribou volcanic field, assuming an extension rate of 1-2% m.y. -•

*Low value used in calculation.

?High value used in calculation.

can be estimated, then the convective component Aq due to magmatic intrusion can be determined by subtraction. Inter- polating from Figure 6c of Lachenbruch [1978] allows estima- tion of conductive heat flow from the mantle to the base of the

crust qn as a function of extension rate s and reduced heat flow qr' Estimates of the heat flux Aq due to transport of basalt across the mantle-crust interface for various values of qr and s are given in Table 4. For a reduced heat flow of 79-90 mW m -2 and an extension rate of 1-2% m.y. -•, qn ranges from 31 to 72 mW m -2, and therefore Aq ranges from 18 to 48 mW m -2. This range compares with 7 mW m -2 calculated from our analysis of basalt influx to the Caribou volcanic field.

This analysis suggests that the amount of basalt transmitted across the mantle-crust interface could be ---3 to 7 times the

amount of basalt we calculated as being required to yield the volume of volcanic rocks at Caribou volcanic field. The addi-

tional basalt presumably lodges in the lower crust as gabbro produced by complete crystallization of basalt or higher in the crust as more silicic (dioritic to granodioritic) plutons derived from the basalt by crystal fractionation, partial melting, and mixing.

Thermal and Mass Implications of Magmatic Evolution: Lassen Volcanic Center

For Brokeoff volcano, cooling and fractional crystallization of basalt provides 148 x 10 TM J (sum of QFRAC values for Brokeoff volcano in Table 1), which is nearly twice the 78 x 1018 J (sum of Brokeoff Q•,M values) required to produce the 27 km 3 of rhyolite melt (sum of Brokeoff lapM values) incorporated into Brokeoff lavas. This relationship is similar to that of Caribou volcanic field. Thus in the andesitic phase of volcanism, either regionally or in the large stratocone, the calculated basalt input is more than sufficient to drive the magmatic system. Indeed, accumulation of substantial excess thermal energy (QFRAC -- Q•'M = 70 x 10 48 J) during development of Brokeoff volcano probably was critical for initiation of the silicic phase of volcanism that predominated after 400 ka.

At Lassen volcanic center as a whole, however, the calculated input of basalt is insufficient to generate the large volume of rhyolitic partial melt required by our petrologic model. Significant volumes of silicic partial melt are incorporated into erupted lavas at Lassen volcanic center, with approximately 65% of the 215 km 3 of erupted magmas consisting of rhyolitic partial melts and the other 35% being variously fractionated

mafic magmas. The total volume of rhyolitic melt (VpM) required at Lassen volcanic center is 141 km 3. The amount of heat Qp;u needed to produce that volume by partial melting is 410 x 10 •8 J. In contrast, the total amount of heat provided by cooling and fractional crystallization of 112 km 3 of basalt (QFRAC) is 157 X 1048 J. Thus in the analysis summarized in Table 1 there is an overall heat shortfall/X Q at Lassen volcanic center of 253 x 10 •8 J.

An additional source of heat to the lower crust, beyond the basalt input we have calculated in the fourth row of Table 1, is needed to provide A Q. We surmise that this A Q is derived from additional input of mantle-derived basalt, but determination of the amount of that additional input depends on assumptions of basalt evolution. The simplest case to consider is one in which A Q is provided by a volume of basalt Vx transmitted from the mantle and completely crystallized as gabbroic intrusions in the lower crust, without further ascent or evolution in the crust. This scenario is the most efficient at keeping basaltic heat in the lower crust in the zone of partial melting.

The volume of this basalt Vx is calculated from the equation

AQ = (Tt.- 900øC)c•p•lax + AH.t•p,•Vx (10) AHrB is latent heat of fusion of basalt, calculated for the composition listed in Table 3. Solving for lax yields a basalt volume of 91 km 3, which has no eruptive expression and is in addition to the previously calculated input basalt volume of 112 km 3 at Lassen volcanic center. This additional volume was

intruded prior to the onset of the major pulses of silicic volcanism at 400 ka (Rockland sequence) and 250 ka (dome field dacites).

The 91 km 3 of basalt is a minimum additional input because we assume complete crystallization to gabbro in the lower crust with no loss of magmatic mass, and thus heat, from the zone of partial melting. Any thermal energy removed from the lower crustal system by conduction and higher-level emplacement of basalt or derivative plutonic rocks would increase the amount of additional basalt that is necessary to provide AQ. More complex scenarios of basalt evolution would lead to more plutonic rocks in the crust. Assuming complete crystallization to gabbro in the lower crust, however, yields a quantifiable minimum basalt input of 203 km 3 (112 km 3 + 91 km 3) that is needed to support erupted lavas at Lassen volcanic center and to satisfy the demands of partial melting.

The minimum basalt input of 203 km 3 to the lower crust is


less than the erupted volume of 215 km 3. This is permitted because of the role of crustal partial melting in magma genesis. If fractional crystallization were the dominant process by which silicic magmas were generated at Lassen volcanic center, then the volume of evolved magma available for eruption progres- sively would decrease as crystallization of the input basaltic magma proceeded, and consequently the extruded volume would be less than the volume of input basalt. However, when generation of silicic magmas is due to extensive partial melting, then crustal material is mobilized, and the input:output ratio can be less than 1. At Lassen volcanic center the input:output ratio is 203 km3:215 km 3 = 0.9.

Basalt-driven partial melting derives from the fact that the melting temperature and heat of fusion of rhyolite are sub- stantially lower than those of basalt. For example, cooling and crystallizing of 1 km 3 of basaltic magma from 1300øC to 900øC provides 2.8 x l0 is J (from equation (10)). Assuming complete heat transfer, that amount of heat ideally can generate 4 km 3 of rhyolitic melt (from equation (8)). This relationship suggests that under conditions of highly effective heat transfer from basalt to the lower crust, substantial volumes of rhyolitic melt can be created.

The effectiveness of basalt intrusion in producing partial melting can be measured by the ratio of the minimum required volume of input basalt to the volume of partial melt. For Lassen volcanic center as a whole, this minimum ratio (203 km 3 divided by 141 km 3) is 1.4. In contrast, at Caribou volcanic field, where the rate of basalt input is lower and partial melting is less important, the ratio is approximately 5. The lack of a major deep partial-melt zone at Caribou volcanic center is consistent with closely spaced mafic to intermediate vents in the field, in contrast to Lassen volcanic center, where mafic vents are absent within the larger focal area.

Basalt intrusion at Lassen volcanic center transfers much

heat across the base of the crust. The net amount of heat

available to warm the lower crust beneath Lassen volcanic

center is the heat provided by cooling and fractional crystallization of basalt (QFR^c, 157 X 10 •s J) plus the additional heat from complete crystallization of basalt magma to gabbro (A Q, 253 x 10 •s J) minus the amount of heat in the erupted rhyolite (QFR 4-Xp2uQLc = 141 X 10 •8 J). This formulation is similar to that for Caribou volcanic field (see equation (9)), except that for Lassen volcanic center the total heat provided by basalt crystallization is equal to Qp2u because of addition of A Q to Q FRAC (see A Q row, Table 1). The net amount of 269 x 10 •s J averaged over the area and life span of the center is equivalent to 82 mW m -2. Such a high value of Aq demon- strates that Lassen volcanic center is a major lower crustal thermal anomaly.

Estimate of Basalt Influx Rate: Lassen Volcanic Center

At Lassen volcanic center, 203 km 3 (Table 1) of mantle- derived basalt is averaged over 190 km 2 and a time interval of 650 kyr to yield a normalized influx value of 1.6 km 3 (km 2 Ma)-•. Clearly, this average value smooths temporal and spatial variations in magmatism within the center. Pulses of silicic magmatism are restricted to a small part of the overall area; conversely, hybrid andesites account for about 40% of the area of the center but less than 5% of the eruptive volume. The influx rate at Lassen volcanic center is less than the extrusion

rate of 1.7 km 3 (km 2 Ma) -•, reflecting augmentation of the extrusive volume by crustal partial melts.

This basalt-influx rate at Lassen volcanic center is a mini-

mum, based on the simple model of complete crystallization of basalt in the lower crust as a source of heat for A Q. We have not attempted a heat flow comparison at Lassen volcanic center like that we applied at Caribou volcanic field in order to estimate the total possible influx of basalt. Lassen volcanic center is not analogous to the Basin and Range settings analyzed by Lachenbruch [1978], and values for qr and s are essentially unconstrained.

Lassen volcanic center has been sustained by basalt influx to the lower crust at a rate at least 5 times that for Caribou

volcanic field. The higher influx rate at Lassen volcanic center likely reflects the coupled effects of higher rate of basalt production in the mantle above the subducting plate and locally higher rate of lithospheric extension that focuses magmatism in the crust. In his analysis of the large Long Valley magmatic system in east central California, Lachenbruch [1976] points out that the crust would thicken unreasonably rapidly owing to intrusion of mantle-derived basalt unless intrusion were ac-

companied by accelerated local spreading. Lachenbruch and Sass [1978] further argue that sustained high basalt supply may facilitate regionally distributed extension because the basaltic- heat contribution prevents cooling and consequent thickening of the lithosphere.

The presence of older volcanic centers in the vicinity of Lassen volcanic center is evidence of temporal and spatial variation in basalt influx on a regional scale. The Maidu and Dittmar volcanic centers (Figure 2) developed during the late Pliocene to early Quaternary within a few tens of kilometers of the site of Lassen volcanic center and had life spans of 1-2 million years [Clynne et al., 1993], suggesting that the rate of basalt influx and/or sites of localized extension also shift on

those scales of space and time. Basalt influx high enough to sustain ample partial melting in

the lower crust is crucial for development of a large silicic locus such as Lassen volcanic center. Caribou volcanic field has

failed to develop into a silicic locus not because basalt rose too quickly to the surface in a more extensional environment, but rather because the basalt supply has been inadequate for substantial partial melting. Indeed, the extension rate at Lassen volcanic center probably is higher in order to accommodate the greater influx of basalt. Extension rates have not been quantified across the Lassen region, although a baseline Global Po- sitioning System network has been established there (D. Dzuri- sin, personal communication, 1992). An important future project would be correlation of measured extension rates with basalt influx and volcanic extrusion rates in the Lassen region.

While emphasizing the role of crustal partial melting in magmatic evolution of the Lassen region, we do not conclude that partial melting has occurred on the scale necessary to cause wholesale mobilization of lower crustal material or em-

placement of plutonic magmas of batholithic proportions. Batholith formation in magmatic arcs may require both a higher plate convergence rate and a longer history of magmatism [Younker and Vogel, 1976] than are found in the relatively immature arc of the southernmost Cascades.

Conclusions

For the Lassen region, a model of basalt-driven magmatic evolution characterized by mixing of variously ffactionated mafic magmas with silicic partial melts of the lower crust is fundamentally reasonable in terms of its broad-scale heat and mass requirements. The calculated basalt input is more than


sufficient to drive the andesitic phases of magmatism at Lassen volcanic center and Caribou volcanic field. At Lassen volcanic

center, accumulation of excess thermal energy during the andesitic phase appears to have been critical for initiation of the later silicic phase of volcanism. For Lassen volcanic center as a whole, heat needed for partial melting dominates the heat budget and significantly leverages the basaltic mass requirements in our model. At least 203 km 3 of mantle-derived basalt

is needed to drive the magmatic system of Lassen volcanic center.

The amount of heat added by intrusion of so much basalt beneath Lassen volcanic center is consonant with a major thermal anomaly in the lower crust. Cooling and crystallization of 203 km 3 of basalt provides heat for 141 km 3 of rhyolitic partial melt. At Caribou volcanic field, where basalt influx is lower and partial melting is less important, basalt input of 52 km 3 produces 10 km 3 of rhyolitic partial melt. Partial melting at Lassen volcanic center mobilizes significant amounts of crustal material and thus augments the magmatic volume available for eruption. Consequently, the calculated input volume of 203 km 3 of mantle-derived basalt required is less than the erupted volume of 215 km 3.

At Lassen volcanic system the minimum rate of basalt input to the lower crust that is required to support the volcanic system and to satisfy the heat and mass demands of our petrologic model is 1.6 km 3 (km 2 Ma) -•, about 5 times greater than the minimum influx to the Caribou volcanic field. Basalt

influx high enough to sustain considerable partial melting in the lower crust, coupled with locally high extension, is a crucial factor in development of a large silicic locus like Lassen volcanic center. In contrast, Caribou volcanic field has failed to develop into a large silicic center primarily because the basalt supply has been inadequate.

Acknowledgments. We consulted Bruce Hemingway on the thermodynamic basis of our analysis and Rosalind Helz on the melting behavior of basalts and benefited from the expertise that they so patiently shared. Many of our petrologic ideas were refined by discus- sions with Tom Bullen. Reviews by George Bergantz, Wes Hildreth, Paul Morgan, Manuel Nathenson, Nick Petford, and Steven Sparks greatly improved the manuscript. This work was supported by the Geothermal Research, Volcano Hazards, and Deep Continental Stud- ies Programs of the U.S. Geological Survey.

References

Basaltic Volcanism Study Project, Basaltic Volcanism on the Terrestrial Planets, 1286 pp., Pergamon, Tarrytown, 1981.

Beard, J. S., and G. E. Lofgren, Dehydration melting and water- saturated melting of basaltic and andesitic greenstones and amphi- bolites at 1, 3, and 6.9 kb, J. Petrol., 32, 365-401, 1991.

Bergantz, G. W., Underplating and partial melting: Implications for melt generation and extraction, Science, 245, 1093-1095, 1989.

Borg, L. E., The origin and evolution of magmas from the Lassen region of the southernmost Cascades, Ph.D. dissertation, Univ. of Tex. at Austin, 1995.

Bullen, T. D., and M. A. C!ynne, Trace element and isotopic constraints on magmatic evolution at Lassen volcanic center, J. Geophys. Res., 95, 19,671-19,691, 1990.

Burns, L., The Border Ranges ultramafic and mafic complex, south- central Alaska: Cumulate fractionates of island-arc volcanics, Can. J. Earth Sci., 22, 1020-1038, 1985.

Clemens, J. D., and D. Vielzeuf, Constraints on melting and magma production in the crust, Earth Planet. Sci. Lett., 86, 287-306, 1987.

Clynne, M. A., Disaggregation of quenched magmatic inclusions contributes to chemical diversity in silicic lavas of Lassen Peak, Califor- nia (abstract), Bull. N.M. Bur. Mines Miner. Resour., 131, 54, 1989.

Clynne, M. A., Stratigraphic, lithologic, and major element geochemi-

cal constraints on magmatic evolution at Lassen volcanic center, California, J. Geophys. Res., 95, 19,651-19,669, 1990.

Clynne, M. A., Geologic studies in the Lassen area of the southernmost Cascade Range, California, Ph.D. thesis, 404 pp., Univ. of Calif., Santa Cruz, 1993.

Clynne, M. A., and L. J.P. Muffler, Geologic map of the Lassen region, Cascade Range, USA, Trans. Geotherm. Resour. Counc., 14, 1387- 1389, 1990.

Clynne, M. A., L. J.P. Muffler, and G. B. Dalrymple, Late Cenozoic volcanism in the Lassen area, southernmost Cascade Range, Cali- fornia (abstract), Geol. Soc. Am. Abstr. •'rograms, 25, 22, 1993.

Cockerham, R. S., Evidence for a 180-km-long subducted slab beneath northern California, Bull. Seismol. Soc. Am., 74, 569-576, 1984.

DeBari, S. M., and R. G. Coleman, Examination of the deep levels of an island arc: Evidence from the Tonsina ultramafic-mafic assem-

blage, Tonsina, Alaska, J. Geophys. Res., 94, 4373-4391, 1989. Ghiorso, M. S., Chemical mass transfer in magmatic processes, I,

Thermodynamic relations and algorithms, Contrib. Mineral. Petrol., 90, 107-120, 1985.

Guffanti, M., and C. S. Weaver, Distribution of late Cenozoic volcanic vents in the Cascade Range: Volcanic arc segmentation and regional tectonic considerations, J. Geophys. Res., 93, 6513-6529, 1988.

Guffanti, M., M. A. Clynne, J. G. Smith, L. J.P. Muffler, and T. D. Bullen, Late Cenozoic volcanism, subduction, and extension in the Lassen region of California, southern Cascade Range, J. Geophys. Res., 95, 19,453-19,464, 1990.

Harrison, T. M., and E. B. Watson, The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations, Geochim. Cosmochim. Acta, 48, 1467-1477, 1984.

Hart, W. K., J. L. Aronson, and S. A. Mertzman, Areal distribution and age of low-potassium, high-alumina olivine-tholeiite magmatism in the northwestern Basin and Range, Geol. Soc. Am. Bull., 95, 186- 195, 1984.

Heiken, G., and J. C. Eichelberger, Eruptions at Chaos Crags, Lassen Volcanic National Park, California, J. Volcan. Geotherm. Res., 7, 443-481, 1980.

Helz, R. T., Phase relations of basalts in their melting ranges at PH2O -- 5 kb, II, Melt compositions, J. Petrol., 17, 139-193, 1976.

Hildreth, W., Gradients in silicic magma chambers: Implications for lithospheric magmatism, J. Geophys. Res., 86, 10,153-10,192, 1981.

Huppert, H. E., and S. J. Sparks, The generation of granitic magmas by intrusion of basalt into continental crust, J. Petrol., 29, 599-624, 1988.

Johnston, A.D., and D. S. Draper, Near-liquidus phase relations of an anhydrous high-magnesia basalt from the Aleutian Islands: Implica- tions for arc magma genesis and ascent, J. Volcan. Geotherm. Res., 52, 27-41, 1992.

Jurewicz, A. G., and E. B. Watson, Cations in olivine, I, Calcium partitioning and Ca-Mg distribution between olivine and coexisting melts, with petrologic application, Contrib. Mineral. Petrol., 99, 176- 185, 1988.

Lachenbruch, A. H., Geothermal setting and simple heat conduction models for the Long Valley caldera, J. Geophys. Res., 81,769-784, 1976.

Lachenbruch, A. H., Heat flow in the Basin and Range province and thermal effects of tectonic extension, Pure Applied Geophys., 117, 34-50, 1978.

Lachenbruch, A. H., and J. H. Sass, Models of an extending lithosphere and heat flow in the Basin and Range province, Mem. Geol. Soc. Am., 152, 209-250, 1978.

Lange, R. L., and I. S. E. Carmichael, Thermodynamic properties of silicate liquids with emphasis on density, thermal expansion and compressibility, Rev. Mineral., 24, 25-64, 1990.

Lapierre, H., L. E. Ortiz, W. Abouchami, O. Monod, C. Coulon, and J.-L. Zimmerman, A crustal section of an intra-oceanic island arc: The late Jurassic-early Cretaceous Guanajuato magmatic sequence, central Mexico, Earth Planet. Sci. Lett., 108, 61-77, 1992.

Mase, C. W., J. H. Sass, A. H. Lachenbruch, and R. J. Munroe, Preliminary heat-flow investigations of the California Cascades, U.S. Geol. Surv. Open File Rep., 82-150, 240 pp., 1982.

McGuire, A. V., and R. J. Stern, Granulite xenoliths from western Saudi Arabia: The lower crust of the late Precambrian Arabian-

Nubian Shield, Contrib. Mineral. Petrol., 114, 395-408, 1993. Mooney, W. D., and C. S. Weaver, Regional crustal structure and

tectonics of the Pacific coastal states: California, Oregon, and Wash-


ington, in Geophysical Framework of the Continental United States, Mem. Geol. Soc. Am., 172, 129-162, 1989.

Oppenheimer, D., et al., The Cape Mendocino, California, earthquakes of April 1992: Subduction at the triple junction, Science, 261, 433-438, 1993.

Peacock, S. M., T. Rushmer, and A. B. Thompson, Partial melting of subducting oceanic crust, Earth Planet. Sci. Lett., 121,227-244, 1994.

Pezzopane, S. K., and R. J. Weldon II, Tectonic role of active faulting in central Oregon, Tectonics, 12, 1140-1169, 1993.

Robie, R. A., B. S. Hemingway, and J. R. Fisher, Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (10 s pascals) pressures and at higher temperatures, U.S. Geol. Surv. Bull., 1452, 456 pp., 1979.

Tatsumi, Y., M. Sakuyama, H. Fukuyama, and I. Kushiro, Generation of arc basalt magmas and thermal structure of the mantle wedge in subduction zones, J. Geophys. Res., 88, 5815-5825, 1983.

Walter, S. R., Intermediate-focus earthquakes associated with Gorda plate subduction in northern California, Bull. Seismol. Soc. Am., 76, 583-588, 1986.

Younker, L. W., and T. A. Vogel, Plutonism and plate dynamics: The origin of Circum-Pacific batholiths, Can. Mineral., 14, 238-244, 1976.

M. A. Clynne and L. J.P. Muffler, U.S. Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, CA 94025.

M. Guffanti, U.S. Geological Survey, 922 National Center, 12201 Sunrise Valley Drive, Reston, VA 22092. (e-mail: [email protected]. usgs.gov)

(Received August 19, 1994; revised October 31, 1995; accepted November 9, 1995.)

Documents

Thermal and mass implications of magmatic evolution in the Lassen volcanic region, California, and minimum constraints on basalt influx to the lower crust