Relationship of the Tarim Craton to the Central Asian Orogenic Belt: insights from Devonian intrusions in the northern margin of Tarim Craton, China

<p>The boundary and relation of the Tarim Craton to the Central Asian Orogenic Belt (CAOB) and its role in the formation history of the CAOB remain controversial. This article presents ages and Hf-in-zircon isotopic and geochemical results for gabbroic, dioritic, and granitic plutons from the northern margin of Tarim Craton (NMTC), and discusses their petrogenesis and tectonic regimes as well as the boundary between the CAOB and the Tarim Craton. These plutons yield zircon ages of 424–385 Ma. In the Quruqtagh zone south of the Xinger Fault, the gabbroic pluton shows enrichment in LREEs and LILEs, depletion in HFSEs and positive <i>ε</i>_Hf(<i>t</i>) values (+4.0 to +11.4), suggesting that parental magmas of gabbros were likely derived by partial melting of a depleted mantle wedge previously metasomatized by slab-derived aqueous fluids. In the Hulashan Zone north of the Xinger Fault, the studied rocks include one dioritic pluton and three granitic plutons. The geochemical characteristics and petrogenesis of the dioritic pluton are similar to those of the studied gabbroic with positive <i>ε</i>_Hf(<i>t</i>) values (+3.0 to +9.4). The three granitic plutons display relative depletion in HFSEs and enrichment in LILEs. Their variable <i>ε</i>_Hf(<i>t</i>) values range from −2.1 to +8.9, with <i>T</i>_DM2 ages of 858–1503 Ma, suggesting complex crustal sources with different proportions of juvenile and ancient materials. This article confirms and evidences an Andean-style active continental margin of the Tarim Craton due to southward subduction of the South Tianshan Ocean. Furthermore, our Hf isotopic data, together with regional data from the literature, show that the Hulashan zone to the north to the Xinger Fault has younger continental materials in deep than these of NMTC south of the fault, and is similar to microcontinental fragments in the CAOB. This suggests that the Xinger fault may be the boundary between the Tarim Craton and Tianshan orogen.</p

Carboniferous porphyry Cu–Au deposits in the Almalyk orefield, Uzbekistan: the Sarycheku and Kalmakyr examples

<p>The Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low ε_Nd(t) values of −1.0 to −0.1 for Kalmakyr and −2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesis.</p

Tracking deep ancient crustal components by xenocrystic/inherited zircons of Palaeozoic felsic igneous rocks from the Altai–East Junggar terrane and adjacent regions, western Central Asian Orogenic Belt and its tectonic significance

<p>The deep crustal continental components and architecture of the western Central Asian Orogenic Belt (CAOB) have long been a matter of debate. This article presents an integrated study of published geochronological and Hf-in-zircon isotopic data for inherited zircons from the Palaeozoic granitoid rocks and associated felsic volcanic rocks of the Chinese Altai, East Junggar, and nearby regions. The aim is to trace the age spatial distribution of deep old crustal components. Our data set comprises 463 published age data obtained by SHRIMP and LA-ICP-MS from felsic igneous rocks in these areas. Among these samples, zircon xenocrysts were observed in 69 granitic rocks and 15 felsic volcanic rocks from the Chinese Altai and 30 granitoid rocks and five felsic volcanic rocks in the East Junggar, respectively.</p> <p>Three major zircon xenocrysts provinces are defined based on the distribution of these inherited zircon ages, combined with Hf-in-zircon isotopes. Province I, mainly situated in the eastern part of the central Chinese Altai, is characterized by the abundant inherited zircons with Meso-Proterozoic and Palaeo-Proterozoic ages (1000–1600 and 1600–2500 Ma, respectively), and variable <i>ε</i>_Hf(<i>_t</i>) values ranging from −15 to +7 with ancient Hf crustal model ages (T_DMC) ranging from 1.5 to 2.9 Ga. A few scattered parts of province I are scattered situated in the East Junggar (individual areas, e.g. Taheir and Shuangchagou). Province II, situated mostly in the central Chinese Altai, is characterized by abundant xenocrystic zircons with Neo-Proterozoic ages (542–1000 Ma), <i>ε</i>_Hf(<i>_t</i>) values ranging from −6.8 to +8.1, and corresponding Hf crustal model ages of ~1.0–1.3 Ga. Province III contains abundant Phanerozoic (<541 Ma) xenocrystic zircons that show highly positive <i>ε</i>_Hf(<i>t</i>) values ranging from +5 to +16 and the youngest Hf crustal model ages (0.4–0.95 Ga). The main part of Province III occupies most areas of the East Junggar and the southernmost and northern parts of the Chinese Altai. Identification of the ancient (pre-Neoproterozoic) Hf crustal model ages in the eastern part of the central Chinese Altai (Province I) supports the suggestions that ancient concealed crustal components exist in the Chinese Altai. In contrast, Province III in the East Junggar predominantly displays young model ages, which indicates that it is mainly composed of juvenile components and likely a typical accretionary belt. Besides, a few small areas with ancient model ages are recognized in the East Junggar, providing evidence for the local existence of Precambrian crust or micro-blocks within the accretionary belt. The zircon xenocrysts provinces are consisted with the Nd isotopic province and provide further evidence for the ancient and juvenile compositions in deep. In addition, the tectonic division of the region is discussed based on the distribution of deep crustal components. The Erqis fault zone can be regarded as the boundary between the Chinese Altai and East Junggar regions and its western extension is constrained to be closer to the Altai–Qinghe Fault than previously considered. The central Chinese Altai can be subdivided into two distinct tectonic units.</p

Tracing Deep Carbon Cycling by Zinc Isotopes in a Peralkaline‐Carbonatite Suite

Sedimentary carbonates are known to be carried into the deep mantle by subducted slabs, and studies on mantle‐derived magmas have attempted to trace the recycled carbonate in their mantle source. However, the final depth of storage of recycled carbonate and the role of recycled carbonate in the partial melting of mantle remain controversial. Peralkaline‐carbonatite suites are considered to have been derived from a carbonated mantle source and are windows to evaluate carbon in the mantle. In this study, we report the Zn isotopic compositions of a peralkaline‐carbonatite suite from the Tarim Large Igneous Province (Tarim LIP). The peralkaline‐carbonatite suite has heavier δ66Zn than normal mantle with δ66Zn of 0.34–0.40 ‰ for nephelinite, 0.35–0.47 ‰ for aillikite, 0.51–0.55 ‰ for nepheline syenite, 0.58–0.67 ‰ for calciocarbonatite and 0.38–0.56 ‰ for magnesiocarbonatite. The heavy Zn isotopic compositions of the peralkaline‐carbonatite suite in the Wajilitag complex suggest the incorporation of recycled carbonate‐bearing materials into the deep mantle. We infer that the calciocarbonatite was produced by the initial partial melting of subducted MgSiO3/MgO + C‐bearing carbonated eclogite, whereas the magnesiocarbonatite, aillikite, and nephelinite are considered as reacted melts between carbonated eclogite‐derived melts and peridotite. The heavy Zn isotopic compositions of the nepheline syenite are attributed to fractional crystallization from nephelinite magma in the magma reservoir. Our study highlights the incorporation of carbonated eclogite as an important agent of recycled carbon in the deep mantle and interactions between carbonated eclogite‐derived melts and peridotite lead to the complex lithological heterogeneities in the peralkaline‐carbonatite suite in Tarim LIP.</p

A snapshot of the transition from monogenetic volcanoes to composite volcanoes: case study on the Wulanhada Volcanic Field (northern China)

The transition processes from monogenetic volcanoes to composite volcanoes are poorly understood. The Late Pleistocene to Holocene intraplate monogenetic Wulanhada Volcanic Field (WVF) in northern China provides a snapshot of such a transition. Here we present petrographic observations, mineral chemistry, bulk rock major and trace element data, thermobarometry, and a partial melting model for the WVF to evaluate the lithology and partial melting degree of the mantle source, the crystallization conditions, and pre-eruptive magmatic processes occurring within the magma plumbing system. The far-field effect of India–Eurasia collision resulted in a relatively high degree (10 %–20 %) of partial melting of a carbonate-bearing eclogite (∼ 3 wt % carbonate; Gt/Cpx ≈ 2 : 8, where Gt denotes garnet and Cpx denotes clinopyroxene) followed by interaction with ambient peridotite. The primary melts ascended to the depth of the Moho (∼ 33–36 km depth), crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C with the melt water contents of 1.1 wt %–2.3 wt %. Part of the primary melt interacted with the lithospheric mantle during ascent, resulting in an increase in the MgO contents and a decrease in the alkaline contents. The modified magma was subsequently directly emplaced into the middle crust (∼ 23–26 km depth) and crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C. The primary melts from the same mantle sources migrated upward to the two-level magma reservoirs to form minerals with complex textures (including reverse and oscillatory zoning and sieve texture). Magma erupted along the NE–SW-striking basement fault and the NW–SE-striking Wulanhada–Gaowusu fault in response to the combined effects of regional tectonic stress and magma replenishment. The crustal magma reservoir in the WVF may represent a snapshot of the transition from monogenetic volcanoes to composite volcanoes. It is possible to form a composite volcano with large magma volumes and complex compositions if the magma is continuously supplied from the source and experiences assimilation and fractional crystallization processes in the magma plumbing system at crustal depth.</p