PETROGENETIC CONNECTIONS BETWEEN ASH-FLOW TUFFS AND INTRUSIVE SUITES - TOWARD AN INTEGRATED MODEL FOR UPPER-CRUSTAL MAGMA SYSTEM EVOLUTION IN THE CALIFORNIA ARC

Paired data from ash-flow tuffs and cogenetic plutons are useful for testing hypotheses that seek to integrate the volcanic and plutonic record. We are analyzing existing and new geochronologic and petrologic data from ash-flow tuffs and intrusive suites in the Mesozoic California arc, in an effort to construct an integrated volcano-plutonic model for silicic magmatism in arc upper crust. Zircon U-Pb ages, immobile element abundances in whole rocks, and trace element abundances in zircons allow us to interpret a shared magmatic heritage between individual tuffs and assembly of components of underlying granodioritic to granitic intrusive suites. Preliminary results for five volcanic sequences and adjacent cogenetic plutons suggest several preliminary conclusions: (1) first-order magmatic variation is episodic or pulsed, as plutonism defines three Mesozoic magmatic epochs of ~40-60 m.y. duration that are largely mimicked by the ash-flow tuff record; (2) in the cases of Triassic and Jurassic magmatic epochs, second-order episodicity on the million-year time scale is recorded in both the intrusive and ash-flow tuff records, suggesting that assembly of incrementally emplaced intrusive suites was occasioned by periodic eruption of ignimbrites; (3) ash-flow tuffs range from dacite to rhyolite in bulk rock composition, and are petrographically similar to modern “monotonous intermediate” dacite to relatively phenocryst-poor low-silica rhyolite; (4) dacitic to rhyolitic tuffs are not, on average, more fractionated than cogenetic intrusive rocks, and often, though not always, contain zircons with similarly complex, multi-stage growth histories. Thus these ash-flow tuffs are generally not instantaneous snapshots of magma systems lacking a precursor intrusive process (‘immaculate conception”). Rather, ash-flow tuffs and plutonic rocks record complementary elements of a lengthy, episodic evolutionary history in these cool and hydrous upper-crustal arc magma systems.