24 Crystal Windows into Igneous Processes (Posters)

Thursday, 16 May 2019
9:00 AM - 6:00 PM
Exhibit Hall B

PETROLOGY OF MAGMATIC ENCLAVES AND MAFIC DIKES IN THE JACK MAIN CANYON INTRUSIVE SUITE, CENTRAL SIERRA NEVADA

Paper number: 24-4
Program: Theme Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Intermediate to mafic magmatic enclaves (MME) and mafic dikes (MD) represent one type of recharge magmas that might be sustaining plutons above the solidus at the emplacement level for 10s of kyrs to myrs. We use field observations, petrography, cathodoluminescence (CL), and X-Ray fluorescence whole-rock element analyses to study such magmas in the Jack Main Canyon intrusive suite (JMCIS) in the central Sierra Nevada. We 1) petrographically and chemically characterize MME magmas; 2) explore physical (crystal mixing) and chemical (melt mixing) exchange between recharge and host magmas; and 3) investigate whether MME and MD magmas are cogenetic.

The ~98-94.5 Ma JMCIS is a NW-migrating set of four plutons. It is composed of, from oldest to youngest and mafic to felsic, the Quartz Diorite of Mount Gibson (Kgi), the Granodiorite of Bearup Lake (Kbu), the Granodiorite of Lake Vernon (Klv), and the Boundary Lake Granite (Kbl). Field mapping reveals that MME are non-uniformly distributed throughout the JMCIS units; MME increase in abundance from the Kbl to the Kgi, Kbu, and Klv.

MME compositions vary between granodiorite, monzonite, monzodiorite, and gabbroic diorite across the JMCIS. All Klv MME samples are of monzodiorite composition. A/CNK increases in MME with younging host units; all MME are peraluminous in the Kbl, per- to metaluminous in the Klv, and metaluminous in the Kbu and Kgi.

MME and MD are mineralogically similar, but texturally heterogeneous across the JMCIS, with grain sizes and textures ranging from fine to coarse-grained and equigranular to Plag and/or Hbl and Bt porphyritic. Their mineralogy includes Plag and Bt ± Hbl, ± Sphene ± Qtz ± Kspar in varying proportions. Hbl abundance typically increases in more mafic host magmas. Although MME with sharp margins are ubiquitous across the JMCIS, MME with mingled and diffuse margins are common. Pillowed and dismembered MD occur in the Kgi, Kbu, and Klv, often frozen while clearly sourcing MME.

Our field, textural, petrography/CL data suggest a) multiple MME/MD populations are present in the JMCIS, and b) physical and chemical mixing occurred between recharge and host magmas at the cm scale. We propose intermediate to mafic recharge magmas to the JMCIS were from different sub-emplacement level sources and/or experienced varied degrees of local mixing with host magmas.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329850

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