Lower Cretaceous turbidites in the Shiquanhe–Namco Ophiolite Mélange Zone, Asa area, Tibet: Constraints on the evolution of the Meso-Tethys Ocean

Turbidites fromthe Shiquanhe–Namco OphioliteMélange Zone(SNMZ)record critical information about the tectonic affinity of the SNMZand the evolutionary history of theMeso-Tethys Ocean in Tibet.This paper reports sedimentologic,sandstone petrographic,zircon U-Pb geochronologic,and clastic rocks geochemical data of newly identified turbidites(Asa Formation)in the Asa Ophiolite Mélange.The youngest ages of detrital zircon from the turbiditic sandstone samples,together with~115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age.The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments,suggesting that the turbidites were mainly derived froma recycled orogen provenancewith a minor addition of volcanic arc materials.The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of 170–120 Ma,300–220 Ma,600–500 Ma,1000–700 Ma,1900–1500 Ma,and~2500 Ma,similar to the ages of the Qiangtang Terrane(age peak of 600–500 Ma,1000–900 Ma,~1850 Ma and~2500 Ma)and the accretionary complex in the Bangong–Nujiang Ophiolite Zone(BNMZ)rather than the age of the Central Lhasa Terrane(age peak of~300 Ma,~550 Ma and~1150 Ma).The mineral modal compositions,detrital U-Pb zircon ages,and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane.Based on our new results and previous studies,we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone,rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane.The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction,accretion,arc-Lhasa collision,and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.

The Ren Co MOR-type Ophiolite in the North-central Tibetan Plateau: A Fast-spreading Ridge Segment of the Meso-Tethys Ocean?

The Meso-Tethys Ocean is generally considered to have opened in the late Early Permian as a result of the Cimmerian continental block drifting away from the Gondwana supercontinent. This ocean is also termed the north branch of the Neo-Tethys Ocean, and is now represented by the Bangong–Nujiang suture zone in the north-central Tibetan plateau. However, it is still unknown for the evolutionary history for as such a huge ancient ocean basin. Ophiolites are remnants of oceanic lithosphere and preserve key information in rebuilding the evolutionary history of ancient oceans. In this study, we undertook detailed geological mapping for the Ren Co ophiolite in the middle part of the Bangong–Nujiang suture zone, and a typical Penrose-type ophiolite sequences was newly documented in the Ren Co area. The Ren Co ophiolite comprises serpentinized peridotite, cumulate rock, gabbro/diabase, sheeted dike, massive and pillow lavas, and minor red chert. These rocks exhibit well-preserved mantle and crust rock suites, and show close similarities to oceanic lithospheres at modern fast-spreading ridges. Zircon U–Pb dating for gabbro and plagiogranite yielded ages of 169–147 Ma, which suggest that the Ren Co ophiolites were formed during the Middle to Late Jurassic. Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle. Mafic rocks(basalt, diabase and gabbro) of the Ren Co ophiolite show geochemical features similar to those of typical N-MORB. Moreover, all samples have positive whole-rock εNd(t), and zircon εHf(t) and mantle-like δ18O values. These features suggest that these rocks were derived from a depleted mantle source, thus the Ren Co ophiolite was typical MOR-type ophiolite. We suggest that the Ren Co ophiolite was formed in a fast-spreading mid-ocean-ridge(MOR) setting, and they most likely represented the ridge segment of the BangongNujiang Meso-Tethys Ocean. The Bangong–Nujiang MesoTethys Ocean was a wide paleo-ocean, and experienced continuous oceanic spreading, subduction, accretion before final Lhasa and South Qiangtang continental assembly.

From arc-continent collision to ocean closure: Lower Cretaceous Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone, central Tibet

One of most hotly debated topics concerning the Late Mesozoic evolution of Tethyan and the Tibetan Plateau is the timing of the closure of the Meso-Tethys ocean,which is represented by the Bangong-Nujiang suture zone.The Upper Jurassic-Lower Cretaceous Shamuluo Formation,which unconformably overlies the older Mugagangri Group accretionary complex,provides important information on the closure of the Meso-Tethys Ocean.This paper precisely confines the depositional age of the Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone,extending it from the Late Jurassic to the Albian.Combined with the results of previous studies,we suggest that the Shamuluo Formation in the Awengco-Baerqiong region mainly contains a bathyal Berriasian-Hauterivian subunit and a shallow-marine Albian subunit.Provenance analysis indicates that the Berriasian-Hauterivian subunit was mainly derived from the Jurassic southern Qiangtang magmatic arc,while the Albian subunit was derived from the coeval volcanic rocks and the Upper Carboniferous-Upper Permian strata in the southern Qiangtang terrane.Thus,the two subunits of the Shamuluo Formation have significant distinct sedimentary facies and provenances,indicating that they were deposited in different tectonic settings.Based on the regional geological data,we suggest that the bathyal Berriasian-Hauterivian subunit and the shallow-marine Albian subunit of the Shamuluo Formation should be interpreted as a record of the oceanic arc-continent collision and the Lhasa-Qiangtang soft-collision,respectively.Thus,the closure time of the Meso-Tethys Ocean is at least limited to the Albian.