Structural evolution of the Sing Peak pendant: Implications for magma chamber construction and mid-Cretaceous deformation in the central Sierra Nevada, California



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Texas Tech University


Metamorphosed strata of the Sing Peak pendant, central Sierra Nevada, CA, record structures formed during the contemporaneous emplacement of adjacent plutons and regional deformation in the mid-Cretaceous. Host rocks in the Sing Peak pendant consist of deformed metavolcanic rocks of the mid-Cretaceous Minarets Caldera sequence and less common Jurassic metasedimentary rocks. Structures in the Sing Peak pendant include: a) a penetrative, subvertical, NNW-striking foliation defined by stretched phenocrysts and recrystallized biotite, epidote and amphibole; and b) a dextral sense ductile shear zone with a shallow/moderate plunging (~40º) NNW-trending lineation. The shear zone can be traced into the Jackass Lakes pluton along the northeastern and northern margins of the pendant. The Jurassic metasedimentary rocks are locally folded with NNW-striking axial planes and the folds are commonly truncated by the Jackass Lakes pluton. Structures within the Jackass Lakes pluton include a well-developed NNW-striking, steeply west dipping magmatic foliation and a moderately north plunging (~40º) lineation defined by elongated biotite and hornblende phenocrysts. The magmatic structures are continuous across compositional zones within the Jackass Lakes pluton and parallel to the elongated axis of mafic enclaves observed within the pluton. Abundant xenoliths (up to 300 m long) of metavolcanic rocks appear to be concentrated near the roof pendant but are also found throughout the pluton. Metamorphic foliations observed within the xenoliths are sub-parallel with those in the host rocks and the magmatic foliation of the pluton. Some xenoliths contain dikes of the Jackass Lakes pluton, up to 1 m in width, that have been folded with axial planes striking sub-parallel to the metamorphic foliation in the xenoliths, host rocks and magmatic foliations in the pluton. These observations suggest that the magmatic and metamorphic foliation as well as the folded dikes were formed during chamber construction and most likely due to regional deformation. Based on the lineation data, the deformation in the ductile shear zone and the ~ 98 Ma Jackass Lakes pluton is the result of regional transpression. Previous work has suggested that regional dextral transpression was not initiated until ~ 90 Ma in the central Sierra Nevada. Therefore, the Jackass Lakes pluton – host rock system represents the earliest documented regional dextral transpression in the central Sierra Nevada.