MAFIC-FELSIC MAGMA MIXING IN CONTINENTAL CRUST FORMATION: AN EXAMPLE FROM LOW-INITIAL Sr GRANITOIDS OF THE NORTHWESTERN PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIABenjamin L. Clausen - Geoscience Research Institute & Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350Douglas M. Morton - U. S. Geological Survey & Department of Earth Sciences, University of California, Riverside, CA 92521Ronald W. Kistler - U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025Cin-Ty A. Lee - Department of Earth Science, Rice University, Houston, TX 77005

1. INTRODUCTION 4. PETROGENETIC MODEL

Selected References

Southern California

SCAMPAreal Mapping Project

DEPARTMENT OFCONSERVATIONCaliforniaGeological Survey

Magma MixingPartial Melting & Fractional Crystallization

3. MAGMA DIVERSIFICATIONSample Calculations

West of Palomar Mountain

2. DATA

ABSTRACT A 1000 km2 area within the western Peninsular Ranges Batholith (PRB) near Escondido, California is made up of granitoids having a uniformly low initial 87Sr/86Sr isotope ratio of Sri<0.704, but a wide range of SiO2 compositions ranging from 46 to 78 wt.%. The systematically sampled Escondido plutons are made up of three distinct groups consisting of 20% gabbros, 35% tonalites, and 45% granodiorites. The linear data pattern on Harker diagrams is interpreted as resulting from the mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to produce magma of intermediate SiO2 composition. These three magma types subsequently fractionated and were emplaced at shallow levels. The early Cretaceous western PRB granitoids formed in association with island arc basalts accreted to the North American craton, and this especially low Sri portion displays negligible contamination from old continental crust. These Escondido granitoids are unique in having undergone a cycle of mantle melting to give arc basalts, a cycle of arc basalt (or gabbroic underplate) melting to give a range of SiO2 granitoids, but no third cycle of continental crust melting and assimilation to yield high Sri. Therefore, these granitoids provide a simplified Phanerozoic example of the petrogenetic process for forming continental crust. Simple partial melting, mixing, and fractional crystallization calculations were performed to quantitatively understand the relative importance of each process during differentiation. Mass balance calculations indicate that the volume of ultramafic restite left after differentiation of the lower crust is about twice the volume of the fractionated Escondido granitoids.

Significance – The Escondido granitoids provide a uniquely simple system to aid in understanding continental crust formation, especially the role of magma mixing: • unique because they contain a wide range of SiO2,

but low initial 87Sr/86Sr (Sri); • simple because this indicates significant fractionation,

but no continental crust contamination.

after: Kistler and Peterman (1973), Anderson (1990), Winter (2001)

• Kistler, R. W. and Z. E. Peterman (1973). Variations in Sr, Rb, K, Na, and initial Sr87/Sr86 in Mesozoic granitic rocks and intruded wall rocks in central California. Geological Society of America Bulletin 84: 3489-3511.

• Kistler, R. W. and Z. E. Peterman (1978). Reconstruction of crustal blocks of California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks. United States Geological Survey Professional Paper 1071.

• Kistler, R. W., B. W. Chappell, D. L. Peck, and P. C. Bateman (1986). Isotopic variation in the Tuolumne Intrusive Suite, central Sierra Nevada, California. Contributions to Mineralogy and Petrology 94: 205-220.

• Kistler, R. W., J. L. Wooden, and D. M. Morton (2003). Isotopes and ages in the northern Peninsular Ranges batholith, southern California. USGS Open-File Report 03-489. 45p.

• Lee, C.-T. A., D. M. Morton, R. W. Kistler, and A. K. Baird (2007). Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism. Earth and Planetary Science Letters 263: 370-387.

• Silver, L. T., H. P. Taylor, and B. W. Chappell (1979). Some petrological, geochemical and geochronological observations of the Peninsular Ranges batholith near the international border of the U.S.A. and Mexico. In Mesozoic crystalline rocks. P. L. Abbott and V. R. Todd, eds., San Diego State University, Department of Geological Sciences: 83-110.

• Sun, S.-s. and W. F. McDonough (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. A. D. Saunders and M. J. Norry, eds. Geological Society of London Special Publication 42: 313-345.

• Tatsumi, Y. (2005). The subduction factory: How it operates in the evolving Earth. GSA Today 15(7): 4-10. • Tulloch, A. J., and Kimbrough, D. L. (2003). Paired plutonic belts in convergent margins and the development of high Sr/Y

magmatism: Peninsular Ranges batholith of Baja-California and Median batholith of New Zealand. In Tectonic evolution of northwestern México and the southwestern USA. S. E. Johnson, S. R. Paterson, J. M. Fletcher, G. H. Girty, D. L. Kimbrough, and A. Martín-Barajas, eds. Geological Society of America Special Paper 374: 275-295.

• Winter, J. D. (2001). An introduction to igneous and metamorphic petrology. Upper Saddle River, NJ, Prentice-Hall.

• Anderson, J. L., ed. (1990). The nature and origin of Cordilleran magmatism. Geological Society of America Memoir 174 • Baird, A. K., K. W. Baird, and E. E. Welday (1979). Batholithic rocks of the northern Peninsular and Transverse Ranges,

southern California: chemical composition and variation. In Mesozoic crystalline rocks. P. L. Abbott and V. R. Todd, eds. San Diego State University, Department of Geological Sciences: 111-132.

• Baird, A. K. and A. T. Miesch (1984). Batholithic rocks of southern California --- a model for the petrochemical nature of their source materials. U. S. Geological Survey Professional Paper 1284.

• Boynton, W. V. (1984). Cosmochemistry of the rare earth elements: meteorite studies. In Rare earth element geochemistry: Developments in geochemistry 2. P. Henderson, ed. Amsterdam, Elsevier: 63-114.

• Busby, C. J. (2004). Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of Baja California, Mexico. Tectonophysics 392: 241-277.

• Cocherie, A. (1986). Systematic use of trace element distribution patterns in log-log diagrams for plutonic suites. Geochimica et Cosmochimica Acta 50: 2517-2522.

• Collins, W. J. (1996). Lachlan Fold Belt granitoids: products of three-component mixing. In The third Hutton symposium on the origin of granites and related rocks. M. Brown, P. A. Candela, D. L. Peck, W. E. Stephens, R. J. Walker, and E.-a. Zen, eds. Geological Society of America Special Paper 315: 171-181.

• DePaolo, D. J. (1981). A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. Journal of Geophysical Research 86(B11): 10470-10488.

• Ehrlich, R. and W. E. Full (1987). Sorting out geology—unmixing mixtures. In Use and abuse of statistical methods in the earth sciences. W. B. Size, ed. Oxford University Press: 33-46.

• Faure, G. (1986). Principles of isotope geology. New York, Wiley. • Gromet, L. P. and L. T. Silver (1987). REE variations across the peninsular ranges batholith: implications for batholithic

petrogenesis and crustal growth in magmatic arcs. Journal of Petrology 28: 75-125. • Johnson, G. W., R. Ehrlich, and W. E. Full (2002). Principal components analysis and receptor models in environmental

forensics. In Introduction to environmental forensics. B. L. Murphy and R. D. Morrison, eds. Academic Press: 461-515.

Sample differentiation calculations on the Escondido granitoids are shown for partial melting (PM) of the mantle and of lower crust gabbro/amphibolite, mixing of gabbro and granodiorite magmas, and fractional crystallization (FC) of tonalite and granodiorite magmas. The two arrows signify the direction of the two differentiation cycles. Tic marks represent melt fraction in 10% steps. Atypical Escondido granitoids are shown as open circles. The eastern PRB samples are taken from east of the San Jacinto Fault. Effective distribution coefficients were used for the major elements:

• partial melting of gabbro DSiO2=0.65, DMgO=7.0, DK2O=0.15, DNa2O=0.45 • fractional crystallization of granodiorite DSiO2=0.93, DMgO=2.5, DK2O=0.01, DNa2O=1.3

0

2

4

6

8

10

45 50 55 60 65 70 75 80

MgO

(wt.

%)

SiO2 (wt. %)

eastern PRBgabbro(gabbro)tonalite(tonalite)granodiorite(granodiorite)mantleN-MORBtholeiitic basaltcalc-alkaline basaltSantiago Pk volclower cont crustBedford Canyon Fmmantle - PMgabbro - PMmixingtonalite - FCgranodiorite - FC

90%

50%

20%

10%

mantle composition at43.4% SiO241.8% MgO

0.0

0.5

1.0

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

K2O

(wt.

%)

Na2O (wt. %)

10%

80%

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(ppm

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Sierra Nevada

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oxid

e (w

t. %

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granitoidsgranitoids - atypicalmantleMORBbasalt - tholeiitebasalt - calc-alkalineSantiago Pk volclower cont crustBedford Cyn Fm.eastern PRB

Fe2O3

CaO

MgO

Al2O3

mantle composition at45% SiO237.8% MgO

gabbro

tonalite

granodiorite

0

2

4

6

45 50 55 60 65 70 75 80SiO2 (wt. %)

Na2O

K2O

low-K tholeiitemedium-K calc-alkali

gabbro(gabbro)tonalite(tonalite)granodiorite(granodiorite).

EM2

EM3immobile

EM1incompatible B477 restite

B489

B493cumulate

B522cumulate

FC

PMFC

mixing

FC

1

10

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100 1000 10000

Co

(ppm

)

Ba (ppm)

Partial melting and fractional crystallization of magma are not sufficient to explain the full range of Escondido granitoid data having 46-78 wt% SiO2. Magma mixing is needed as well, as evidenced by:

• the linear data trend on Harker diagrams • the gap in data at 65-70 wt% SiO2 • data plots of compatible .vs. incompatible elements

Such magma mixing has been suggested as generally needed to explain the calc-alkaline andesite/tonalite average composition of the continental crust. A principal production mechanism is thought to be mixing of mantle-derived basaltic magma and arc crust-derived felsic magma (Tatsumi, 2005). Deep crustal level mixing has been suggested for the Lachlan Fold Belt (Collins, 1996) and Sierra Nevadas (e.g., Kistler et al., 1986). Because the Escondido granitoids have no continental crust contamination, it allows a quantitative determination of the importance of this mixing.

Major elements – Harker diagrams showing the major oxide data for the Escondido granitoids compared to data from the eastern PRB and to various possible source areas. Good straight-line least squares fit to the data provide evidence for magma mixing. The gap in the data at 65-70% SiO2 suggests two different processes for forming tonalites and granodiorites.

Polytopic vector analysis – Escondido granitoid samples plotted with respect to the three end members (EM) resulting from polytopic vector analysis (Ehrlich and Full, 1987; Johnson et al., 2002). Partial melting (PM) of the mantle yields a gabbroic melt. Gabbro and granodiorite magmas mix to give tonalite, after which fractional crystallization (FC) moves the magma composition away from the compatible and immobile corners. A tonalite and a granodiorite sample represent cumulates from fractional crystallization.

Trace and rare earth spidergrams are shown for two cycles of partial melting and fractional crystallization. Standard calculational parameters were used for the original concentration and distribution coefficients. The trace elements are normalized to Sun and McDonough (1989) and rare earth elements to Boynton (1984).

2nd cycle – From the calculations, the Escondido granodiorites can be modeled as resulting from 25% partial melting (PM) of lower crustal gabbroic rocks followed by up to 70% fractional crystallization (FC) of the resulting magma. The tonalities can be modeled as resulting from mixing between gabbroic and tonalitic magma. The mean and standard deviation of the eastern PRB data is plotted in the REE spidergram to highlight its steeper curve. This demonstrates that only in the east was the magma source deep enough to leave behind REE in garnet (Gromet and Silver, 1987).

1

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Cs Rb Ba Th U Nb Ta Sr Zr Hf Ti Y

Sam

ple

/ cho

ndrit

e

c-a

th

SPv

gabbro

(a)

10% PM of mantle

1

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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Sam

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ndrit

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(b)

10% PM of mantle

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Cs Rb Ba Th U Nb Ta Sr Zr Hf Ti Y

Sam

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(a)

25% PMof gabbro

70% FC of granodiorite

tonalite

granodiorite

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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Sam

ple

/ cho

ndrit

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(b)

25% PMof gabbro

70% FC of granodiorite

tonalite

granodiorite

B502

eastern PRB

Histogram showing the three-part SiO2 distribution for the Escondido granitoids. The data are fit using three Gaussian peaks centered at 51.9% SiO2 for gabbro, 61.9% SiO2 for tonalite, and 72.8% SiO2 for granodiorite.

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# of

sam

ples

SiO2 (wt. %)

gabbro (1/6)

tonalite (1/3)

granodiorite (1/2)

Map for the northern Peninsular Ranges Batholith (PRB) in southern California showing major faults and the suture and shear zones. Circles show the location of samples collected by Baird et al. (1979, 1984): black, blue, and red circles locate the Escondido samples that are the subject of this report, with atypical samples shown by open circles; open gray circles locate the remaining samples, with those northeast of the San Jacinto Fault being emphasized here. Sample locations are also shown for published Sm-Nd (DePaolo, 1981) and U-Pb (Kistler et al., 2003) isotope data. The updated δ18O contours use data from Silver et al. (1979), DePaolo (1981), and Kistler et al. (2003).

Temecula

Riverside

Palm Springs

Escondido

San Diego

Mexico

PacificOcean

San Andreas Fault

San Jacinto Fault

Agua Caliente Fault

Elsinore Fault

6‰ δ18O 8‰

SUTU

RE ZON

ESH

EAR ZO

NE

34.0° N

33.5°

33.0°

32.5°117.5° W 117.0° 116.5°

California25 mi

40 km

0

0

x

Santia

go P

k

Bedfor

d Cyn

JT - volcanics

J - marine

K - granitoids Baird Escondido gabbro Escondido tonalite Escondido granodiorite other northern PRB thin sections isotopes Sm-Nd U-Pb δ18O

R

westernPRB

easternPRB

N

Peninsular RangesBatholith

CoastBatholith

IdahoBatholith

Sri=0.706line

N500 km

Sierra NevadaBatholith

The petrogenetic process generating the Escondido granitoids includes partial melting (PM), fractional crystallization (FC), and magma mixing in two cycles of differentiation from mantle to lower crust and lower crust to upper crust. The resulting granitoid volumes are one sixth gabbro, one third tonalite, and one half granodiorite. Restite from the second cycle has a volume about twice that of the granitoids.

Jurassic

Mid-Cretaceous

Oceanic crust

Sediments

Continental crust, including underplate and magma chambers

Continental crust, including underplate and magma chambers

island arcvolcanics

suturezone

myloniteshear zone

Mantle

Early Cretaceousmoderateextension

compression

(eastern PRB in North American craton)

(western PRB in island arc volcanics)

(MORB)

(flysch)

Peninsular Ranges Batholith

SWNE

craton

0.702

0.704

0.706

0.708

0.710

0.712

0.714

0.716

0.0 0.5 1.0 1.5 2.0 2.5 3.0

87Sr

/86Sr

87Rb/86Sr

1a 1bultramafic

2a2c

3a

3c

felsic

mafic

cycles#1 = MORB, IAB ... Santiago Pk volcanics#2 = uncontaminated, Precambrian, A-type … Escondido PRB#3 = contaminated, Phanerozoic, … eastern PRB

3b

2b

1c

UPPER CRUST (5-10 km depth) gabbro [1/6] tonalite [1/3] granodiorite [1/2]

.FC FC FC

mafic magma mixing 20-30% PM .cumulate .

LOWER CRUST (20-30 km depth) arc basalt, underplate … amphibolite .

FC . 5-15% PM restite [2x]

..

MANTLE (60-90 km depth) peridotite delamination.

fluid migrates up .

SUBDUCTED OCEANIC SLAB dehydration .

1st cycle

2nd cycle

• Southern California batholiths – About 500 granitoid samples were collected by Baird et al. (1979) on a grid that is accurately representative of the batholiths in southern California. Each sample was made up of eight specimens – two collected at each of the four corners of a square, 400 ft on a side.

• Peninsular Ranges Batholith – Of the 500 or so samples, 287 were collected in the northern Peninsular Ranges Batholith (PRB).

• Escondido granitoids – On the northwestern edge of the PRB, 67 of the granitoid samples contain low Sri (<0.704) and a range of SiO2 compositions (46-78 wt%).

The low-Sri values in the Escondido granitoids

The original source for island arc basalts is usually considered to be partial melting of an ultramafic mantle yielding mafic magma with a 87Sr/86Sr ratio of less than 0.704 (Faure, 1986). For the Escondido area, this magma produced the low-Sri (Jurassic/Triassic) Santiago Peak island arc basalts and then underplated and intruded the volcanic arc with low-Sri melts. As additional mantle magma intruded, melted, and mixed with the arc basalts, the composite retained its low-Sri composition. These low-Sri mafic melts then differentiated yielding low-Sri felsic plutons.

None of these steps involved contamination from high-Sri upper continental crust or back arc basin marine sediments, such as the (Jurassic) marine Bedford Canyon Formation. The Escondido sample locations show few associated marine sedimentary rock from which they could be contaminated, in contrast to sample locations to the north and east that have higher Sri values.

1st cycle – The Escondido gabbros and Santiago Peak volcanic basalts (SPv) are closer to a calc-alkaline (c-a) than a tholeiitic basalt (th) composition. The gabbros and basalts can be modeled as resulting from 10% partial melting (PM) of the mantle.

Trace elements – This bivariate Co-Ba diagram plots a compatible against an incompatible element. The mantle composition is off the diagram at 105 and 6.6 ppm, respectively. The log-log version of this plot provides the best discrimination between partial melting with a concave up curve, fractional crystallization with a straight line, and mixing with a concave down curve (see Cocherie, 1986). The DCo=1.5 and DBa=0.2 for fractional crystallization calculations on tonalite.

The Rb-Sr concentrations demonstrate that the Escondido granitoids differentiated from a much more tholeiitic source than the eastern PRB or Sierra Nevada granitoids (Kistler and Peterman, 1978). The DSr=1.2 and DRb=0.01 for fractional crystallization calculations on tonalite.

The evolution of Sr isotopes with time. The Sri (y-intercept) ratio, the Rb concentration, and the 87Sr/86Sr ratio begin with low values in the mantle and increase through several differentiation cycles. Each cycle consists of three parts:

a) with melting and diffusion, Sri becomes constant throughout the magma due to 87Sr and 86Sr equilibration;

b) during partial melting and fractional crystallization, the relative concentration of the incompatible, mobile element Rb increases due to fractionation;

c) after solidification, the 87Sr/86Sr ratio increases over time due to the decay of 87Rb.

A schematic cross section model showing the three cycles for formation of the northern PRB (modified from Tulloch and Kimbrough, 2003; Busby, 2004; Lee et al., 2007): 1) formation of an island arc, then accretion to the North American craton; 2) formation of western PRB granitoids [during moderate tectonic extension] by intrusion into continental

crust made up of island arc volcanics; 3) formation of eastern PRB granitoids [during tectonic compression] by intrusion into the continental craton.

Sri SiO2 examples continental crust formation

1st cycle forms "uncontaminated lower" crust < 0.704 < 55%oceanic crust MORB, OIBisland arc basalt Marianas, Scotia Sea, S Shetland Islands by accretion

… Santiago Peak volcanicslower continental crust amphibolites by underplating and intrusion

2nd cycle forms "uncontaminated upper" crust < 0.704 50-75%arc volcanics Philippines, Aleutians, Fiji, Marianas, Japan; Andes, Cascades by accretionarc granitoids - Precambrian Africa, Canadian shield, basement of western USA ubiquitous, but under different conditions than present dayarc granitoids - Phanerozoic plutons found in: Newfoundland, Japan, New Zealand, Himalayas, this study - present day conditions without contamination by previous crust

Lachlan, Andes, Aleutians, Br Columbia, Idaho, Klamath, Sierras … Escondido granitoids

anorogenic granitoids Sudan, Antarctic, Oslo, New Hampshire, Carolinas minimal

3rd cycle forms "contaminated upper" crust > 0.704 50-75%arc volcanics N. Lesser Antilles, Taiwan by accretionarc granitoids - Phanerozoic common ubiquitous, but complicated with contamination by previous crust

… eastern PRB granitoids

CYCLES 1st cycle

• Oceanic slab subducted under crust dehydrates and yields fluids that migrate into the mantle wedge. • Hydrous mantle partially melts yielding mafic magma. • Melt is extruded as volcanics, invades the lower crust, and underplates the crust.

2nd cycle

• Additional mafic magma from the mantle heats the lower crust. • Partial melting yields a felsic magma. • Mafic and felsic magma can mix at depth and/or be emplaced at shallow levels as gabbro, tonalite, and

granodiorite or be extruded as volcanics. o Bimodal magmatism predominates in extensional regimes. o Mixing predominates in compressional regimes.

• Restite left after partial melting of the lower crust delaminates back into the mantle.

3rd cycle • Same as second cycle, but contaminated by continental crustal.