Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone:Implications for the Opening of the Neo-Tethys Ocean

In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).

Conodont Biostratigraphy and Age of the Early Triassic Fish-Bearing-Nodule Levels from Nanjing and Jurong, Jiangsu Province, South China

Fish-bearing-nodules are found in Early Triassic marine strata in many regions, including East Greenland, East Spitsbergen, Northwest Madagascar, Canada, Angola, and South China. A new Olenekian(Early Triassic) conodont biostratigraphic study for stratum that contains fish-bearing-nodule levels is conducted based on the Longtan(LT) Section of Nanjing City and the Qingshan(QS) Section of Jurong County, Jiangsu Province, South China. A total of 101 samples were collected at the two sections and three conodont zones were recognized: in ascending order, they are Scythogondolella(Sc.) milleri Zone, Novispathodus(Nv.) pingdingshanensis Zone, and Triassospathodus(Tr.) aff. homeri Zone. The Scythogondolella milleri Zone is globally recognized as the uppermost conodont zone of the Smithian and the base of the Novispathodus pingdingshanensis Zone was previously suggested as the marker of the Smithian-Spathian boundary in the Yangtze region. The fish-bearing-nodule levels of Nanjing and Jurong are within the conodont Scythogondolella milleri Zone, laterally correlated well with the upper part of the Novispathodus waageni Zone in Chaohu of Anhui Province that also contains fish-bearing-nodule levels. The conodont biostratigraphic correlation confirms the Early Triassic fish-bearing-nodule levels in the Lower Yangtze region are coeval, with an end-Smithian in age.

U-Pb zircon ages and petrogeochemistry and tectonic implications of gabbro and granite in southwest Lahad Datu area of Sabah, Malaysia

The southwest Lahad Datu felsic rocks were previously thought to have formed in the late Triassic as part of the microcontinental crystalline basement.Based on U-Pb ages,geochemistry,and the Hfisotopes of zircon from the southeastern Sabah gabbro and granite,in this study,the tectonic properties of the Sabah area during the Triassic were investigated.The weighted average U-Pb zircon ages of the gabbro and granite samples were determined to be(230.9±2.5)Ma and(207.1±3.3)Ma,respectively.The granite had SiO_(2) contents of 66.54%-79.47%,low TiO_(2) contents of 0.08%-0.3%,Al_(2)O_(3) contents of 10.97%-16.22%,Na_(2)O contents of 5.91 %-6.39%,and low K_(2)O contents of 0.15%-0.65%.The chondrite-normalized rare earth element(REE) patterns exhibit light REE enrichment,with right-sloping curves.The primitive mantle-normalized trace element spider diagrams exhibit Th,U,La,Sr,and Zr enrichment and Nb,Ta,P and Ti depletions,i.e.,the geochemical characteristics of typical island arc igneous rocks.The tectonic discriminant diagram indicates that the granite is a volcanic arc granite.The Hf isotopic an alysis of gabbro zircon revealed that the zircons have ε_(Hf)(t)values of 12.08-16.24(mean of 14.32) and two-stage model ages(t_(DM2)) of 223-491 Ma(mean of 347 Ma).This indicates that the diagenetic magma of the gabbro was mainly derived from melting of newly formed crustal materials.The ophiolite in southeast Sabah has existed since the early Late Triassic.The crystalline basement granite in southeastern S abah was emplaced lasted from late Triassic to early Cretaceous.Based on previous studies and global plate reconstruction models,it is speculated that the southeastern Sabah granite may have been formed in an island arc setting,i.e.,where the oceanic crust of the Paleo-Tethys Ocean collided with the oceanic crust of the Panthalassa Ocean.

LYSTROSAURS OF XINJIANG, CHINA

Since the first report on Chinese lystrosaurs made by Yuan and Young in 1934, abundant material has been collected from the Early Triassic of Jiuchaiyuan Formation of Jimusar (Fuyuan), Junggar basin and Taoshuyuan, Turpan basin. Up to now 7 species of Lystrosaurus——L. broomi, L. hedini, L. weidcnreichi, L. youngi, L. robustus, L. latifrons and L. shichanggouensis, have been erected in China. All the species were described based on almost complete skulls, except L. weidenreichi, which was on some materials of postcranial skeleton. It was thought generally that L. broomi and L. youngi are closely related to the South African L. murrayi and L. curvatus respectively. Furthermore, the two Chinese species were considered as synonymous with their counterparts of South Africa by Colbert in 1974, when he revised this genus. The characters of the Chinese lystrosaurs, however, appeared to give no support to Colbert’s suggestion. A comprehensive study of the Chinese lystrosaurs based on the type specimens and the additional materials, and the revised diagnoses. for 4 species are given in the present paper.

Carbon isotope composition and comparison of Lower Triassic marine carbonate rocks from Southern Longmenxia section in Guang＇an, Sichuan Basin

The Early Triassic was a period of ecological restoration for the earth's system after the end-Permian mass extinction.Geochemical records linked to the variations in marine ecosystems during this period have attracted the interest of geologists for many years.Based on petrographic analysis of samples and evaluations of their reliability as proxies for original seawater,this study investigated the carbon and oxygen isotopic compositions of 350 carbonate rock samples from the Lower Triassic(and adjacent strata)in the southern Longmenxia section of Guang'an,eastern Sichuan Basin.The results indicate that the Triassic carbonate rocks from the southern Longmenxia section favorably preserved the original seawater'sδ^(13)C signal.Furthermore,carbon and oxygen isotopic compositions are found to be poorly correlated,with a determination coefficient as low as 0.0205 and only 44 rock samples show results of Mn/Sr>2 and/orδ^(18)0<-6.5‰.The complete carbon isotopic curve for the Lower Triassic is established using the data from the remaining 306 samples with Mn/Sr<2 and/orδ^(18)O>-6.5‰.This curve presents favorable comparability on a global scale,specifically in theδ^(13)C minima near the Permian-Triassic boundary,at the top of the Jia1 and within the the Jia3,as well as in theδ^(13)C maxima at the tops of the Ye1 and Ye4,at the base of the Jia2 and at the top of the Ye3.The peaks and troughs corresponding to these maxima and minima all reflect global signals.By comparing these results to previous research on coeval carbon isotopic curves established within the chronostratigraphic framework,the ages of these critical evolution points are determined.The results show that the Ye1 roughly corresponds to the Griesbachian substage;the Ye2 through Ye4 correspond to the Dienerian substage;Jial corresponds to the Smithian substage;from the Jia2 to the lower part of the Jia4 correspond to the Spathian substage;and the middle and upper parts of the Jia4 belongs to the Aegean Substage of the Middle Triassic.Around the boundary between the Jial and Jia2(which represents the Smithian-Spathian boundary(SSB)),the value ofδ^(13)C increases rapidly from-0.911‰to 3.679‰.The span during which the seawater's carbon isotope experiences this drastic change may be less than36 kyr.The oxygen isotope,which is more sensitive to sedimentary environmental changes,exhibits changes prior to the carbon isotope near the SSB,indicating a significant increase in the salinity of the seawater before a sharp rise in the carbon isotope;this event leads to the formation of evaporites and dolomites.