ZIRCON AGES AND TRACE ELEMENT COMPOSITIONS LINK FRYS POINT GRANITE AND MIGMATIZATION IN THE SEQUOIA PENDANT, SEQUOIA NATIONAL PARK

In the Sequoia Pendant, migmatized biotite schists of the Kings Sequence are exposed along the Marble Fork of the Kaweah River. The proximity of the migmatites to the Fry’s Point Granite leads to an hypothesis for the cause of migmatization. Mafic components (mostly diorite) associated with the Frys Point pluton partially melted the adjacent sedimentary rocks, resulting in formation of distinct melano- and leucosomes in the Marble Fork biotite schists, with melting of deeper portions of the Sequoia pendant producing the Frys Point Granite, which intrudes both the diorite and the schist in the Marble Fork region.

Comparison of new SHRIMP RG U/Pb ages and trace element compositions of zircon from both the migmatites and the Frys Point Granite test this hypothesis. The migmatite yielded few zircons. SHRIMP-RG U/Pb analyses of six zircons yielded a range of ages from 1800-105 Ma. The youngest zircon, of the same age as the dioritic and granitic phases of the Frys Point pluton, had euhedral prismatic terminations suggesting growth during migmatization. The Frys Point granite zircons commonly contain inherited cores. One core was dated at ca. 1100 Ma indicating the granite likely incorporated a component of the metamorphosed sedimentary rocks. Furthermore, REE profiles of zircons from the Marble Fork biotite schist and the Frys Point Granite are indistinguishable.

These preliminary data support the proposed hypothesis. The matching ca. 105 Ma ages of the granite and the euhedral zircon in a migmatite leucosome indicate that their formation was coeval. The inherited zircon cores in the granite and the similarity of REE patterns supports incorporation of biotite schist into the granite magma. The ca. 105 Ma intrusion age for the diorite allows it to be a heat source at the time of migmatization.