LINKED CRETACEOUS PLUTONIC-HYPABYSSAL-VOLCANIC FIELDS IN THE CENTRAL SIERRA NEVADA, CALIFORNIA: A PLUTONIC VIEW BENEATH FOCUSING VOLCANIC SYSTEMS

Magmatic focusing is defined as the inward migration and homogenization of multiple volcanic centers, and by inference, of plutons underlying them. Studies of focused volcanic systems suggest that magmatic focusing in the volcanic field is the surface expression of the growth and maturation of a deeper, vertically extensive magma plumbing system, which in active arcs can only be imaged geophysically.

The central Sierra Nevada (CSN) is a superb example of the plutonic footprint of a ca. 115 Ma-85 Ma magmatically focused system. In the CSN, an extensive Cretaceous plutonic belt intrudes slices of contemporaneous metavolcanic rocks, linked by an increasingly recognized widespread belt of hypabyssal, porphyritic stocks and sheets that form pluton-porphyry-volcanic systems that provide vertical constraints on magma focusing processes and rates. One example is the Tioga Pass triad (ca. 97-98 Ma), which includes a dacite porphyry unit with large internal lithologic and textural complexity. This unit is mutually intrusive with the Tioga Pass monzodiorite and overlaps in age with a nearby rhyolite.

Regionally, plutons in the CSN are magnesian, calc-alkaline and metaluminous to peraluminous, hornblende-biotite granodiorites-granites. In most plutonic rocks, and particularly in plutons younger than 95 Ma (e.g. the Tuolumne Intrusive Complex), Sr values remain constant over a range in Sr_i between 0.705 and 0.708, indicating the importance of mixing. In comparison, porphyry and metavolcanic fields overlap considerably with pluton compositions, suggesting that they are part of the larger, linked magmatic system. Porphyries, however, trend towards peraluminous and alkali-calcic type and are more compositionally restricted than plutons, containing between 62-76 wt% SiO₂. While compositions of all three fields are dominated by fractionation of amphibole and clinopyroxene, (e.g., Dy/Dy* of <0.05) both porphyry and metavolcanic rocks have a stronger fractionation signature, seen in lower whole-rock Rb/Sr and restricted Sr_i values, relative to plutonic samples. CSN pluton-porphyry-volcanic systems thus preserve a record of magma mixing and fractionation processes that occurred along the vertical, evolving pathway of magmas in a focusing magmatic system.