Geochemistry and origin of mafic schists from the Pelona, Orocopia, and Rand Schists; structure and metamorphism of the Orocopia Schist, southern California

摘要

The Orocopia thrust of southern California is a segment of a regional system termed the Vincent-Chocolate Mountains thrust, which has been interpreted as a Late Cretaceous subduction zone. The Orocopia Schist of the lower plate is thought to represent an accretionary wedge. Element abundances indicate the mafic schists of the Orocopia Schist of the Orocopia Mountains (OMS) are tholeiitic and alkalic basalts generated from several sources. Most samples resemble normal to enriched mid-ocean ridge basalts (N-MORB to E-MORB). Variation of MORB-like samples can be accounted for by low-pressure crystal fractionation. A second group of samples resembles non-depleted basalts (P-type) and may have formed as an ocean island or seamount. A third group has elemental abundances similar to E-MORB, however, Y, Yb, Lu, and P are more depleted. Muscovite and amphibole compositions in the OMS metabasalts are similar to calcic series amphiboles found in high-pressure terranes. The OMS is metamorphosed to epidote amphibolite facies at temperatures of approximately 500°C. An inverted metamorphic zonation was not evident. The earliest deformational fabric of the OMS is preserved as a foliation of graphite in pseudomorphed porphyroblasts. This foliation (S[subscript]1) has been transposed at least twice during deformation to produce the current matrix. Early foliations and relict crenulations in the matrix indicate that a process of foliation regeneration produced the fabric in the OMS. The youngest structures are open to tight folds (\u22style 2 folds\u22) which fold schistosity and compositional layering. Style 2 folds, in some cases, overprint isoclinal folds (\u22style 1 folds\u22). Both styles of folds, as well as stretching lineations, have variable orientations. Some samples show evidence of limited recrystallization and retrograde metamorphism during the formation of late folds. Examples include garnet retrograded to chlorite in fractures oriented perpendicular to schistosity and epidote altered to muscovite which is crenulated. Evidence from the Orocopia Mountains indicates that movement direction, is some areas, was oblique to an earlier shear strain and, at the time of style 2 folding, was generally eastward. However, the lack of mylonite and extensive mineralization at the fault contact suggest reactivation of the Orocopia thrust.