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The Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA) is an exposure of six compositionally diverse intrusive lithologies with clear crosscutting relationships that permit a focused investigation of magma source characteristics and the relative roles of petrogenetic processes on the evolution of the system. We use new field observations, zircon U-Pb dates, major and trace element data, and Sr-Nd-Pb isotopic data to develop a model that can be applied to similar SNB intrusive suites. Stage 1 units, emplaced ca. 105 Ma, consist of two gabbros, a gabbrodiorite, and a granite. Stage 2 and stage 3 units were emplaced ca. 104 Ma and ca. 103 Ma, respectively, and are granites. We suggest that stage 1 gabbroids were derived by partial melting of lithospheric mantle, whereas coeval felsic magmas were derived by partial melting of a mafic, juvenile crustal source. Stage 2 and stage 3 granitoids were derived from similar sources that generated stage 1 granitoids, but there was greater input from evolved crust. Fractionation and/or assimilation played only a minor role in system evolution. Past studies of SNB magmas have come to conflicting conclusions about the petrogenesis of intermediate magmas that dominate the batholith; we hypothesize that mafic and felsic end members of the AMC could represent end members in mixing processes that generate these magmas. The timing of emplacement of the AMC coincides with a transition of magmatic style in the SNB, from smaller volume magmatic suites with mixed mantle and crustal sources to larger volume magmatic suites derived from greater proportions of crust.
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Mafic intrusions record mantle inputs and crustal thickness in the eastern Sierra Nevada batholith, California, USA
Contributions of heat and/or mass from mafic magmas are commonly invoked in models of voluminous granodiorite and andesite generation in magmatic and volcanic arcs worldwide. However, mafic intrusions are a volumetrically minor component in most arc batholiths. This is the case in the Sierra Nevada batholith, California, USA, where gabbro and diorite plutons are smaller and less abundant than the granitoid suites that make up the bulk of the batholith. Here, we constrain the timing and geochemistry of mafic intrusions in the Sierra Nevada batholith to assess the role of these compositions in arc batholith construction. Previous detailed studies on a limited number of mafic intrusions demonstrate that they formed penecontemporaneously with the felsic batholith, but there is a need for a broader survey of mafic plutons using modern geochronological techniques. New U-Pb zircon ages for 13 gabbro to diorite plutons and geochemistry from 17 mafic intrusions in the eastern Sierra Nevada batholith document two main episodes of mafic magmatism in the eastern Sierra Nevada batholith, from 168 Ma to 145 Ma and from 100 Ma to 89 Ma. These episodes overlap with the ages of granitoid plutons in the eastern Sierra Nevada batholith, including the Late Jurassic Palisade Crest and Late Cretaceous John Muir intrusive suites, in addition to other felsic plutons dated in the eastern Sierra Nevada batholith. Non-primitive mineral compositions in the mafic bodies indicate that their parental magmas are the evolved products of mantle-derived basalts that first differentiated in the lower crust prior to ascent and crystallization in the upper crust. The presence of rocks with cumulate textures, as well as a wide range of bulk-rock compositions (SiO2 wt% 38−64, Mg# 39−74), show that magmatic differentiation continued within each mafic body after intrusion into the upper crust. Sr/Y ratios in melt-like (i.e., non-cumulate) mafic samples suggest that the crustal thickness of the Sierra Nevada batholith was roughly 30 km in the Early Jurassic and increased to ∼44 km by the Late Cretaceous. Concomitant intrusion of mafic melts along with voluminous granitoid plutons supports mantle melting as a major contributor of heat and magmatic volumes to the crust during construction of the eastern Sierra Nevada batholith.
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- Award ID(s):
- 2105371
- PAR ID:
- 10537717
- Publisher / Repository:
- Geological Society of America Bulletin
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B36646.1
