Very low grade Metamorphism in the Sierra Nevada, California: An overview and new results
Abstract
The Sierra Nevada exposes a metamorphosed Paleozoic - Jurassic basement intruded by the Cretaceous Sierra Nevada batholith. Within the basement, a large metaperidotite and thrust fault separate an eastern, near-continental sequence of Paleozoic - Jurassic sedimentary and volcanic rocks, and a western oceanic assemblage of largely Mesozoic, fault-bounded, accreted terranes of volcanic arcs, ophiolites and sedimentary rocks. The basement terrane exhibits multiple episodes of Jurassic and older deformation and greenschist or sub-greenschist facies metamorphism that reflect the complex accretionary history of the orogen. Facies boundaries commonly are through-going regional scale faults, suggesting that most of the regional metamorphism is older than the latest stages of deformation. Volcanic rocks within the accretionary assemblage exhibit four distinct styles of pre-orogenic metamorphism: (1) hydrothermal metamorphism associated with early intrusions; (2) coeval(?) regional metamorphism; (3) prograde metamorphism in late shear zones; (4) contact metamorphism by late- and post-tectonic granitoid plutons. Regional and late contact metamorphic assemblages in mafic to intermediate volcanic rocks commonly are transitional between the prehnite-pumpellyite and greenschist facies. In rocks that contain Ab, Ep, Chl, Qtz, and an aqueous fluid, actinolite is stable in the most magnesian rocks, pumpellyite in ferroan rocks, and both Pmp and Act are stable in intermediate compositions. Pmp and Chl are replaced by Act with increasing grade by continuous reactions. In rocks lacking Pmp, Act appears first by a dehydration reaction between the Chl-rich matrix and previously unreacted igneous clinopyroxene. Although prehnite is common, its role in the equilibria is unclear.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....7112D