Ages, geochemistry and tectonic settings of the Paleozoic-Mesozoic rocks along the Hegenshan-Heihe suture zone in the eastern segment of the Central Asian orogenic belt

The Hegenshan-Heihe belt (HHB) represents a suture zone resulting from the collision of the Xing’an and Songliao blocks located in the eastern segment of the Central Asian Orogenic Belt (CAOB). The formation of the HHB marks the destruction of the oceanic lithosphere and the closure of the Paleo-Asian Ocean (PAO). It has been a long-time debate regarding the tectonic evolution of the Xing’an and Songliao blocks and the intervening ocean during Paleozoic time, particularly the final closure timing of the PAO along the HHB. This project is aimed to address these issues by integrating the geochronology and geochemistry of the Paleozoic volcanic rocks and their tectonic setting with the detrital zircon data. The geochronological and geochemical data for two suites of the Paleozoic volcanic and sedimentary rocks suggest two long-lived island-arc systems along the southern margin of the Xing’an block and the northern margin of the Songliao Block. LA-ICP-MS zircon U-Pb dating results indicate that the Ordovician volcanic rocks developing at the southern margin of the Xing’an block erupted at ca. 447Ma and geochemically resemble the arc-related volcanism with strongly enriched LREE and LILE. The zircon ages also revealed the basement information of the Xing’an block by yielding age populations similar to 2.5, 1.8, 1.2, and 0.8 Ga from the inherited zircons, most likely shed from the deep continental crust during the magma migration and contamination. Similar features were also discovered in the locally deposited Silurian-Devonian sedimentary strata. This study suggests an Andean-type continental margin developing at the southern margin of the Xing’an block, associated with the northward subduction of the PAO from Ordovician to Devonian (ca.465-392Ma). The Carboniferous volcanic rocks developed at the northern margin of the Songliao block, associated with the coevally deposited sedimentary strata, showing a close affinity with oceanic island-arc, and the sedimentary strata formed in a back-arc/forearc basin. This study also suggests a Western Pacific-type subduction and arc-basin system developing at the northern margin of the Songliao block during Ordovician to Carboniferous time (ca. 452-315 Ma). The detrital zircon analysis suggests that a provenance transfer for the sedimentary strata developing in the HHB took place between the Late Carboniferous and Permian, with the Carboniferous strata sourcing Paleozoic detritus from the isolated island-arcs and the Permian strata sourcing Precambrian detritus from the approximate continent. Integrated with the previously reported data, this study suggests that the final closure of the PAO along the HHB occurred at some time between 303-312 Ma. This study also reports a suite of Mesozoic bimodal volcanic rocks in Zhalantun area, which was previously thought to be Permian in age, namely the Dashizhai formation. This suite of volcanic rocks formed in a regional extensional environment, which was mostly likely induced by the post-collisional extension of the eastern CAOB. On the basis of new data presented in this thesis combined with previous studies, I present a new tectonic model for the Paleozoic subduction and collision followed by a Mesozoic extension for the Hegenshan-Heihe suture zone.published_or_final_versionEarth SciencesDoctoralDoctor of Philosoph

The occurrence of Precambrian amphibolites from the Xinghuadukou Complex from NE China: Implications for the evolution of the Xinlin-Xiguitu Ocean, the NE branch of the Paleo-Asian Ocean

One of the geological attractions of the Xinghuadukou Complex resides in a suite of well exposed amphibolites outcropped locally in the northern part of the Xinlin-Xiguitu Suture zone between the Erguna and the Xing’an blocks in NE China. This suite of amphibolites occurs with gneisses, leptynite, schists, migmatitie and marbles from the Xinghuadukou Complex. Previously, only geochronological studies have been carried out on these amphibolites, suggesting a Late Neoproterozoic to Early Cambrian protolithic age (i.e. 547 ± 46 Ma). However, with only geochronological studies, it is insufficient to solve the controversy surrounding the tectonic settings of these amphibolites, such that the Precambrian evolution of the Xinlin-Xiguitu Suture cannot be further constrained. In this study, our new lithological and geochemical data indicate that these amphibolites are tholeiitic basalts with E-MORB affinities, which were most likely derived from the partial melting of a lithologically-mixed source of fusible eclogite and peridotites within the asthenosphere mantle under lithosphere thinning. Together with the previously reported Mid-Neoproterozoic suprasubudction-related ophiolites (i.e. 620–690 Ma) and ca. 516 Ma Toudaoqiao blueschists from this region, these amphibolites are suggested to be generated on the oceanic crust formed from the back-arc extension due to the intra-oceanic subduction. This interpretation provides new constraints not only on the tectonic implication of the Xinghuadukou Complex, but also the tectonic evolution of the Xinlin-Xiguitu Ocean within the Paleo-Asian Ocean regime

Geochronology and geochemistry of Permian to Early Triassic granitoids in the Alxa Terrane: constraints on the final closure of the Paleo-Asian Ocean

As a pivotal junction between the North China and Tarim cratons, the Alxa Terrane provides an ideal window to constrain the final closure of the middle segment of the Paleo-Asian Ocean (PAO). This study carried out petrological, whole-rock geochemical, and zircon U-Pb-Hf isotopic investigations on four major granitic plutonic complexes in the Alxa Terrane. The Bayan Nuru and Yabulai plutonic complexes are I-type granitoids that yield crystallization ages of 281–268 Ma and 277–270 Ma, respectively, with negative zircon εHf(t) values (−11.5 to −3.2), primarily sourced from the Neoproterozoic rocks in the region. The Nuergai granitoids yield crystallization ages of 281–268 Ma and show I-type affinities and positive zircon εHf(t) values (+1.6 to +6.4), indicating an origin related to magma mixing. Emplaced at ca. 241 Ma, the Oliji granitoids display varying zircon εHf(t) values from −0.9 to +11.5, which necessitate a predominant source of mantle-derived materials. A compilation of zircon εHf(t) and whole-rock εNd(t) values of the magmatic rocks in the Alxa Terrane illustrates a decreasing trend from the Late Carboniferous to the Early Permian and an increasing trend during Middle Permian to Triassic time. The marked shift with a large variation of zircon εHf(t) and whole-rock εNd(t) values at 280–265 Ma indicates a tectonic switch from subduction to post-collision tectonic regimes in the Alxa Terrane, marking the final closure of the middle segment of the PAO. Comparable isotope variations are also identified from 260 to 245 Ma magmatic counterparts on the northern margin of the North China Craton, hence suggesting a progressively eastward closure of the PAO

Geochronology and geochemistry of the Yilan greenschists and amphibolites in the Heilongjiang complex, northeastern China and tectonic implications

The Heilongjiang complex, extending along a suture zone between the Jiamusi and Songliao blocks in Northeast China, is composed mainly of blueschists, greenschists, meta-ultramafic rocks, quartzites, muscovite–albite schists and two-mica schists. Controversy has long surrounded the ages and tectonic settings of mafic rocks from the complex, which are crucial part of the complex. The lithological associations and their major and trace element compositions indicate that the mafic protoliths of the Yilan greenschists can be subdivided into alkali and tholeiitic basalts, which were derived from partial melting of a garnet-bearing and spinel-bearing mixed source, whereas the protoliths of the amphibolites are tholeiitic and were generated from the partial melting of spinel peridotite. Magmatic zircons from a tholeiitic amphibolite sample yielded a 206Pb/238U age of 256 ± 2 Ma, interpreted as its protolithic age. The sample also contains small amounts of older inherited zircons up to 344 Ma, which, together with its origin from shallow lithospheric mantle, indicate that the tholeiitic rocks were generated in a continental rift. The geochemical data suggest that further rifting led to the formation of an ocean between the Jiamusi and Songliao blocks, in which some oceanic islands developed, represented by the alkali basaltic protoliths of the Yilan greenschists. Magmatic zircons from an alkaline greenschist sample yielded a 206Pb/238U age of 162 ± 3.9 Ma, which, together with protolithic age of 141.8 ± 1 Ma previously obtained for the Yilian blueschist, support the model that the ocean between the Jiamusi and Songliao blocks closed at some time after ~ 141 Ma, not earlier at 210–180 Ma as previously considered

Ages and Hf isotopes of detrital zircons from the Permian strata in the Bengbatu area (Inner Mongolia) and tectonic implications

The Central Asian Orogenic Belt (CAOB) was built up through protracted accretion and collision of a variety of terranes/micro-continents during Neoproterozoic–Mesozoic time. To understand potential links among Paleozoic subduction and accretionary processes that were operative during the development of the southeastern CAOB, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from previously defined Devonian, Carboniferous and Early Permian strata in the Bengbatu area, Inner Mongolia. Detrital zircons from (meta-) sandstones in these strata commonly yield major Paleozoic age populations at ca. 300–261 Ma, 351–300 Ma and 517–419 Ma, and also give several Precambrian ages that range from 2687 Ma to 544 Ma. The youngest ages redefine the deposition of all these strata to be in the Middle Permian (Wordian–Capitanian) or later, much younger than previously considered. These ages, coupled with regional magmatic records, support an interpretation of most surrounding areas as possible detritus sources, including the Mongolian arcs to the north, the Northern Accretionary Orogen to the south, and the intervening Erenhot–Hegenshan Ophiolite Belt. Zircons with magmatic ages of ca. 500–350 Ma and ca. 300–261 Ma display a large range of εHf(t) values (−13.97 to +15.31), whereas ca. 350–300 Ma zircons are dominated by positive εHf(t) values (+0.14 to +16.00). These results support the occurrence of two significant shifts of the zircon εHf(t) values, which has tectonic implications for the understanding of the Carboniferous–Permian evolution of the southeastern CAOB. A marked shift from mixed to positive zircon εHf(t) values at 350–330 Ma likely manifests the incipient opening of the Hegenshan Ocean, due to the slab rollback of the subducting Paleo-Asian Oceanic lithosphere. Another shift from positive to mixed zircon εHf(t) values at ca. 300 Ma likely corresponds to a tectonic switch from syn-orogenic subduction-related to post-orogenic extensional setting, genetically related to the tectonic collapse of a formerly overthickened crust. Keywords: Detrital zircon, U-Pb dating, Hf isotope, Central Asian Orogenic Belt, Subduction, Back-arc basi

A broadscale shift in antarctic temperature trends

During the second half of the twentieth century, the Antarctic Surface Air Temperature (SAT) trends are characterized by fast warming over West Antarctica but mild cooling over East Antarctica. However, after 2000, the warming over several stations in the Antarctic Peninsula slowed down, whereas the South Pole experienced fast warming. These reversed SAT trends show strong regionality and seasonality, together with large uncertainty and disagreement among different observational and reanalysis datasets, which makes it difficult to achieve a comprehensive understanding of the multi-decadal Antarctic SAT trend and its reversal. In this study, we use the Combined Maximum Covariance Analysis (CMCA) method to extract the most coherent modes of the Antarctic SAT trends among six reanalysis datasets and 26 station-based observations. Further analysis shows that the reversals of the SAT trends before and after 2000, especially for austral spring and summer, are mainly attributed to the reversed trends of the leading CMCA modes and their related atmospheric circulation and thermal advection patterns over Antarctica. For austral spring, the reversal of the west-warming-east-cooling pattern over Antarctica is closely related to the changes of thermal advection induced by the anomalous circulation center over the Antarctic Peninsula–Weddell Sea region. For summer, the post-2000 reversal of the Antarctic Peninsula-warming-East Antarctic-cooling is attributed to the stratospheric ozone recovery over the Antarctic, and the associated adjustment of the southern annular mode. The CMCA decomposition better combines the information from different measurements, clarifying the long-term SAT trend and its reversal for different seasons