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Origin of Andesitic and Granitic Magmas in the Northern Sierra Nevada, California

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Origin of Andesitic and Granitic Magmas in the Northern Sierra Nevada, California ANNA HIETANEN U.S. Geological Survey, Menlo Park, California 94025 Origin of Andesitic and Granitic Magmas in the Northern Sierra Nevada, California ABSTRACT Paleozoic and Mesozoic time below the west side of the northern Sierra Nevada. The meta- The early magmas of the northern Sierra morphic complex includes an andesitic suite of Nevada, calc-alkaline andesite of island-arc island-arc type that is tentatively correlated type and its derivatives, all low in potassium, with Devonian metavolcanic rocks in the were generated during the Devonian(?) Taylorsville area about 30 km to the northeast period, possibly along an eastward-dipping sub- (McMath, 1966) and with the suite of andesite duction zone. These magmas could have been in the Klamath Mountains (Davis, 1969; derived from mantle peridotite of the con- Burchfiel and Davis, 1972). This andesitic tinental plate by introduction of water from suite is a key to understanding the major the descending oceanic plate. Later, during the structural events and geochemical changes that Permian(?) period, the magmas became accompanied the formation of later granitic basaltic, with potassium-rich silicic derivatives magmas, which intruded the metamorphic indicating anhydrous conditions and a deeper complex in Late Jurassic and Early Cretaceous level of magma generation. Plutonism began in time. A second episode of andesitic volcanism Jurassic time, at the end of a period of intense was pyroclastic and occurred in the Tertiary; it deformation and metamorphism. The earliest was preceded and followed by basaltic eruptions intrusive rocks are gabbro and diorite. At the (Hietanen, 1972). end of the Jurassic period, large granitic plutons were emplaced. These grade from hornblende OUTLINE OF GENERAL GEOLOGY quartz diorite at the borders to monzotonalite The Feather River area in the northern at the centers. Trondhjemite occurs as the Sierra Nevada (Fig. 1) is in the inner part of an latest product of crystallization differentiation arcuate segment of the Nevadan orogenic belt of plutonic magmas. Exchange of elements be- (Hietanen, 1973). Here, northerly trends, tween plutonic and metamorphic rocks sug- typical of the Sierra Nevada south of the area gests that the plutonic magmas were composite. of Figure 1, turn to the northwest and in The partial melts of the downfolded volcanic places to the west, curving around small and sedimentary rocks were modified by partial plutons that were emplaced after the Paleozoic melts from the mantle and the subducted metasedimentary and metavolcanic rocks were oceanic lithosphere below. Relative amounts of deformed and recrystallized during the Jurassic. material contributed by each of the three Five major high-angle faults disrupt the struc- sources of plutonic magma changed with time, tural and lithologic continuity and divide the and these changes, along with differentiation metamorphic rocks into four belts, in which processes, were responsible for the diversity in rarely more than one formation is exposed. The composition of magmas. west sides of the faults were downdropped rela- tive to the east sides, and isoclinal folds typical INTRODUCTION of all local metamorphic rocks are overturned Petrologic and structural studies in the com- to the west (Fig. 2C). These structural features plex plutonic and metamorphic terrane of the are consistent with the idea that a subduction Feather River area in the northern Sierra zone lay beneath and west of this area during Nevada have revealed many features that bear the Paleozoic and Mesozoic and that the faults on the origin of andesitic and granitic magmas and overturned folds are surface expressions of in orogenic belts. The structures in this area the subduction process (Fig. 2). are consistent with the hypothesis that an east- The oldest rocks in the area are interbedded ward-dipping subduction zone existed during metachert and phyllite, and thus similar to Geological Society of America Bulletin, v. 84, p. 2111-2118, 3 figs., June 1973 2111 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/2111/3428913/i0016-7606-84-6-2111.pdf by guest on 28 September 2021 2112 ANNA HIETANEN metamorphosed equivalents of sediments de- rocks and a few carbonate layers, was probably posited on ocean floors. These rocks, the Cala- deposited during intermit): sntly waning vol- veras Formation, of Paleozoic (Silurian?) age, canic activity in shallow v/ater between the are faulted against a belt of peridotite and island arc and the continent. serpentine in the east, and are overlain by meta- The large round plutons shown in Figure 1 andesite believed to be the lowest part of the are postkinematic. They were emplaced over a Franklin Canyon Formation (Hietanen, 1973). period of time at the end of Jurassic time. The Similar potassium-poor meta-andesite and in- oldest of these plutons, the Lumpkin pluton terlayered metadacite and metamorphosed and associated intrusive bodies, consists of sodarhyolite, all with pyroclastic structures and gabbro and quartz diorite that has more interbedded with metamorphosed tuffaceous hornblende and less quartz than the quartz layers, are exposed southwest of the Calaveras diorite in the other plutons. Most of the other Formation in a belt that is 4 to 9 km wide and plutons are monzotonalite grading outward to is bordered by faults. These rocks are also quartz dioritic border zones, 1- to 2-km wide. assigned to the Franklin Canyon Formation Exceptions are the Bucks Lake pluton, which and tentatively correlated with the Devonian includes a mass of pyroxene diorite in its center, metavolcanic rocks in the Taylorsville area and the Bald Rock pluton, which grades to about 30 km to the northeast (McMath, 1966). trondhjemite in its north-central part. A They are overlain by the Horseshoe Bene tonguelike body of trondhj emite extends to the Formation, a heterogeneous sequence of inter- northern part of the neighboring Cascade bedded metavolcanic and metasedimentary rocks including thin layers of limestone of pluton, proving that the Bald Rock pluton is probable Permian age. The oldest rocks in the the younger of the two. Trondhjemite occurs Horseshoe Bend Formation are metabasalt as a late differentiate in the Merrimac pluton with discontinuous thin layers of potassium- also and as dikelike bodies in the south-central feldspar-bearing metarhyolite, rhyolitic meta- part of the Cascade pluton. tuff, and quartzite. The middle part of the ORIGIN OF ANDESITIC MAGMA formation consists mainly of phyllite anc quartzite, with a little limestone and some Three major hypotheses to explain the pos- metavolcanic rocks. Another layer of basaltic sible origin of calc-alka!ic andesite magma have rocks with some quartzitic layers overlies the been advanced during recent years. Before the metasedimentary sequence and forms the upper advent of the plate-tector.ics concept, andesite part of the Horseshoe Bend Formation. Small was generally considered to be derived from bodies of metagabbro, metadiorite, and meta- basaltic magma either through fractionation or trondhjemite are most common in the Franklin by contamination by sialic material, even Canyon Formation and are considered to be though, in many areas such as island arcs, there deep-seated and hypabyssal equivalents of the is no evidence for either. In the light of plate metavolcanic rocks. These early intrusive rocks tectonics, two other possibilities have been were deformed and metamorphosed with the proposed for the generation of calc-alkalic metavolcanic rocks, which they resemble in andesite magma: partial melting of either their mineralogy and chemical composition, downgoing oceanic lithosphere (Dickinson, including the trace-element content. 1970) or of mantle perioditite in hydrous con- ditions (Yoder, 1969; McBirney, 1969a). The sequence of metamorphosed chert and Kuno (1968, 1969) has discussed the pos- shale (Calaveras Formation), overlain by sibility that andesitic magma may form by andesitic volcanic rocks (Franklin Canyon fractionation of basaltic rnagma. He advanced Formation) intruded by gabbroic and trondhje- this hypothesis on the basis of field associations mitic rocks, and in fault contact with a belt of and the frequency of eruption of basaltic ultramafic rocks of mantle origin, is similar andesite associated with basalt and andesite. In to rock assemblages in the oceanic lithosphere. the Cascade Range and in northern California, This similarity and the low potassium content large quantities of basaltic andesite were of the andesitic suite supports the contention erupted in Cenozoic time:. In the Feather River by Burchfiel and Davis (1972) that an island- area of northern Sierra Nevada, late Tertiary arc system extended from the Klamath Moun- hypersthene andesite was derived by crystal- tains to the northern Sierra Nevada in Devon- lization differentiation from a tholeiitic basalt ian time. The overlying Horseshoe Bend magma (Hietanen, 1972). There, two-pyrox- Formation, which includes metasedimentary ene basaltic andesite occurs as gradational Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/2111/3428913/i0016-7606-84-6-2111.pdf by guest on 28 September 2021 ANDESITIC AND GRANITIC MAGMAS, SIERRA NEVADA 2113 IZI'OO' / \/] Moniotonal/te llltramajic rocks rea o•j map Lakz Oroville Figure X. Sketch map of study area in the northern mian?). Figure 2C shows a schematic cross section Sierra Nevada: (1) Mainly rocks of the Calaveras Forma- along A-A'. U and D on faults indicate relative up- tion
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