Mesozoic Tectonic Evolution Of The Qiangtang Block: Insights From Geochronology, Geochemistry And Isotopes Of Volcanic Rocks

This thesis presented the geochronology, geochemistry and Nd isotopic signatures of Mesozoic volcanic rocks from four areas in the Qiangtang block, in an attempt to reconstruct the tectonic evolution of the Qiangtang block during the Mesozoic.The volcanic rocks from the Meiriqiecuo region have a Late Triassic (205±4 Ma) zircon ²⁰⁶Pb/²³⁸U mean average age, which is considered to roughly represent the crystallizational age. These Late Triassic volcanic rocks contain low Ti potassic alkali basalts and are characterized by lowεNd_i (-4.79～- 5.35). The volcanic rocks from the Qiagangcuo region also give a fine Late Triassic zircon U-Pb concordant age (202±4 Ma), and are characterized by low Ti, high Mg #, calc-alkali andesites and dacites, with negative Eu anomalies to different extents (0.61 - 0.69), and low eNd_i values (-9.24～-9.79). The volcanic rocks from the Juhuashan region obtain an Early Jurassic ⁴⁰Ar/³⁹Ar plateau age (177.21±1.69 Ma) of whole-rock sample, and are made up of calc-alkali dacites that are characterized by negative Eu anomalies, high eNd_i (1.8～-0.6) and initial ⁸⁷Sr/⁸⁶Sr values (0.7077 - 0.7092). The volcanic rocks from these three areas all display strong enrichment of LREE and LILE relative to HREE and HFSE, respectively. The volcanic rocks from the Riajunlama region have an Early Jurassic ⁴⁰Ar/³⁹Ar plateau age (179.19±2.88 Ma) of whole-rock sample,and are characteristic of slight enrichment of LREE relative to HREE weak positive Eu anomalies, low ratios of (La/Ta)_N and (Th/Nb)_N (0.73, 1.08), and Nb/U and Ce / Pb ratios (35 - 40, 7.3 - 8.0) close to MORB and OIB, as well as positive eNd_i values.The Qiangtang block, during the Mesozoic, could have been interacted by continental subduction and denudation of the UHP rocks along the Shuanghu suture, continental collision with the Songpan-Ganzi complex along the Jinsha suture, and oceanic subduction and subsequent collision with the Lhasa block along the Bangong-Nujiang suture. These tectonic activities were all accompanied by a large scale of volcanism. The formation of the Late Triassic volcanic rocks could be related to the break-off of oceanic slab and partial thinning of sub-continental lithospheric mantle (SCLM) during the eastward subduction of the western Qiangtang plate. The Early Jurassic calc-alkali dacites could have resulted from the partial melt of mantle and subsequent AFC magmatic process during northward subduction of the Bangong-Nujiang ocean. The Early Jurassic alkali basalts from the Riajunlama region probably represent the oceanic island basalt within the plate of the Bangong-Nujiang ocean. The Early Cretaceous OIB-type basalts near the Bonggong-Nujiang suture and the intermediate-acid volcanic rocks in the northern Lhasa block could be a consequence of the back-arc extension during the northward subduction of the Neo-Tethys.