Kai-Jun Zhang

North and South China collision along the eastern and southern North China margins

Abstract Lithofacies distribution and petrography and intracontinental deformation in the North China block (NCB) and the South China block (SCB) show an active continental margin flanked by the eastern NCB, and a passive continental margin was present along the west side of the SCB, both margins initially trending N-NE. The Tanlu fault possibly initiated as part of the subduction zone during Paleozoic time along the eastern margin on the NCB. The NCB and the SCB were divided along the eastern–southern North China margin by the Qinling–Dabie–Tanlu–Sulu–Imjingang–Yanji zone from southwest to northeast. These two continental blocks made contact first in the northeast during late Early Permian time, finally in the southeast at Late Triassic time, and the clockwise collision probably lasted to Middle Jurassic time. The eastern suture, the Yanji–Imjingang–Sulu–Tanlu zone, was dominated by contractional deformation. The southern suture, the Qinling–Dabie zone, was a transpression belt dominated by right-lateral strike-slip.

The blueschist-bearing Qiangtang metamorphic belt (northern Tibet, China) as an in situ suture zone: Evidence from geochemical comparison with the Jinsa suture

Metasiliciclastic rocks and metabasalts from the blueschist-bearing Qiangtang metamorphic belt and the Jinsa suture zone were analyzed for major and trace elements in an attempt to evaluate the affinities of these two tectonic entities. Tholeiitic mid-oceanic-ridge basalts (MORBs) from the Jinsa suture can be distinguished from metabasalts of Qiangtang, which have alkalic compositions and exhibit a range of characteristics typical of many within-plate oceanic islands. The Qiangtang metasiliciclastic rocks were derived from a passive continental margin source, whereas those from the Jinsa suture zone were sourced from a continental island arc or an active continental margin source. The geochemical distinction of metasiliciclastic rocks and metabasalts of the Qiangtang metamorphic belt from their counterparts within the Jinsa suture indicates that there is no affinity between these two tectonic entities and that the Qiangtang metamorphic belt could not have been underthrust from the Jinsa suture. It most likely represents an exhumed accretionary complex composed of sediments derived from a passive continental margin and fragments of seamount sequences, and marks an in situ suture zone that separates northern and southern Qiangtang terranes.

Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet: Comment and Reply: COMMENT

[Kapp et al. (2000)][1] suggested that the Qiangtang metamorphic belt consists of underplated blueschist-bearing melange and was exhumed by low-angle normal faults, and that both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the

U–Pb and Lu–Hf isotope systematics of detrital zircons from the Songpan–Ganzi Triassic flysch, NE Tibetan Plateau: implications for provenance and crustal growth

We conducted in situ U–Pb and Lu–Hf isotope analyses of 401 detrital zircons collected from the Songpan–Ganzi Triassic turbidite complex in an attempt to understand the provenance variations of the siliciclastic rocks and the crustal growth history of central China. These detrital zircons exhibit a wide age spectrum with three major peaks at 1.7–2.0 Ga, 750–1050 Ma, and 210–500 Ma. They are dominated by negative ϵHf(t) values with a large range. Synthesis of the zircon U–Pb and Lu–Hf isotopic data indicate that the Triassic Songpan–Ganzi turbiditic succession could have been derived dominantly from the Tibetan terrains + the Kunlun and Qinling orogens. Our samples are characterized by a common, prominent group of Hf crust formation model ages at 0.8–4.1 Ga with a peak at 2.7–3.4 Ga. This fact indicates that (1) Phanerozoic magmatism in central China could have been predominantly products of crustal reworking with insignificant formation of juvenile crust and (2) the Neoarchaean was an important period of continental growth in central China. In addition, our data set also reveal that three widespread tectonothermal events could have occurred in the region during the late Mesoproterzoic, Palaeozoic, and early Mesozoic, respectively.

Lawsonite- and glaucophane-bearing blueschists from NW Qiangtang, northern Tibet, China: mineralogy, geochemistry, geochronology, and tectonic implications

The occurrence of high-pressure (HP) blueschists within the central Qiangtang terrane of northern Tibet has a significant bearing on plate-suturing processes. In order to contribute to the ongoing debate on whether the central Qiangtang metamorphic belt represents an in situ suture within the Qiangtang terrane, we examined lawsonite- and glaucophane-bearing blueschists from the northwest Qiangtang area (84° 10′–85° 30′ E, 34°10′–34° 45′ N). All studied rocks are metapelites, metasandstones, or metabasalts, characterized by lawsonite + glaucophane + phengite, lawsonite + glaucophane + epidote + albite + quartz, or glaucophane + phengite + quartz assemblages. The meta-mafic rocks contain very high TiO2 and low Al2O3 contents. They are typified by abundant ferromagnesian trace elements, and an absence of Eu anomalies and Nb–Ta deletions; all the above features indicate that these mafic rocks represent oceanic island basalt (OIB) protoliths. Most of the metasediments contain high SiO2, moderate Al2O3 + K2O, and low TiO2 + Na2O. They display high CIA (chemical index of alteration) values (74% ± 5%) and distinctly negative Eu anomalies (Eu/Eu* = 0.64 ± 0.05). This, along with their high field strength elemental characteristics, indicates that they were deposited in a passive continental margin environment, intercalated with OIB-type basalts. We estimate the peak metamorphic conditions for these blueschists as T = 330–415°C and P = 9–11.5 kbar. This HP event occurred at ca. 242 Ma, indicated by a well-defined 40Ar/39Ar plateau age for glaucophane. Retrograde metamorphism occurred at T = 280–370°C, P = 6.5–9.5 kbar, t = ca. 207 Ma (40Ar/39Ar dating of phengite). Therefore, a cold subduction (geotherm ∼8°C/km) attended the passive continental margin during the Triassic when the eastern Qiangtang collided with the western Qiangtang. The northwest Qiangtang HP metamorphic belt is an extension of the central Qiangtang metamorphic belt that defines the suture between eastern and western Qiangtang, and indicates an anticlockwise, diachronous closure of the Shuanghu Palaeo-Tethys.

Diversified Provenance of the Songpan-Ganzi Triassic Turbidites, Central China: Constraints from Geochemistry and Nd Isotopes

The Songpan-Ganzi Complex (SGC) in central China is one of the largest turbidite basins on Earth, but the origin of slates and sandstones is still open to debate. Petrographic, geochemical, and Nd isotope data for representative turbidites were collected, and the SGC was thus divided up into four main zones. The western and middle zones are characterized by easterly paleocurrents, abundant high-pressure (HP)-related dense minerals, and low chemical index of alteration (CIA) and Th/U values. The samples from the western zone have strongly negative ϵNd values, old Nd model ages, and low ferromagnesian but high felsic contents. The sediments from the middle zone have intermediate ϵNd values and Nd model ages and trace elemental ratios. No HP-related dense minerals have been identified in the eastern zone, which is dominated by westerly paleocurrents, with intermediate Nd isotopic and geochemical values. The northernmost zone is characterized by high εNd (0) values, young TDM ages, and high ferromagnesian but low felsic contents. Geochemical and Nd isotopic data all are suggestive of an overall silicic provenance of the sediments, which have not been homogenized by recycling. Several source rocks for the samples are identified, and they include sedimentary rocks, ophiolites, silicic igneous rocks, and HP metamorphic rocks. Nd isotopes and geochemical indices least susceptible to change by sedimentary processes indicate that turbidites in the SGC were dominantly derived from central Qiangtang, Dabie-Qinling, and Kunlun. The low CIA values and Th/U ratios indicate that the source areas for the western SGC could have been elevated by tectonism and dominated by cool and/or arid climates.

Geochronology and geochemistry of the c. 80 Ma Rutog granitic pluton, northwestern Tibet: implications for the tectonic evolution of the Lhasa Terrane

The Rutog granitic pluton lies in the Gangdese magmatic arc in the westernmost part of the Lhasa Terrane, NW Tibet, and has SHRIMP zircon U-Pb ages of c. 80 Ma. The pluton consists of granodiorite and monzogranite with SiO2 ranging from 62 to 72 wt % and Al2 O3 from 15 to 17 wt %. The rocks contain 2.33-4.93 wt % K2O and 3.42-5.52 wt % Na2O and have Na2O/K2O ratios of 0.74-2.00. Their chondrite-normalized rare earth element (REE) patterns are enriched in LREE ((La/Yb)n = 15 to 26) and do not show significant Eu anomalies (δEu = 0.68-1.15). On a primitive mantle-normalized trace element diagram, the rocks are rich in large ion lithophile elements (LILE) and poor in high field strength elements (HFSE), HREE and Y. Their Sr/Y ratios range from 15 to 78 with an average of 30. The rocks have constant initial 87 Sr/ 86 Sr ratios (0.7045 to 0.7049) and slightly positive eNd(t) values (+0.1 to +2.3), similar to I-type granites generated in an arc setting. The geochemistry of the Rutog pluton is best explained by partial melting of a thickened continental crust, triggered by underplating of basaltic magmas in a mantle wedge. The formation of the Rutog pluton suggests flat subduction of the Neo-Tethyan oceanic lithosphere from the south. Crustal thickening may have occurred in the Late Cretaceous prior to the India-Asia collision.

Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

ABSTRACT We report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas Type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than Type 1 ones. Type 2 eclogites are significantly more enriched in TiO2, P2O5, and light rare-earth elements, but more depleted in Al2O3 contents and Mg numbers. P–T estimates for both types of eclogites are consistent with minimum pressures of 23–25 kbar at 830–920°C, with the retrograde amphibolite facies assemblage of 15.2–17.5 kbar and 500–732°C. Zircon U–Pb dating indicates that the protolith of Type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at ~260 Ma, whereas the protolith of Type 2 eclogites, geochemically familiar with oceanic-island basalt (OIB), erupted at ~242 Ma. 40Ar/39Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize eclogite-bearing high-pressure rocks were exhumed at ~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of ~85 km along the southern margin of the Qiangtang block during the Late Triassic.

Late Permian–Triassic siliciclastic provenance, palaeogeography, and crustal growth of the Songpan terrane, eastern Tibetan Plateau: evidence from U–Pb ages, trace elements, and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.

Genesis of the Late Mesozoic Great Xing’an Range Large Igneous Province in eastern central Asia: A Mongol–Okhotsk slab window model

The late Mesozoic Great Xing’an Range Large Igneous Province (XRLIP), with an area of >3 × 105 km2, is a prominent, enigmatic feature in eastern central Asia. The province is characterized by extensive within-plate magmatism, including a >4 km-thick sequence of volcanic rocks and voluminous plutons emplaced during an interval of ~40 million years from Late Jurassic through Early Cretaceous times (~150–110 Ma). The igneous activities are characterized by widespread adakitic rocks, alkalic basalts, and A-type granitoids with largely intraplate geochemical signatures, emplaced in a normal continental crustal setting. A Mongol–Okhotsk ridge subduction model is proposed for petrogenesis of the igneous rocks. Partial melting of young, hot, subducting oceanic slabs close to the ridge formed the adakitic rocks. A slab window that opened during ridge subduction triggered alkalic basaltic to A-type granitic and minor calc-alkaline magmas, as well as large-scale, metallogenic mineralization and subsequent basin formation.