T140 The Dance of Arcs: Causes of Arc Migrations, Flare-Ups, and Magmatic Focusing II

Monday, 23 October 2017
1:30 PM - 5:30 PM
Skagit 4

LITHOSPHERIC STRUCTURE CONTROL ON CHANGES IN MAGMA SOURCE OVER TWO MAGMATIC PULSES IN THE WHITE–INYO RANGE, CA

Paper number: T140-9
Program: Technical Sessions
Time: 3:35 PM – 3:50 PM

Authors

Abstract

Plutons of the White–Inyo Range (WIR) represent the easternmost extent of Mesozoic subduction magmatism in California. Over twenty recognized granitic to gabbroic plutons intruded during two magmatic events at ca. 180–167 Ma and 102–86 Ma into greenschist facies Neoproterozoic and Cambrian sedimentary units. This study utilized Sr, Nd, and Pb isotope systems to understand changes in magma source and to interpret the tectonic evolution of the underlying lithosphere.

Jurassic plutons of the WIR are characteristically amphibole-bearing gabbros to granites. Whole rock ƐNdi values of Jurassic plutons reflect a relatively juvenile source with values ranging from –0.15 to –8.32, 87Sr/86Sri ranging from 0.7045 to 0.7074, and 206Pb/204Pbi ranging from 18.55 to 19.26. Comparatively, Cretaceous plutons are dominantly aluminous, two-mica granitoids. Isotopic ratios for the Cretaceous plutons fall into two ranges, and the plutons are spatially distributed. In the southern WIR, the ƐNdi values range from –18.6 to –20.1, 87Sr/86Sri from 0.7110 to 0.7122, and 206Pb/204Pbi from 17.59 to 17.70. These ratios are atypical of the region in that they reflect an older, Paleozoic to Proterozoic crustal source. Contrastingly, Cretaceous plutons of the northern WIR have ƐNdi values that range from –5.76 to –9.19, 87Sr/86Sri from 0.7071 to 0.7080, and 206Pb/204Pbi from 18.52 to 19.11, reflecting a source like that for the Jurassic plutons.

We propose that the lower lithosphere below the WIR during the Jurassic was juvenile, but inhomogeneous in major and trace elements, perhaps due to underplating of compositionally heterogeneous material to the lithosphere. In contrast, the isotopic ratios of southern Cretaceous plutons suggest melting of upper-crustal sedimentary units with Proterozoic provenance. The sedimentary units in the WIR have ƐNdi values of –18.0 to –20.9 that match those of the plutons. The transition to a sedimentary source for some plutons in the WIR between the Jurassic and Cretaceous eras suggests heating of the crust, perhaps due to thickening during tectonic compression.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305357

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