STRUCTURE AND CONSTRUCTION OF THE CRETACEOUS SONORA PASS INTRUSIVE SUITE, A LARGE ZONED INTRUSION OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA

The ~92-89 Ma Sonora Pass Intrusive Suite is the most northern of four large, Late Cretaceous zoned intrusive suites within the central Sierra Nevada batholith. It consists of the older, marginal, equigranular Kinney Lakes hornblende-biotite granodiorite (Kkl) and the younger, inner, porphyritic Topaz Lakes biotite granodiorite (Ktl). The SPIS intruded the ~109 Ma Bummer’s Flat granodiorite, Jurassic diorites and gabbros, and Precambrian to Cambrian metasedimentary rocks of the Snow Lake block. Contacts with host rocks are gradational, show evidence of incorporation of older units, and been intruded by felsic dikes. In the SPIS, the main internal contact between the Kkl and Ktl is sharp, has variable but consistently steep dips, and is defined by a 1.5 m-wide zone of rounded mafic enclaves, schlieren, and euhedral mafic mineral accumulations. Near this contact, the Kkl contains abundant zones of schlieren, which strike parallel to, but dip shallowly away from the contact, discordant to magmatic foliations. Topaz Lake dikes (10 cm to 1m wide) with sharp boundaries finger into the Kkl for >100 m from the contact.

Both units of the SPIS are heterogeneous, but in varying degrees and styles. The Ktl was previously thought to be largely homogenous, but magmatic pipes, diapirs, schlieren, and packed megacryst zones are widespread. In the margin of the Topaz Lake unit, K-feldspar phenocrysts are 1-2 cm long and grade inwards to >9 cm. In the Kkl, heterogeneities are more concentrated and abundant, and include ≥1 km long zones each defined by one of the following: schlieren, elongate mafic enclave swarms, and host rock xenoliths (10 cm to 300 m long) with felsic dike swarms.

Foliations within the SPIS range from magmatic to solid-state with mostly steep dips. In the margin of the Kkl, foliations are parallel to host rock contacts, but discordant to parallel to host foliations. Inward from the contact, foliations strike mostly N to NW, compatible with Late Cretaceous regional strain. These inferred regional fabrics are truncated by the Topaz Lake unit, in which magmatic foliation strikes are mainly N to NW, but locally E-W.

From field relationships discussed above, particularly heterogeneities and their distributions, the Topaz Lake and Kinney Lakes units are inferred to have been constructed by different mechanisms.