James D. Hoover | Geology Reports

USGS1990

Chapter 19: Magmatic components of a tilted plutonic system, Klamath Mountains, California

The Slinkard pluton (SP) and Wooley Creek batholith (WCB) are the lower and upper parts, respectively, of a tilted Middle Jurassic magma system. The SP and lower WCB intruded structurally lower ophiolitic mélange of the Marble Mountain terrane; the upper WCB intruded successively structurally higher metavolcanic and metasedimentary rocks of the western and eastern Hayfork terranes. The predominant volume of the system comprised a two-layer chamber in which an upper dacitic magma crystallized to form tonalite to granite in the upper WCB and a lower andesitic magma crystallized to form gabbro to tonalite in the lower WCB and SP. The upper part of the system had Sr i , = 0.7043 and a range of δ 18 O from +8.7 to +11.2%o; the lower part had average Sr i = 0.7046 and δ 18 O from +8.1 to +8.8%o The two layers of the system are separated by a transition zone that is intermediate in isotopie composition. The compositional differences between upper and lower parts of the system can be explained as (1) the result of intrusion of two separate pods of noncogenetic magma, or (2) the product of in situ assimilation-fractional crystallization. The second explanation requires that a relatively 87 Sr-rich contaminant such as the structurally lower Marble Mountain terrane was assimilated in the lower part of the system, whereas an 18 O-rich, generally 87 Sr-poor contaminant such as the structurally intermediate western Hayfork terrane was assimilated by the upper part. Trace-element evidence suggests that gradational upward zoning (from gabbro to granite) resulted from an upward decrease in the efficiency of crystal-melt segregation and crystal accumulation. H 2 O-rich basaltic magma preceded development of the two-layer system, and basaltic pulses into the lower part of the system continued during most of its solidification history. Most basaltic rocks display evidence of some degree of fractional crystallization and interaction with crustal rocks; however, a few have low Sr i and high concentrations of Cr and Ni, characteristics of undifferentiated mantle melts. Two-mica granite of the western Slinkard pluton cannot be related to the remainder of the system by fractional crystallization. High δ 18 O, high Ba, and low Sr abundances suggest that the two-mica granite is probably a partial melt of crustal material.

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USGS1990

Geochemistry and intrusive history of the Ashland pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex that intruded the western Paleozoic and Triassic belt of the Klamath Mountains in late Middle Jurassic time. The pluton comprises a series of compositionally distinct magma pulses. The oldest rocks are hornblende gabbro and two-pyroxene quartz gabbro with initial 87 Sr/ 86 Sr = 0˙7044, δ 18 O = 8˙7%, and REE patterns with chondrite normalized La/Lu = 7. These units were followed by a suite of tonalitic rocks (La N /Lu N = 7) and then by a suite of K 2 O- and P 2 O 5 rocks of quartz monzodioritic affinity (La N /Lu N = 13–21; La N /Sm N = 2˙4–3˙) The quartz monzodioritic rocks were then intruded by biotite granodiorite and granite with lower REE abundances but more fractionated LREE(La N /Lu N = 13–19; La N /Sm N = 4˙3–6 and they, in turn, were host to dikes and bosses of hornblende diorite. The latest intrusive activity consisted of aplitic and granitic dikes. Combined phase equilibria and mineral composition data, indicate emplacement conditions of approximately P total = 2˙3kb, P H2O between 1˙5 and 2˙2 kb, and f O2 between the nickel-nickel oxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO 2 together with increasing δ 18 O and decreasing initial 87 Sr/ 86 Sr. The isotopic data are consistent with either (1) combined fractional crystallization of andesitic magma and concurrent assimilation of crustal material characterized by low Sr 1 and high (δ 18 O or, more probably, (2) a series of partial melting events in which sources were successively less radiogenic but richer in 18 O Each intrusive stage displays evidence for some degree of crystal accumulation and/or fractional crystallization but neither process adequately accounts for their compositional differences. Consequently, each stage appears to represent a distinct partial melting or assimilation event. The P 2 O 5 -rich nature of the quartz monzodiorite suite suggests accumulation of apatite. However, the suite contains abundant mafic microgranitoid enclaves and most apatite in the suite is acicular. These observations suggest that magma mixing affected the compositional variation of the quartz monzodiorite suite. Mass balance calculations are consistent with a simple mixing process in which P 2 O 5 -rich alkalic basalt magma (represented by the mafic microgranitoid enclaves) was combined with a crystal-poor felsic magma (represented by the tonalite suite), yielding a quartz monzodioritic magma that then underwent differentiation by crystal fractionation and accumulation.

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