Cambrian-Permian Ophiolites

Longmuco－Shuanghu－Lancang suture zone is an important tectonic boundaryin Qinghai－Tibet plateau. The middle and western parts of the suture zone arelocated in Qiangtang, and the research data is limited, especially on ophiolites.Therefore, some key aspects about the suture, such as its nature of oceanic basinrepresented by the suture, its evolution and whether representing the north boundaryof Gondwanaland, are still open to intense debate. This has constituted an obstacle forunderstanding the formation and evolution of early period of Qinghai－Tibet plateau.In this thesis, intensive studies, including fieldwork, petrology, petrography,geochemistry and zircon U－Pb geochronology (SHRIMP, LA－ICP－MS,CAMECA) and Lu－Hf isotope, were conducted on Taoxinghu andGuoganjianianshan ophiolitic melanges in middle and western parts ofLongmuco-Shuanghu-Lancang suture zone. Cambrian-Permian ophiolitic melangeswere defined. Combined with the previous studies, the author investigated the natureand southeast extension of Longmuco-Shuanghu-Lancang suture zone and finallyconstructed the evolution model.Cambrian ophiolitic mélange is mainly made of metaperidotite, cumulate(cumulate gabbro, cumulate pyroxenites, cumulate plagioclasite), plagiogranite,gabbro dike and basalt. U-Pb ages of zircon from cumulate gabbro and plagiograniteare504.9±4Ma,517.1±3.8Ma and491.6±1.5Ma,497.2±4.8Ma, respectively. As themantle unite, metamorphic peridotite are originated as the residue from melting at a ridge with7–20%degree of fraction melting and are subsequently modified byinteraction with mafic melt on subduction zone. Cumulate may be originated fromdepleted mantle. Lu-Hf isotopes and geochemistry show that plagiogranite areprobably the products of melting of cumulate gabbro.Ordovician-Silurian ophiolitic mélanges mainly include metaperidotite, cumulate,plagiogranite, gabbro dike, basalt and siliceous. U-Pb dating of zircons from gabbroand plagiogranite has yielded ages of459.8±7.7Ma,453.2±6.1Ma and440.5±1.6Ma,441.6±3.4Ma. The cumulate have MORB affinity.The gabbro from Devonian ophiolitic mélange has U-Pb age of367.2±3.3Ma.The geochemistry of gabbro suggests that they could have formed under a middleoceanic ridge environment, and interacted with aqueous fluid during later tectonicemplacement.The Carboniferous ophiolitic mélange is lack of mantle unit, and composed ofcumulate, plagiogranite, gabbro dike and basalt. The cumulate gabbro andplagiogranite have ages of358.0±12Ma and354.8±2.4Ma based on zircon U-Pbmethod. The most of basalt and gabbro geochemically display characteristic featuresof MORB and IAT, and some basalts show OIB affinity. Plagiogranite were probablyoriginated from shear melting of gabbric rock during oceanic crust moving.The Permian ophiolitic mélange mainly consists of cumulate, mafic dike andbasalt. By zircon U-Pb dating, the gabbro and basalt have ages of274.7±3.9Ma,272.9±1.8Ma and279.8±3.6Ma. The geochemistry show that Permian ophioliticmelange was probably formed in mid-oceanic ridge setting and effected by aqueousfluid during later moving and tectonic emplacement.According to the Cambrian to Permian ophiolitic relicts and related data inEastern Tibet and three-river area, the thesis proposes that Longmuco-Shuanghu-Lancang suture zone probably goes through north of Baqing-Quemoco-southeast ofGeladan-Changdu in the east of Shuanghu, link to Changning-Menglian suture zonein three-river area, and southward extends to Nanleihe, Fang in northern Thailand,Qingmai, Qinglai and Bentong-Laowu in Malaysia. The eastern Lancang-Changning-Menglian-Qingmai-Qinglai-Bentong-Laowu suture zone may have the same evolutional history as western part of Longmuco-Shuanghu-Lancang suture zonesince Devonian.The thesis first propose Longmuco-Shuanghu-Lancang ocean, which is locatedbetween Gondwanaland and Eurasia and represented by Longmuco-Shuanghu-Lancang suture zone. The evolution of this ocean can be divided into five stages:1,the ocean started rifting and oceanic crust formed in early Cambrian similar to RedSea stage in Wilson Cycle;2, the ocean evolved to be mature oceanic basin fromSilurian to early Carboniferous similar to Atlantic stage;3, the oceanic basin startedshortening and subducting northward from late Carboniferous to late Permian similarto Pacific stage, and a plume formed in the northern margin of Gondwanaland, whichaccelerates the subduction northward;4, the relict ocean was in Mediterranean stagefrom late Permian to middle Triassic;5, Southern and Northern Qiangtang blockswere amalgamated during middle-late Triassic and the western and eastern parts ofocean were closed at the same time. After late Triassic, Southern and NorthernQiangtang blocks received unified sediments.