Xiaoping Long

The source and tectonic implications of late Carboniferous–early Permian A-type granites and dikes from the eastern Alataw Mountains, Xinjiang: geochemical and Sr–Nd–Hf isotopic constraints

ABSTRACT Zircon U–Pb ages, and geochemical, Sr–Nd and zircon Hf isotopic compositions are reported for the A-type granites and dikes in the Alataw Mountains of the northwestern Tianshan Orogenic Belt (NTOB), with the aim of investigating the sources and genesis of A-type granites and dikes. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of A-type granites yielded a concordant weighted mean 206Pb/238U age of 297.4 ± 1.5 and 300.6 ± 0.9 Ma, respectively, defining a late Carboniferous–early Permian magmatic event. Geochemically, the granitic intrusions and dikes are characterized by high SiO2 and total alkalies (K2O + Na2O), high Zr, Nb, Ta content, and Ga/Al ratio with prominent negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate that the granitic intrusions and dikes in the eastern Alataw Mountains are of an A-type affinity. The depleted Nd isotope compositions of the granitic intrusions and dikes are consistent with those of the Carboniferous volcanic rocks in the Alataw Mountains, especially Carboniferous adakites (εNd(t) = +3.6 to +6.6), suggesting that they were likely generated by partial melting of less evolved crustal materials, such as oceanic crust stored in the middle and/or lower crust or Carboniferous volcanic arc crust. The widespread late Carboniferous–early Permian magmatism in the NTOB may have been related to a ridge subduction accompanied by slab roll-back of the subducting plate of the North Tianshan Ocean.

Arc magmatism associated with steep subduction: Insights from trace element and Sr–Nd–Hf–B isotope systematics

Subduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318–312u2009Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction-like geochemical signatures, together with relatively high Mg#, high eNd(t) and eHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction-modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid-ocean ridge basalt-like Sr–Nd isotopes and high zircon eHf(t), similar to slab-derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from −7.7 to −6.4‰, whereas the adakitic dykes have relatively high δ11B values from −6.9 to −4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B-depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high-angle dip of the subducting oceanic slab.

Sr-Nd-Pb isotopic compositions of the lower crust beneath northern Tarim: insights from igneous rocks in the Kuluketage area, NW China

The composition of lower crust of the Tarim Craton in NW China is essential to understand the petrogenesis of the ~290–275xa0Ma Tarim basalts and associatedxa0intermediate-felsic rocks. However, it remains poorly constrained because extremely sparse granulite terrains or granulite xenoliths have been found in the Tarim Craton. New trace element and Sr-Nd-Pb isotopic data are reported for the Neoarchean and Neoproterozoic igneous rocks widely distributed in the northern margin of the Tarim Craton. The Neoarchean granitic gneisses show fractionated REE (rare earth element) patterns [(La/Yb)Nxa0=xa012–58, YbNxa0=xa010.6–36] with pronounced negative Nb-Ta and Ti anomalies. These features, together with negative εNdi (−0.7 to −3.2) suggest that they were derived from melting of mafic lower crust. The Neoproterozoic biotite granodiorites are strongly depleted in HREE with (La/Yb) N up to 55. They are characterized by high Sr (671–789xa0ppm) but very low Y (7.10–8.06xa0ppm) and Yb contents (0.47–0.58xa0ppm), showing typical features of adakitic rocks. The samples with different SiO2 contents display identical 87Sr/86Sri (0.7101–0.7103), εNdi (−14.1 to −15.7) and Pb isotopes (208Pb/204Pbixa0=xa036.94–37.07). These features together with arc-like trace element patterns suggest that they were derived from melting of thickened lower crust. In comparison, the Neoproterozoic hornblende-biotite granodiorites have similar trace element compositions except for weaker depletion in HREE and have lower 87Sr/86Sri (0.7078) and initial Pb isotopes, and higher εNdi (−12.3 to −12.7). This suggests that they were formed by melting of old lower continental crust at a shallower depth than the biotite granodiorites. These rocks were derived from the lower crust, thus providing valuable information on the nature of the lower crust beneath northern Tarim. Combined with published data, the 87Sr/86Sri, εNdi, 206Pb/204Pbi and εHfi of the northern Tarim lower crust ranges from 0.7055 to 0.7103, from −12 to −17, from 16.20 to 16.65, and from −7 to −19, respectively, at ~785xa0Ma. These data also suggest vertical compositional heterogeneity of the northern Tarim lower crust.

Ultrahigh-temperature metamorphism in the Helanshan complex of the Khondalite Belt, North China Craton: Petrology and phase equilibria of spinel-bearing pelitic granulites

Handling Editor: Donna Whitney Abstract To better understand the formation of ultrahigh‐temperature (UHT) metamorphic rocks, we present a detailed petrological study of the recently discovered spinel‐ bearing garnet–sillimanite granulites in the Helanshan complex of the Khondalite Belt in the North China Craton. In calculated P–T pseudosections, isopleths of grossular content in the peak assemblage field of garnet+K‐feldspar+sillimanite+spinel+ilmenite+quartz+melt suggest that the metamorphic peak occurred at ∼960–1,030°C and 6.3–7.3 kbar. Using ternary feldspar thermometry, a minimum temperature limit of the peak metamorphic conditions is calculated to be ∼910– 955°C at 6.5 kbar, with a weighted mean of ∼940°C. Thus, all these results point to a very steep geothermal gradient well into the UHT field. In addition, a clockwise P–T evolution is determined, which involves pre‐Tmax decompression followed by nearly isobaric cooling. Based on these newly discovered UHT pelitic granulites, which do not contain index minerals typically considered diagnostic of UHT metamorphism, and the high‐P pelitic granulites exposed in the Helanshan and Qianlishan complexes, we propose that the Khondalite Belt is an ultrahot metamorphic orogen formed by collision between the Yinshan and Ordos Terranes. The style of this continental collision was rather different from Phanerozoic collisions, but was similar to the two‐sided hot collision model during the Proterozoic. Two‐sided hot collision involves shallow slab breakoff during collision, which leads to extension and the development of a wide plateau‐like orogen, which is underlain by melt‐bearing mantle that maintains a hot environment at the collision zone.