Chunjing Wei

EPMA U-Th-Pb MONAZITE AND SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF HIGH-PRESSURE PELITIC GRANULITES IN THE JIAOBEI MASSIF OF THE NORTH CHINA CRATON

The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet + kyanite +biotite + muscovite + rutile (M1), garnet + kyanite + ternary-feldspar + biotite + muscovite + rutile (M2), garnet + sillimanite + biotite + perthite + anti-perthite (M3), and biotite + muscovite + sillimanite + plagioclase (M4), of which M2 is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism. The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M4) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.

Petrogenesis of Mesozoic granitoids in the Dabie UHP complex, Central China: trace element and Nd–Sr isotope evidence

Post-collisional magmatism in the Dabie Orogen is characterized by the near contemporaneous intrusion of voluminous granitoids and minor mafic magmas in the late Mesozoic (135–110 Ma). The granitoids are represented by three major batholiths in Northern Dabie: Zhubuyuan, Baimajian and Tiantangzhai. The largest Zhubuyuan Batholith comprises hornblende-bearing intermediate rocks (Penghe suite) and hornblende-free granitic rocks (Huangbai suite). The Huangbai granites show limited variation in chemical composition, but are highly variable in textures, ranging from medium to coarse grained. They have rather homogeneous Sr and Nd isotopic compositions, with (87Sr/86Sr)i=0.708–0.709 and eNd(T)=−19 to −24. The Penghe intermediate suite covers a wide spectrum of rock types, including diorite, monzodiorite, quartz monzonite to monzogranitic rocks, and all of them contain hornblende and mafic enclaves. The existence of compositional and textural disequilibrium (e.g., calcic and resorbed cores of plagioclase) and the nature of chemical variation in these rock types indicate that magma mixing between an evolved mafic and a granitic magma was involved in their genesis. Fractional crystallization (FC) could have coupled with the process of magma mixing but with minor contamination by the upper continental crust during magma ascent. Isotopically, the intermediate rocks show a variation in eNd(T) values from −13.7 to −18, while their (87Sr/86Sr)i ratios vary from 0.7074 to 0.7088. The genesis and evolution of the Dabie post-collisional magmatic rocks may be summarized as follows. Intrusion of mantle-derived mafic/ultramafic magmas into thickened continental crust induced dehydration melting of the lower crust rocks with intermediate compositions and subsequent production of large amounts of granitic melts (Huangbai suite). Extensive interaction between the crustal and mantle-derived melts through mixing produced a hybrid parental magma which subsequently underwent a FC process en route to higher crustal levels producing a wide variety of rock types as shown by the Penghe intermediate rock suite. Injection of successive pulses of basic magma into upward mobile magma chambers could have resulted in the formation of the mafic enclaves as well as mafic dykes.

Discovery and implications of the high-pressure pelitic granulite from the Jiaobei massif

The high pressure pelitic granulite with peak assemblage of garnet + kyanite + ternary feldspar + muscovite + rutile was discovered in Qixia area in the Precambrian Jiaobei massif, where high pressure basic granulites are widely distributed. The metamorphic peak conditions for the pelitic granulite were calculated asT = 800–840°C andP = 1.0–1.25 GPa on the basis ofP-T pseudosection. The post peakP-T path is characterized first by an isothermal decompression (ITD) pattern and then by an isobaric cooling (IBC) pattern, indicating a geodynamic process related to thinning of thickened continental crust.

A Brief Review of UHP Meta-ophiolitic Rocks, Southwestern Tianshan, Western China

We here summarize recent studies of the occurrence, petrology, geochemistry, and geochronology of a new UHP metamorphic belt in the southwestern Tianshan. Eclogites here are grouped into three types: (1) lenticular eclogite bodies surrounded by blueschists; (2) eclogites with relict pillow structures; and (3) banded eclogite layers within marbles. They contain inclusions of coesite pseudomorphs in garnet, coesite exsolution rods in omphacite, and the possible stable association of aragonite + magnesite after dolomite. Rocks record three discrete stages of recrystallization: a peak UHP eclogite stage at 560-600°C and 4.9-5.0 GPa, a main UHP eclogite stage at 598-496°C and 2.6-2.7 GPa and retrograde overprinting under epidote blueschist-facies conditions. Protoliths possess geochemical characteristics of ocean-island basalts that originated from an enriched mantle with εNd = -1.4 to -0.4, NMORB from depleted mantle with εNd = +6.7 to +7.4, and EMORB from enriched mantle with εNd = -2.5 to +3.2. SHRIMP dating of zircon separates shows that the oceanic basaltic protoliths formed in a pre-Carboniferous (>310 Ma) seamount environment, began northward subduction beneath the Yili-central Tianshan craton at the end of the Permian (280-290 Ma), and were subjected to HP-UHP metamorphism in the Triassic (220-230 Ma). Based on the occurrence of this Triassic HP-UHP belt together with discovery of coeval low-P granulite-facies rocks to the north, a paired metamorphic belt tectonic model is proposed for the southwestern Tianshan, western China.

Determination and geological significance of the eclogites from the northern Dabie Mountains, central China

Numerous eclogite-amphibolite blocks have been found along the Mayanling fault in the northern Dabie complex. Most of the eclogites are strongly retrograded and contain garnet, rutile and the amphibole-plagioclase symplectite. The occurrence of these eclogites may have something to do with the inverted movement of the Mayanling thrust fault.

Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P−T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P−T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.

Mesozoic high-K granitic rocks from the eastern Dabie Mountains, Central China and their geological implications

The Mesozoic high-K granitic intrusions from the eastern Dabie Mountains, Central China, can be divided into three superunits namely the Yaohe, Penghe and Huangbai superunits. The Yaohe superunit is compositionally dominated by quartz monzonite extending as a band in NW direction which is differently foliated, contains numerous dioritic enclaves and has been dated as 174 Ma. The Penghe superunit, widely distributed in the field, varies in composition but is dominated by quartz monzonitic and granitic rocks, which is massive in structure, has well developed with dioritic enclaves and is aged in 125-127 Ma. The Huangbai superunit is mainly composed of granitic composition which is massive in structure, rarely contains dioritic enclaves and is aged in 120-111 Ma. These three superunits of granitic intrusions can also be clearly distinguished in geochemistry. They have recorded an orogenic process of the Dabie Mountains from the end of regional metamorphism to the overprinting of the circum-Pacific tectonic regime.

Discovery of granulite from the Fuping area in southern Qinling Mountains and its geological significance

Two-pyroxene-bearing granulites were discovered for the first time in the Hercynian-lndosinian metamorphic belt of the southern Qinling Mountains. The granulites occur In the lower part of the Fuping complex and are dominated by intermediate-acidic rocks intercalated with a small amount of bands and lenses of basic granulites. The main metamorphic minerals include orthopyroxene, clinopyroxene, biotite, plagioclase and quartz, and orthopyroxenes are often retrogressively transformed Into amphiboles. The metamorphic conditions have been estimated to beT = 720–780°C andP = 0. 6 GPa. The granulite-bearing Fuping complex probably belongs to early Proterozoic in age. Whether it occurs as the crystalline basement of the southern Qinling Hercynian-lndosinian orogenic belt or as a thrusted slice in the collisional process needs further study.

Characteristics and geological significance of the eclogites from the Tongbai Mountains, Henan Province

Some eclogites and the related high-P metamorphic belt have been found for the first time from the Tongbai Mountains, Henan Province, where the eclogites and their probable retrogressive products —garnet amphibolites—occur as lenses in mica-schists. The fresh eclogites comprise garnet, omphacite, quartz, rutile, phengite, barroisite, etc. The compositional features of garnet and available geothermobarometers indicated that the eclogites belong to the low-T type. The mica-schists contain phengite, quartz, garnet and rutile that are also attributed to high-P assemblage. The eclogites and related high-P belt probably belong to the Early Paleozoic in age, which is distinct from the lndosinian Dabie highand ultrahigh-P metamorphic belt.

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

The Shirokaya Salma eclogite-bearing complex is located in the Archean-Paleoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz-amphibole, orthopyroxene, and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages – (1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase +quartz+rutile+ilmenite±orthopyroxene – are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite- and granulite-facies metamorphism that occurred at 16-18 kbar, 750-800 °C and 11-15 kbar, 820-850 °C, respectively. The quartz-amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P-T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Paleoproterozoic high-grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite-facies metamorphism in the Belomorian Province. This article is protected by copyright. All rights reserved.