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The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record high-silica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe–reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134–136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and δ18O; the Early Jurassic rhyolite shows less evidence of crustal influences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal influences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic flux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This finding likely reflects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.
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Revised geologic map and structural interpretation of the Mineral King pendant, southern Sierra Nevada, California (USA): Evidence for kilometer-scale folding and structural imbrication of a Permian to mid-Cretaceous volcanosedimentary assemblage
Abstract The Mineral King pendant is an ~15-km-long, northwest-striking assemblage of Permian to mid-Cretaceous metavolcanic and metasedimentary rocks that form a steeply dipping wall-rock screen between large mid-Cretaceous plutons of the Sierra Nevada batholith (California, USA). Pendant rocks are generally well layered and characterized by northwest-striking, steeply dipping, layer-parallel cleavage and flattening foliation and steeply northwest-plunging stretching lineation. Northwest-elongate lithologic units with well-developed parallel layering and an absence of prominent faults or shear zones suggests a degree of stratigraphic continuity. However, U-Pb zircon dating of felsic metavolcanic and volcanosedimentary rocks across the pendant indicates a complex pattern of structurally interleaved units with ages ranging from 277 Ma to 101 Ma. We utilize a compilation of 39 existing and new U-Pb zircon ages and four reported fossil localities to construct a revised geologic map of the Mineral King pendant that emphasizes age relationships rather than lithologic or stratigraphic correlations as in previous studies. We find that apparently coherent lithologic units are lensoidal and discontinuous and are cryptically interleaved at meter to kilometer scales. Along-strike facies changes and depositional unconformities combine with kilometer-scale tight folding and structural imbrication to create a complex map pattern with numerous discordant units. Discrete faults or major shear zones are not readily apparent in the pendant, although such structures are necessary to produce the structural complications revealed by our new mapping and U-Pb dating. We interpret the Mineral King pendant to be structurally imbricated by a combination of kilometer-scale tight to isoclinal folding and cryptic faulting, accentuated by, and eventually obscured by, pervasive flattening and vertical stretching that preceded and accompanied emplacement of the bounding mid-Cretaceous plutons. Deformation in the Mineral King pendant represents a significant episode of pure-shear-dominated transpression between ca. 115 Ma and 98 Ma that adds to growing evidence for a major mid-Cretaceous transpressional orogenic event affecting the western U.S. Cordillera.
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- PAR ID:
- 10533544
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 20
- Issue:
- 4
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 1190–1223
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02748.1
