摘要:

Structural studies of the circa 1.42 Ga Sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the “anorogenic” label associated with this and other 1.4 Ga granites in the southwestern United States. The SE margin of the pluton is a 1‐ to 2‐km‐wide NW dipping ductile shear zone. Field and microstructural observations (melt‐filled shear bands, high‐temperature dynamic recrystallization of K‐feldspar megacrysts, and crosscutting pegmatite dikes) indicate that top‐to‐the‐NW (normal) movement in the shear zone took place in the presence of melt. Subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top‐to‐the‐NW shear sense, suggesting that over large regions of the pluton, magmatic flow mimicked solid‐state strain. In the northern aureole, contact metamorphic aluminosilicate porphyroblasts grew during the formation of a NE striking crenulation cleavage (S3) and related folds of late‐stage pegmatite dikes. These features document the synchroneity of magma emplacement, shortening, and metamorphism and indicate that the Sandia pluton is syntectonic, not anorogenic. We interpret the kinematic consistency of structural elements from the base of the pluton, the interior of the pluton, and the northern aureole to reflect a regional (larger than the pluton) strain field and suggest that the “orogeny” recorded in and around the Sandia pluton involved a three dimensional strain field with subhorizontal extension (N–S) and contraction (E–W) directions. N–S extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high‐strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole. East to SE shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated S3crenulation cleavage, and east to SE shortening (or least extension) directions in the pluton proper are documented by the intersections of orthogonal dikes. Thus emplacement of the Sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin, rather

ISSN:0278-7407    

DOI:10.1029/94TC02699

年代:1995

数据来源: WILEY

摘要:

The Grenville Province in western Labrador comprises three lithotectonic terranes, structurally stacked from northwest to southeast, whose geometric relationships evolved over a time interval from the Early Proterozoic to the Grenvillian Orogeny (circa 1.0 Ga). Gagnon terrane lies immediately southeast of the Grenville Front and is dominated by Early Proterozoic sediments that were deposited unconformably on the Archean basement of the Superior Province. It is overlain to the southeast by the Labradorian magmatic arc of Molson Lake terrane which is itself overlain by the highest structural deck comprising Labradorian migmatites and intrusive rocks of Lac Joseph terrane. The Labradorian magmatic arc of Molson Lake terrane was founded on the margin of mid‐Proterozoic Laurentia and was fueled by northwest directed subduction of oceanic crust that existed between Molson Lake and Lac Joseph terranes. Labradorian magmatic rocks of the exotic Lac Joseph terrane were produced in a second, independent subduction zone of undetermined dip direction. No post‐Labradorian magmatic arc is known to have existed in western Labrador, implying that all three terranes were proximal (part of the same plate) by the end of the Labradorian Orogeny. Rifting of the post‐Labradorian configuration, perpendicular to the Labradorian Front led to the emplacement of the Shabogamo‐Michael Gabbro dyke swarm in a linear belt across the future northern Grenville Orogen. The present terrane geometry of western Labrador is a product of northwest directed thrusting that followed an initial period of crustal thickening during the Grenville Orogen at circa 1000–995 Ma, an event accompanied by penetrative deformation and metamorphism only in Gagnon and Molson Lake

ISSN:0278-7407    

DOI:10.1029/94TC02018

年代:1995

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/94TC02023

年代:1995

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/94TC02022

年代:1995

数据来源: WILEY