科研之友
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摘要
The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78-1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84-1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed epsilon(Hf(t)) values, primarily between +5 to -5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile e Hf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74-1.71 Ga yield juvenile e Hf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains (e Hf(t) of -5 to +10) as well as xenocrystic Archean and 1.85 Ga grains (e Hf(t) of -12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an similar to 200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.
- 出版日期2015-9
