Paper No. 5
Presentation Time: 10:00 AM
FIELD OBSERVATIONS FROM THE SONORA PASS INTRUSIVE SUITE: STRUCTURE AND CONSTRUCTION OF A CRETACEOUS ZONED INTRUSIVE SUITE IN THE CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA
We present preliminary field observations from the poorly studied mid-Cretaceous Sonora Pass Intrusive Suite (SPIS), one of the classic zoned suites of the Sierra Nevada batholith, and its host rock, the Bummer’s Flat granodiorite. The SPIS is made up of two intrusions: the ca. 91 Ma Kinney Lakes (Kkl) unit, comprising the outer part of the SPIS, and the inner Topaz Lakes unit (Ktl). The Kkl ranges from mafic to felsic granodiorite and exhibits complex zones of sheeting, diking, and mixing. The Ktl is a biotite granodiorite with conspicuous, ~2-6 cm K-feldspar phenocrysts. The host rock of the ca. 101 Ma Bummer's Flat intrusion is a porphyritic biotite granodiorite with ~1-2 cm K-feldspar and quartz phenocrysts. The unit also contains widespread coeval bodies of diorite and gabbro, and xenoliths of metasedimentary rocks. The
granodiorite is cut by the ≥300 m-wide, ductile Toe Jam Lake shear zone, which strikes N to NW and extends for at least 3 km along strike. This shear zone has moderate to steep dips and down-dip lineations; it mainly records reverse slip.
The Kkl contact with the Bummer's Flat rocks is marked by a broad domain of N to NW-trending sheets ranging in thickness from 10s of cm to 100s of m. Multiple Kkl sheets of variable composition surround and isolate a km-wide Bummer's Flat “xenolith” from the main intrusion. The enclosing Kkl is characterized by domains of generally planar schlieren, ranging from many 10s to 100s of m thick. The large xenolith is intruded by dozens of felsic, N-trending KKl dikes, and contains mafic bodies and solid-state fabrics of the Toe Jam Lake shear zone. These observations provide evidence that the Kkl, was constructed in part by thick sheets associated with large-scale isolation of wall rocks into the younger intrusion. The SPIS is broadly contemporaneous to the nearby Tuolumne Intrusive Suite and our ongoing work is evaluating similarities and differences between these suites.