MULTI-STAGE EVOLUTION OF THE LATE JURASSIC ENGLISH PEAK PLUTON, KLAMATH MOUNTAINS, CA

The composite, calc-alkaline English Peak plutonic complex invaded shallow crustal levels of the Klamath Mountain province in Late Jurassic time. Initial magmatism formed the hornblende-phyric Uncles Creek pluton (NE; <60 wt% SiO2) and the Heiney Bar pluton (south; 47-69 wt% SiO2). Textural and geochemical data on hornblende from the Uncles Creek pluton suggest an H2O- & Sr-rich, Y-depleted (adakitic?) magma. These intrusions were followed by emplacement of early-stage magmas of the English Peak (EP) pluton, s.s. The early stage (54-60 wt% SiO2) is heterogeneous at outcrop scale and consists of biotite ± hornblende ± pyroxene gabbro to tonalite, with no well-defined compositional trends. Late-stage rocks underlie the northern and northwestern half of the pluton and consist of gradationally inwardly-zoned calc-alkaline hornblende biotite tonalite to granite (56-74 wt% SiO2) along a well-defined trend. Synplutonic dikes and swarms of mafic enclaves are common in the northern part of the late-stage. Pseudosections and Al-in-hornblende barometry indicates that crystallization began in a mid-crustal reservoir at (ca. 400 MPa, 750 °C); magmas ascended into shallower crust and were stranded at ca. 200 MPa, 600 °C.

The EP is coeval with the upper zone of the adjacent Wooley Creek batholith (WCb), and the two units are geochemically similar, with early-stage EP rocks comparable to tonalitic parts of the upper WCb. The inward zoning trend of late-stage EP rocks is identical to upwardzoning in upper-zone WCb rocks. However, although hornblende zoning in the upper- WCb indicates vertical zoning in a unified magma body, hornblende zoning in late-stage EP rocks defines three distinct magma pulses.

The ⁸⁷Sr/⁸⁶Sr (0.7040-0.7053) and δ¹⁸O (7.7-10.4 per mil) of the complex suggest deep-crustal assimilation of/mixing with metasedimentary rocks/melts. Thus, the plutonic complex represents multi-stage magma evolution, with contamination of mafic magmas in the lower crust, magma evolution in the middle crust, and piecemeal emplacement of diverse magma batches in the shallow crust.