Depositional age, provenance and tectonic significance of Precambrian metasedimentary rocks from the Dunhuang Complex, NW China: Evidence from field investigation, zircon U-Pb geochronology and whole-rock geochemistry

摘要

The Precambrian Dunhuang Complex comprises five distinct rock units: (1) the Archean (ca. 3.1-2.5 Ga) TTG gneisses; (2) the Paleoproterozoic felsic orthogneisses crystallized at ca. 2.30 Ga and 2.05-1.83 Ga; (3) the Precambrian metasedimentary rocks in this study; (4) the Archean (ca. 2.5 Ga) and late Paleoproterozoic (ca. 1.80-1.73 Ga) monzonitic and potassic granites; and (5) the Paleoproterozoic (ca. 2.06-2.00 Ga and ca. 1.65-1.60 Ga) basic igneous rocks. An integrated study of field investigation, petrology, geochemistry and geochronology was conducted on representative Precambrian metasedimentary rocks from Dunhuang Complex to understand the tectonic evolution of the Dunhuang tectonic belt (DTB) during Paleoproterozoic. Field investigation revealed that these metasedimentary rocks mainly consist of marbles and metamorphosed clastic rocks. Zircon U-Pb dating results yield age populations at ca. 2.52 Ga, ca. 2.36 Ga, ca. 2.03 Ga, ca. 1.96 Ga, ca. 1.86 Ga and ca. 1.83-1.81 Ga. Based on zircon morphological features and geological facts, the maximum depositional age and metamorphic age of these metasedimentary rocks can be constrained at ca. 2.03-1.86 Ga and ca. 1.83-1.80 Ga, respectively. Low-moderate CIA values (54-74), higher ICV values (1.20-1.89), as well as relatively positive correlation between Zr/Sc and Th/Sc ratios indicate low maturity and insignificant weathering. Whole-rock geochemical data indicate that the provenance of these metasedimentary rocks was dominated by felsic to intermediate rocks deposited in a basin adjacent to a continental arc. Considering the reported early Precambrian geological events within the DTB, the provenance is possibly late Archean-Paleoproterozoic intermediate-acidic igneous rocks from the Dunhuang Complex and deposited in an active continental margin setting during late Paleoproterozoic. The Paleoproterozoic metasedimentary rocks and Paleoproterozoic collisional orogenic events documented in the DTB are consistent with th