Multiple Stages Of Magmatism From ~375 To ~200 Ma In Central Qiangtang, Tibet

The Longmu Co–Shuanghu–Lancangjiang suture zone(LSSZ), located in central Qiangtang, SW China, is interpreted to have formed as the boundary between a Gondwana-derived block and Eurasia, following subduction of the Longmu Co– Shuanghu–Lancangjiang oceanic crust. Subduction-related volcanism is common in central Qiangtang, forming a large magmatic zone composed of volcanic rocks and a granitic belt parallel to the LSSZ. This paper presents the results of a study of the magmatic rocks in the central Qiangtang. Based on new fieldwork, petrography, zircon U–Pb dating, geochemistry, and Lu–Hf isotopes, in combination with existing data, this study investigates the dynamic mechanism of magmatism in central Qiangtang and the relation between magmatism and the evolution of the Longmu Co–Shuanghu– Lancangjiangocean. A model is proposed for the evolution of the magmatic arc.Zircons from early Carboniferous basalts and andesites yielded crystallization ages of 357.7±1.9 Ma and 358.5±2.0 Ma, respectively, while a Middle Triassic granodiorite porphyry yielded zircon U–Pb ages of 230.1±1.9 Ma and 236.3±0.9 Ma. Crystallized and inherited zircons from Late Triassic andesites yielded ages of 223.9±1.3 Ma and 364.7±1.9 Ma, respectively, and Late Triassic diorites yielded a zircon U–Pb age of 222.3±1.2 Ma. These age data, combined with those from previous studies, indicate the magmatic early Permian(ca. 272 Ma), Middle Triassic(ca. 245–226 Ma), and Late Triassic(ca. 225–200 Ma).Late Devonian to early Carboniferous magmatic rocks are exposed mainly in the Riwanchaka, Laxiongco, and Guoganjianianshan areas of central Qiangtang. The magmatic rocks are classified into two groups based on petrography and geochemistry: calc-alkaline volcanic rocks and adakitic plagio-granite. The former include basic to acidic rocks; i.e., basalts, andesites, dacites and rhyolites. The negative Nb, Ta, and Ti anomalies of the calc-alkaline volcanics are consistent with typical volcanic arc rocks. The basalts were generated by partial melting of the mantle wedge that was metasomatized by slab-derived components. The andesites can be divided into two groups based on geochemistry: a group derived from partial melting of the mantle wedge, and another produced by fractional crystallization of the coeval basaltic parent magmas. The dacites and rhyolites were the products of highly fractionated basaltic magmas. The adakitic plagio-granites were derived from partial melting of the subducted oceanic slab. The lithological assemblage and geochemical characteristics of the magmatic rocks indicate a continental island-arc setting that formed during the initial northward subduction of the Longmu Co–Shuanghu–Lancangjiang oceanic crust. The formation of coeval SSZ-type ophiolites in the Hongjishan area indicates that initial subduction resulted in extension and the formation of a back-arc basin that led to the formation of the Hongjishan Ocean.Early Permian magmatic rocks are found in the Hongjishan area of central Qiangtang. The Hongjishan granites intruded coeval ophiolites, and their geochemical characteristics show adakitic affinities, indicating they were derived from partial melting of an oceanic slab. Based on these observations and the occurrence of mafic magmatic rocks in the Yushu area, it is suggested that early Permian magmatism in the northern Qiangtang terrane was triggered by northward subduction of the Longmu Co–Shuanghu– Lancangjiang oceanic crust. Upwelling asthenospheric mantle caused extension in the overlying Hongjishan Ocean lithosphere.Middle Triassic magmatic continental rocks are found in the Tuohepingcuo, Riwanchaka, Gaco, and Yanshiping areas, including basic and acidic rocks. The Yanshiping basalts can be grouped into two types: normal continental basalts and Nb-enriched continental basalts. The former were derived from the asthenospheric mantle, contaminated by the mantle wedge, while the latter were a product of partial melting of asthenospheric mantle, contaminated by the overlying crust. The Riwanchaka granodiorite porphyries are classified as normal calc-alkaline I-type granitoids, and they were generated by partial melting of mafic igneous lower crust of the southern Qiangtang terrane. Based on these results and regional geological data, it is proposed that the Middle Triassic magmatic rocks in central Qiangtang formed during continental collision and deep subduction in response to slab roll-back.Late Triassic magmatic rocks are found on both sides of the LSSZ within central Qiangtang. The magmatic rocks are classified into four groups according to petrography and geochemistry: normal calc-alkaline volcanic rocks, normal calc-alkaline intrusive rocks, adakitic rocks, and Riwanchaka andesites. The normal calc-alkaline volcanic rocks include basalts, dacites and rhyolites. The basalts were formed by partial melting of asthenospheric mantle, contaminated by crustal material. The dacites and rhyolites show similar characteristics to I-type granites and were generated by partial melting of mafic igneous lower crust of the northern Qiangtang terrane. The normal calc-alkaline intrusive rocks are divided into highly fractionated S-type and I-type granitoids. The former were the products of partial melting of argillite in the lower crust of the southern Qiangtang terrane and subsequently experienced extensive fractional crystallization, while the latter were derived from the igneous lower crust. The adakitic rocks had two different magma sources: an oceanic slab and thickened lower crust. The Riwanchaka andesites formed by the mixing of melts derived from the lower crust of southern Qiangtang terrane and Late Devonian dioritic magma in shallow magma chambers. It is proposed that both the Late Triassic magmatic flare-up and the coeval exhumation of metamorphic rocks in the central Qiangtang were triggered by slab breakoff in an extensional setting.Recent research has identified another tectonic mélange belt in the Rawanchaka area, in the southern Qiangtang terrane, some 70 km from the LSSZ. The rocks of the Rawanchaka tectonic mélange belt(RTMB) have an affinity with the LSSZ and the northern Qiangtang terrane, indicating that the RTMB was derived from the north and emplaced in the southern Qiangtang terrane during the Late Triassic. It is proposed that the RTMB was detached from the edge of the northern Qiangtang terrane along the Hongjishan tectonic mélange. Retreat of the southern Qiangtang terrane, triggered by slab break-off, carried the RTMB away from the trench and separated the RTMB from the LSSZ and the northern Qiangtang terrane, thereby forming the present-day geological structure in central Qiangtang. The geological features described above indicate that Late Triassic geological events in the central Qiangtang, including a magmatic flare-up event, the exhumation of metamorphic rocks, and detachment of RTMB away from the LSSZ and northern Qiangtang terrane, were all part of a chain reaction of events triggered by slab breakoff of the northward-subducting Longmu Co–Shuanghu–Lancangjiang oceanic lithosphere. Therefore, the central Qiangtang is an ideal area in which to investigate the process of slab breakoff based on geological observations.Our results, together with existing data on ophiolites, metamorphic rocks, stratigraphy, and paleomagnetism, are used to develop a new model of the evolution of the Longmu Co–Shuanghu–Lancangjiang Ocean. It is suggested that the four groups of magmatic rocks identified from zircon dating record four distinct phases of the development of the ocean, as follows:(1) initial northward subduction during the Late Devonian–early Carboniferous;(2) ridge subduction during the early Permian;(3) continental collision and subduction in the Middle Triassic; and(4) post-collision slab breakoff and exhumation in the Late Triassic.