STRUCTURE AND CONSTRUCTION OF CRETACEOUS PLUTONS IN THE SONORA PASS REGION OF THE NORTH-CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA

Cretaceous plutons in the Sierra Nevada batholith and their interactions with host rocks provide insights into processes operating during construction and emplacement of large magmatic systems. We summarize field observations on the southern margin of the ~ 95-87 Ma Sonora Pass Intrusive Suite (SPIS) and its plutonic host rock. The voluminous, undated Poopenaut Valley granodiorite, which may be part of the Early Cretaceous Fine Gold Intrusive Suite, is inferred to be the oldest pluton in the study area on the basis of contact relationships. It is separated from the ~ 109 Ma granodiorite of Bummers Flat to the north by a WNW-trending, ~100 m wide meta-sedimentary screen. Poopenaut Valley rocks contain multiple zones of strong magmatic foliation that grade into typical low intensity foliation to the south. Strongly foliated Poopenaut Valley xenoliths, ranging from 1-20 m in length, occur 10s of m into the main, weakly foliated body of granodiorite. The Bummers Flat unit is a highly texturally and compositionally variable biotite granodiorite containing K-feldspar phenocrysts. Abundant mafic xenoliths and dikes, combined with magma mingling and sheeting, record complex timing relationships with the host granodiorite. The ~ 95-92 Ma granodiorite of Kinney Lakes is the marginal unit of the SPIS and intrudes the other plutons. It is a relatively homogeneous, medium-grained, hornblende-biotite granodiorite commonly containing elongate enclaves (aspect ratios up to 20:1) that are generally parallel to magmatic foliation. The shift in degree of heterogeneity between the Bummers Flat and Kinney Lakes magmas indicates a major transition of Cretaceous magmatic systems in the region.

The dominant outcrop-scale structure in the plutons is steep magmatic foliation. Foliation in the Kinney Lakes granodiorite is typically stronger in its southern margin, discordant to pluton contacts, and parallel to the regional foliation. The major solid-state structure in the area is the >300 m wide, NW-trending, reverse-slip Toe Jam Lake shear zone, which cuts the Bummers Flat granodiorite. This shear zone narrows into multiple zones of L>>S fabrics to the SE and is truncated by Kinney Lakes rocks. Collectively, these structural patterns support a constant NE-SW shortening strain field before and during emplacement of the SPIS.