Search

The early Paleozoic Wuyi-Yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the Yunkai and Wugong domains located in the western Cathaysia block and in the Wuyi domain located in the central part of the Cathaysia block. In order to investigate the evolution of the Wuyi-Yunkai orogen, elemental and Sr-Nd isotopic analyses were performed for granites from the Baoxu pluton in the Yunkai domain and from the Enping pluton in the central part of the Cathaysia block. The Baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite, granodiorite and diorite, and the Enping pluton is mainly composed of massive granodiorite. Biotite granites (441 ± 5 Ma) and gneissic granite xenolith (443 ± 4 Ma) of the Baoxu pluton are all weakly peraluminous (A/CNK = 1.05–1.10). They show high Sr/Y and La/Yb ratios and have negative bulk-rock ε Nd (t) values (−7.0 to −4.4), which are similar to coeval gneissic S-type granites in the Yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source. Granodiorites (429 ± 3 Ma) from Enping and granodiorite xenolith (442 ± 4 Ma) from Baoxu are metaluminous and have REE patterns with enriched light REE and flat middle to heavy REE, possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level. Diorite xenolith from Baoxu is ultrapotassic (K 2 O = 4.9 wt%), has high contents of MgO (7.0 wt%), Cr (379 ppm) and Ni (171 ppm) and shows pronounced negative Nb, Ta and Ti anomalies. This xenolith also has negative ε Nd (t) value (−3.6) and low Rb/Ba and high Ba/Sr ratios, and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite. Early Paleozoic I- and S-type granites in the Wuyi-Yunkai orogen mostly have negative ε Nd (t) values and do not have juvenile components, consistent with genesis by an intracontinental orogenic event. These early Paleozoic granites occur near the ancient suture zone between the Yangtze and Cathaysia blocks and have high La/Yb and Sr/Y ratios, likely due to the existence of residual garnet in the source, suggesting the thickened crust at ca. 440 Ma. The 450–440 Ma gneissic S-type granites near the suture zone are earlier than those in the central part of the Cathaysia block (∼430 Ma). The crustal thickening along the ancient suture zone at 440 Ma propagated into the central part of the Cathaysia block as evidenced by the 430 Ma granites. Early Paleozoic I-type granites near the suture zone clearly show involvement of significant mantle-derived materials, in contrast to granites in the central part of the Cathaysia block. The ancient suture zone may have acted as channels for the emplacement of mafic magmas during the collapse of an intracontinental orogen. [ABSTRACT FROM AUTHOR]

Intensive Devonian felsic magmatism is recorded within the southwestern Mongolian collage system of the Central Asian Orogenic Belt (CAOB). The voluminous magmas have isotopic compositions of juvenile materials from the mantle, thus manifesting significant mantle–crust interaction and continental growth at this time. Here, we present systematic Sr–Nd–Hf–Pb isotopic data for the Devonian mafic intrusions in the Chinese Altai, a key region within the southwestern Mongolian collage system to decipher the evolution of the mantle during this important tectonothermal event. The Keketuohai gabbro (409 ± 5 Ma) and type I mafic dykes (376 ± 5 Ma) within the Habahe complex have high ( 87 Sr/ 86 Sr) i , ( 206 Pb/ 204 Pb) t , ( 207 Pb/ 204 Pb) t and ( 208 Pb/ 204 Pb) t ratios, and decoupled Nd–Hf isotopic compositions; e.g., low ε Nd (t) values (− 2.5 to + 5.4) combined with high ε Hf (t) (+ 2.6 to + 15.1) values. These rocks have low Ba/La and high La/Yb and Th/Yb ratios, and are enriched in Pb, the light rare earth elements (LREE) and Th. They formed from magmas generated from the depleted lithospheric mantle metasomatised by hydrous melts from subducted sediments. In comparison, the gabbroic samples from the Habahe complex (369 ± 3 Ma) are enriched in the LREE, Th and Ba and have high La/Yb, Th/Yb and Ba/La ratios. They do not show significant Pb anomalies, and have depleted isotopic compositions that include low initial 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios and high ε Nd (t) (+ 7.4 to + 7.8) and ε Hf (t) (+ 13.4 to + 15.3) values. These rocks are thought to have formed from magmas derived from the lithospheric mantle metasomatised by hydrous melts from subducted oceanic crust. The type II mafic dykes within the Habahe complex are depleted in the LREE and Th, have high Ba/La ratios, and are enriched in Pb, Ba, Sr, and U. They have positive ε Nd (t) (+ 7.6 to + 8.1) and ε Hf (t) (+ 14.1 to + 15.4) values, high initial 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios, and are thought to have formed from a magmas generated by the partial melting of the depleted lithospheric mantle metasomatised by fluids derived from subducted sediments. The melting and dehydration of subducted oceanic crust and sediments indicate the Devonian subduction of a hot slab beneath the Chinese Altai, which could be related to a ridge subduction model previously proposed. The involvement of hydrous melts and fluids promoted the melting of the lithospheric mantle which was responsible for intensive felsic magmatism with abrupt increase of juvenile component and high temperature metamorphism in the region. [ABSTRACT FROM AUTHOR]

Granitic rocks are crucial in revealing continental crust formation and evolution. The classic S- and I-type granitoid classification (based upon metasedimentary vs. metaigneous protolith) is widely adopted, but is not always straightforward when diagnostic minerals (e.g., cordierite for S-type and amphibole for I-type) are absent. In this study, zircon oxygen isotope and water content were measured simultaneously with Secondary Ion Mass Spectrometry (SIMS) on I- and S-type granitoids from various tectonic settings, including the New England Orogens (NEO), the Diancangshan–Ailaoshan and the Himalayan orogens, and the South China Block. Our results indicate the S-type samples are characterized by lower water-in-zircon content (median 14–170 ppm) than the I-type samples (median 449–885 ppm). Also, inherited I-type zircon cores in S-type samples from the NEO have higher water-in-zircon content than the zircon rims, confirming the different water content between the two granitoid types. We propose that water-in-zircon content is a potential tool to distinguish I- and S-type granitoids. Nonetheless, the low water-in-zircon content of S-type granitoids has likely no correlation with high magma water content. [ABSTRACT FROM AUTHOR]

The Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well‐enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Paleozoic deposits. From the Cambrian to the middle Ordovician, this region was characterized by geographically extensive, flat‐bedded siliceous mudstone, indicating the existence of a large ocean basin. The oceanic plate entered the convergence phase in terms of the Wilson Circle during the Middle Ordovician, when numerous magmatic arcs formed along two opposite sides of the ocean. The magmatic arcs became the widest during the Silurian, suggesting that the Hongliuhe‐Niujuanzi‐Xichangjing Ocean (HNX; a southern branch of the Paleo Asian Ocean) was reduced to a small residual ocean in the central Beishan region by that time, and probably lasted till the Carboniferous or later by newly published data. [ABSTRACT FROM AUTHOR]

This paper investigates the petrogenesis of the Xinxian granitic batholith and its tectonic implications for the Dabie orogen. Two Xinxian granites yield Early Cretaceous U/Pb ages of 122.7 ± 1.3 Ma and 123.6 ± 2.2 Ma. The granites are metaluminous differentiated I-type granites, with A/NKC ranging from 0.90 to 0.96. The chondrite-normalised REE patterns display significant LREE/HREE enrichments with moderate negative Eu anomalies (Eu/Eu = 0.30 ~ 0.85). The granites show enrichments in large-ion lithophile elements (LILEs, e.g. Rb, K, Th and U) and depletions in Sr, Ba and high field strength elements (HFSEs, e.g. Nb, Ta and Ti). Inherited zircons of different age (Neoproterozoic, one Early Proterozoic, one Archean and two Triassic inherited metamorphic zircons) are identified. The Early Cretaceous magmatic zircons contain enriched Hf isotopes with ε ( t) values ranging from −26.9 to −18.7 (average = −22.5), more enriched than their probable alleged source, i.e. the Dabie Complex, represented by the Neoproterozoic and Triassic inherited zircons. Thus, the Archean and Early Proterozoic crustal materials may represent an enriched end member source for the parental magma of the Xinxian granites. Provenance analysis of the magmatic zircons in the North China and Yangtze blocks demonstrates that the Early Proterozoic inherited zircon in Xinxian has North China affinity. Thus, we proposed a multiple-sourced petrogenetic model for the Xinxian granitic batholith, which suggests that the batholith was formed by the remelting of a mixed crustal assemblage, including the Archean and Early Proterozoic crust of the North China Block, the Neoproterozoic crust of the Yangtze Block, as well as some Triassic collision-related ultra-high pressure (UHP) metamorphic rocks. [ABSTRACT FROM AUTHOR]

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D 1 is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D 2 is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D 2 foliations are folded by ∼ NW–SE folds (F 3 ) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D 3 ). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D 1 –D 3 deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D 3 fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D 3 transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D 1 –D 3 at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D 1 ), collapse (D 2 ), and transpressional deformation (D 3 ) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D 3 ), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic. [ABSTRACT FROM AUTHOR]

Abstract: We undertook LA-ICP-MS U–Pb dating of detrital zircons from Late Paleozoic sedimentary rocks in eastern Heilongjiang Province, NE China, with the aim of constraining the tectonic evolution of the eastern segment of the Central Asian Orogenic Belt (CAOB). Detrital zircons from quartz sandstone of the Devonian Heilonggong Formation in the Zhangguangcai Range yield age populations of ∼2503, ∼1833, ∼903, ∼802, and 551–403Ma. Zircons from quartz sandstone of the Devonian Baoquan Formation in the Lesser Xing''an Range yield age clusters at ∼2500, ∼800, and 489–425Ma. These ages reveal that: (1) rocks from the Heilonggong and Baoquan formations were deposited after 391 and 413Ma, respectively; (2) these rocks were sourced from regions with rocks as old as ∼2500Ma, with dominant age populations of ∼2500, ∼1833, 903–800, and 551–403Ma; and (3) in addition to new lower crust accreted during the Phanerozoic (as revealed by voluminous Phanerozoic granitoids with positive εNd values within the CAOB), it appears that several remnants of an ancient Precambrian basement occur adjacent to these Late Paleozoic basins (e.g., the Shangzhi, Tangyuan, and Baoqing basins) that occur within microcontinental massifs including the Songnen–Zhangguangcai Range, Xing''an, and Erguna massifs. Detrital zircons from quartz sandstone of the Devonian Heitai Formation on the eastern margin of the Jiamusi Massif yield peak age populations of 569, 542, 509, and 484Ma, similar to those obtained from zircons within the Mashan Group of the Jiamusi Massif. These age data indicate that: (1) deposition of the Heitai Formation on the eastern margin of the Jiamusi Massif had started by 484Ma, and (2) these sediments were sourced entirely from the Jiamusi Massif. The widespread occurrence of detrital zircons with ages of 480–550Ma within Late Paleozoic basins in the Lesser Xing''an–Zhangguangcai Ranges and the Jiamusi Massif, combined with the presence of a Caledonian (445–499Ma) I-type granitoid belt in the Lesser Xing''an–Zhangguangcai Ranges, indicates that amalgamation of the Songnen–Zhangguangcai Range and Jiamusi massifs took place before the Early Devonian, probably during the late stage of the Early Paleozoic (after ∼445Ma). [Copyright &y& Elsevier]

• Devonian felsic igneous rocks in the Chinese Altai show negative zircon ε Hf (t) values. • The Ordovician Altai accretionary wedge contains ancient crustal components. • Hf-in-zircon spatial-temporal mapping shows isotopic heterogeneity in the Altai wedge. A debate exists on whether the deep crust of the Altai Orogenic Belt contains ancient crustal components. To further address this issue, geochemical analyses and zircon hafnium isotopic measurements were conducted on five Devonian magmatic rocks from the Southern Chinese Altai. Three volcanic rocks have rhyolitic compositions and two other intrusions have granite and granodiorite compositions, respectively. These samples all have calc-alkaline, peraluminous compositions and "arc-related" trace element signatures, marked by enrichment in large-ion lithophile elements relative to many of the high field strength elements. Hf-in-zircon isotopic data for these rocks show variable ε Hf (t) values ranging from −25 to +5, with corresponding T DM c model ages varying from ca. 2.7 to ca. 0.9 Ga. These findings represent the first report describing negative zircon ε Hf (t) values for the Devonian magmatic rocks of the region, documenting magmatic reworking of ancient crustal components. The very similar trace elemental patterns and indistinguishable zircon Hf isotopic characteristics, between the studied magmatic rocks and the pre-existing Ordovician accretionary wedge sediments (the Habahe Group), supports the notion that the magmatic rocks are derived from magmatic reworking of the wedge that contains ancient crustal components. The studied magmatism is synchronous with Middle Devonian anatexis of the Ordovician wedge at low-pressure/high-temperature conditions and extrusion of bimodal volcanic rocks occurred in Devonian extensional basins. These events have been collectively taken as proxies of Devonian lithospheric extension in the Chinese Altai. Integrated mapping of Hf isotopic data for the bulk S-type Devonian magmatism of the Chinese Altai shows different domains with dissimilar Hf isotopic signatures. This further indicates that the magma source, i.e., the Ordovician wedge, is compositionally heterogeneous and contains ancient crustal components. [ABSTRACT FROM AUTHOR]