In the eastern South China Block (SCB), the presence of middle Paleozoic volcanics and mafic rocks is poor in spite of the extensive occurrence of granites in response to the synchronous (Kwangsian) ...intracontinental orogen event. The nature of the lithosphere and its associated orogenic process are poorly known for such a major orogen. In this paper, a set of new U–Pb zircon geochronological, elemental and Sr–Nd isotopic data are presented for several gabbroic plutons, which were recently identified in the Cathaysia Block. The representative samples of the Longhugang, Xinchuan and Xinsi plutons yielded the weighted mean 206Pb/238U ages of 423±8Ma, 434±6Ma and 420±3Ma, respectively, and their gneissoid country-rocks are dated at 442–466Ma. This evidences the presence of Silurian mafic rocks in the Cathaysia Block. These gabbroic rocks can be geochemically classified into three groups. Group 1 from the western Longhugang pluton exhibits flat REE patterns represented by (La/Yb)cn=1.1–1.7, (Gd/Yb)cn=0.99–1.15, and δEu=1.29–1.51 with negative Nb–Ta and P and markedly positive Sr anomalies. It shows enrichment in LILEs and 87Sr/86Sr(t) ratios of 0.70412–0.70727 and εNd(t) values of −2.6 to −0.6. Group 2 from the Xinchuan, Zhouya and Yunlu plutons is characterized by high MgO (11.4–15.8wt.%), mg-number (66–74) and Cr and Ni contents. It is marked by significantly negative Nb–Ta, Zr–Hf and P–Ti anomalies with high 87Sr/86Sr(t) (0.70920–0.71072) and low εNd(t) values (−3.9 to −7.9). Group 3 from the Xinsi and eastern Longhugang plutons has εNd(t) values from −2.8 to −5.7 and shows high Nb content of 7.05–9.89ppm with pronounced enrichment in LILE and weak depletion in HFSE, resembling Nb-enriched arc basalt in HFSE composition. A synthesis of these geochemical data points to the following petrogenesis: Groups 1 and 2 derived from a spinel-bearing, plagioclase-rich source and a garnet-bearing, orthopyroxene-rich source, respectively, proportionally modified by slab- and sediment-derived melts plus fluid fluxing from the earliest Neoproterozoic subduction. The HFSE composition for Group 3 inherited from a metasomatized mantle wedge column with the input of large-proportional slab melts and the absence of residual rutile during partial melting. In conjunction with other available observations, it is proposed that there preserved a paleosubduction-modified wedge column beneath the Cathaysia Block, which might be undisturbed until the Kwangsian intracontinental orogen. During the Silurian post-orogenic collapse of the event, the elevated lithosphere thermal boundary facilitated the partial melting of the column to generate these Silurian gabbroic rocks.
► Three gabbro plutons from Yunkai–Nanling are dated at 420–434Ma, confirming the presence of Silurian mafic rocks in SCB. ► These mafic rocks originated from the wedge column previously modified by subduction-derived components. ► The post-collisional collapse of the Kwangsian intracontinental orogen is proposed as the petrogenetic model of the gabbro. ► The earliest Neoproterozoic subduction-related metasomatism might occur along the southern Cathaysia Block.
► Two metabasites from the northern Yunkai gave the weighted mean ages of 978–997Ma. ► Group 1 is similar geochemically to Saunders island-arc and Group 2 to Nb-enriched basalt. ► Both groups ...originated from MORB source newly modified by subduction-derived fluid/melt. ► A Grenvillian arc–back-arc basin was developed along the southern Cathaysia Block.
The tectonic nature of the Cathaysia Block during earliest Neoproterozoic period has been long disputed and attracted considerable attentions. One reason for this problem is rare exposure and few data for the igneous rocks of Grenvillian age in this Block. In this paper, a set of new U–Pb zircon geochronological, elemental and Nd isotopic data are presented for the metabasites in the Yunkai Group which shows WNW-trending extension with a length of more than 100km along the northern Yunkai Domain of the Cathaysia Block. The plagioclase amphibolite and metabasite from Shiwo (Beiliu) and Tantu (Xinyi) from the northern Yunkai Domain, representative of Groups 1 and 2, yielded the 206Pb/238U weighted mean ages of 997±21Ma and 978±19Ma, respectively, evidencing the presence of the Grenvillian mafic rocks within the Cathaysia Block. Based on the Nb, Nb/La and Nb/U, these samples can be geochemically subdivided into two groups (Groups 1 and 2). Group 1 has Nb/La=0.33–0.54 and Nb=1.71–2.85ppm, and exhibits similar geochemical signatures to those of Saunders island-arc. Group 2 gives Nb/La of 0.57–0.86 and Nb of 7.58–12.70ppm, resembling to those of Phanerozoic Nb-enriched basalts, and shows the geochemical affinity to the Okinawa back-arc basin basalt. Both groups have positive ɛNd(t) values, ranging from +3.84 to +5.36 and +3.93 to +6.55, respectively. These geochemical and isotopic characteristics suggest that Group 1 might have been originated from a MORB-like source modified by subduction-derived fluids. The source of Group 2 Nb-enriched basalt might be metasomatized by melt released from a newly subducted slab. In conjunction with other geological observations, it is identified for the Grenville-aged (ca. 1.0Ga) arc and back-arc basin along the northern Yunkai Domain of the Cathaysia Block, analog to the modern West Pacific. The South China Block probably located at the margin of Rodinia supercontinent between west Australia and east India.
•The metabasic rocks in Cathaysia yield U–Pb ages of 969–984Ma and TDM(Hf) of 0.92–1.44Ga.•They originated from a subduction-modified MORB-like source linked to an arc–back-arc setting.•The South ...China Block was created by episodic amalgamation of a series of arc fragments between ∼970 and 820Ma.•The South China Block is an exterior accretionary orogen along the periphery of Rodinia rather than in the interior.
U–Pb geochronology along with elemental and Nd–Hf–Os isotopic data from the earliest Neoproterozoic metabasic rocks within the Cathaysia Block of the South China Block (SCB) constrain the tectonic setting and paleogeography of the block within the Rodinia supercontinent. The metabasic rocks give zircon U–Pb ages of 969–984Ma, ɛHf(t) values of +1.8 to +15.3 and Hf model ages of 0.92–1.44Ga. They are subalkaline basalts that can be geochemically classified into four groups. Group 1 has low Nb contents (1.24–4.33ppm), highly positive ɛNd(t) values (+4.3 to +5.2), and REE and multi-elemental patterns similar to fore-arc MORB-type basalt. Group 2 has Nb contents ranging from 3.13ppm to 6.48ppm, ɛNd(t) of +3.1 to +6.2, low Re and Os contents and high initial Os isotopic ratios, and displays an E-MORB geochemical signature. Group 3 has Nb=7.18–29.87ppm, Nb/La=0.60–1.40, Nb/U=5.0–37, Ce/Pb=1.1–6.6, ɛNd(t)=+2.9 to +7.0, 187Re/188Os=5.87–8.87 and γOs (t)=178–772, geochemically resembling to the Pickle Nb-enriched basalt. Group 4 has strong LREE/HREE and HREE fractionation and high ɛNd(t) values (+2.3 to +5.6), and is characterized by similar element patterns to arc volcanic rocks. Serpentinites coeval to Group 4 show 187Os/188Os of 0.1143–0.1442 and γOs (t) of −7.8 to +0.1. Groups 1 and 2 are interpreted to originate from the N-MORB and E-MORB-like sources with the addition of an arc-like component, genetically linked to fore- and back-arc settings, respectively. Groups 3 and 4 show inputs of newly subduction-derived melt and fluid in the wedge source. These geochronological and geochemical signatures fingerprint the development of an earliest Neoproterozoic (∼970Ma) arc–back-arc system along the Wuyi-Yunkai domain of the Cathaysia Block. Regional relationships indicate that the Wuyi-Yunkai arc–back-arc system was one of a series of separate convergent margin settings, which included the Shuangxiwu (∼970–880Ma) and Jiangnan (∼870–820Ma) systems that developed in the SCB. The formation and closure of these arc–back-arc systems resulted in the northwestwardly episodic amalgamation of various pieces of the Yangtze and Cathaysia to finally form the SCB. These signatures require the SCB to occupy an exterior accretionary orogen along the periphery of Rodinia during 990–820Ma, rather than to have formed through Mesoproterozoic Sibao orogenesis within the interior of Rodinia.
This paper presents a set of new SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd–Pb isotopic data for the Wusu and Yaxuanqiao basaltic rocks (the Mojiang area) along the Ailaoshan tectonic ...zone. The Wusu basaltic sequence is dominated by SiO
2-poor, MgO- and TiO
2-rich basalts with a major mineral assemblage of plagioclase
+
clinopyroxene. These rocks gave a SHRIMP zircon U–Pb age of 287
±
5
Ma (MSWD
=
0.58). In contrast, the Yaxuanqiao basaltic sequence is predominantly composed of high-Al basaltic andesite, which gave a SHRIMP zircon U–Pb age of 265
±
7
Ma (MSWD
=
0.34). The analyzed samples for both sequences exhibit significant enrichment in LILEs and depletion in HFSEs with (Nb/La)n of 0.38–0.81, similar to arc-like volcanics. They have positive εNd(t) values (+
3.52 to +
5.54). In comparison with MORB-derived magmatic rocks, the Wusu basalts are more enriched in LILEs and REEs, and the Yaxuanqiao samples are more enriched in LILEs but variably depleted in Ti, Y and HREE. The Wusu samples show high Pb isotopic ratios, similar to the Tethyan basalts, whereas the Yaxuanqiao samples plot in the field of the global pelagic sediments. The geochemical and Sr–Nd–Pb isotopic characteristics suggest that the Wusu basalts originated from a MORB-like source metasomatised by slab-derived fluids, while the Yaxuanqiao rocks have a fluid-modified MORB source with the input of subducted sediments. The geochemical affinity to both MORB- and arc-like sources, together with other geological observations, appears to support the development of a Permian arc–back-arc basin along the Ailaoshan–Song Ma tectonic zone in response to the northward subduction of the Paleotethys main Ocean. The final closure of the arc–back-arc basin took place in the uppermost Triassic due to the diachronous amalgamation between the Yangtze and Simao–Indochina Blocks.
► The Wusu and Yaxuanqiao basaltic sequences gave the zircon U-Pb ages of 287 and 265 Ma, respectively. ► Both sequences exhibit the geochemical affinity to both MORB- and arc-like sources. ► They originated from a fluid-modified or with the input of subducted sediments. ► A Permian arc-back-arc basin along the Ailaoshan-Song Ma tectonic zone.
Gneissoid granites were traditionally thought to be the components of the Precambrian basement in the eastern South China Block, but twenty-four gneissoid granite samples from the Wugong, northern ...Wuyi, southern Wuyi and Yunkai Domains gave zircon U–Pb ages of 424–456Ma, 410–457Ma, 426–438Ma and 415–450Ma, respectively. The existence of the abundant Kwangsian gneissoid granites, which have incorrectly been assigned to the Precambrian basement, indicates that the “so-called” Precambrian stratigraphic sequence of the Cathaysia Block should be termed “Complex” rather than “Group”. These gneissoid granites are peraluminous granites with relatively high Al2O3, MgO, TiO2, FeOt and CaO/Na2O but low CaO, Al2O3/(MgO+FeOt) and Rb/Sr ratios. They exhibit strongly negative Ba, Sr, Nb, P and Ti and positive Pb anomalies in primitive mantle-normalized diagrams, and have initial 87Sr/86Sr ratios ranging from 0.70924 to 0.72935 and negative εNd(t) values from −6.4 to −11.4, similar to those of paragneissic enclaves and Precambrian sedimentary rocks in the eastern South China Block. The zircons crystallized in the Kwangsian gneissoid granites gave εHf(t) values ranging from +2.4 to −19.4 with the peak at −4 (almost all grains clustering at −1 to −12) and Hf model ages of 1.1–2.1Ga. The synthesis of these geochemical and in-situ zircon Hf isotopic data indicates that the Kwangsian gneissoid granites dominantly originated from Proterozoic metapelitic and meta-igneous components with insignificant input of juvenile mantle-derived magmas. A model of two-stage crustal anatexis is proposed for the Kwangsian granitic magma in the eastern South China Block, with (1) the formation of 460–430Ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust, and (2) the generation of the 430–400Ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust. The crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far-field response to the assembly of the Australian–Indian plate with the Cathaysia Block during middle Paleozoic (Kwangsian) time.
► The gneissoid granites from the Wugong, Wuyi and Yunkai Domains are crystallized at the Kwangsian (410–457 Ma). ► The gneissoid granites mainly originated from the Proterozoic metapelitic and metaigneous source. ► The source have poor input of the juvenile mantle component. ► The crustal anatexis occurred in an intracontinental regime during middle Paleozoic in the eastern South China Block.
Zircon U‐Pb geochronological data on over 900 zircon grains for Cambrian to Silurian sandstone samples from the South China Block constrain the pre‐Devonian tectonic setting of, and the ...interrelationships between, the constituent Cathaysia and Yangtze blocks. Zircons range in age from 3335 to 465 Ma. Analyses from the Cathaysia sandstone samples yield major age clusters at ∼2560, ∼1850, ∼1000, and 890–760 Ma. Zircons from the eastern and central Yangtze sandstone samples show a similar age distribution with clusters at ∼2550, ∼1860, ∼1100, and ∼860–780 Ma. A minor peak at around 1450 Ma is also observed in the Cathaysia and central Yangtze age spectra, and a peak at ∼490 Ma represents magmatic zircons from Middle Ordovician sandstone in the eastern Yangtze and Cathaysia blocks. The Cambrian and Ordovician strata show a transition from a carbonate‐dominated succession in the central Yangtze Block, to an interstratified carbonate‐siliciclastic succession in the eastern Yangtze Block, to a neritic siliciclastic succession in the Cathaysia Block. Paleocurrent data across this succession consistently indicate directions toward the W‐NNW, from the Cathaysia Block to the Yangtze Block. Our data, together with other geological constraints, suggest that the Cathaysia Block constitutes a fragment on the northern margin of east Gondwana and both Cathaysia and east Gondwana constituted the source for the analyzed early Paleozoic samples. The similar age spectra for the Cambrian to Silurian sandstone samples from the Yangtze and Cathaysia blocks argue against the independent development and spatial separation of these blocks in the early Paleozoic but rather suggest that the sandstone units accumulated in an intracontinental basin that spanned both blocks. Subsequent basin inversion and Kwangsian orogenesis possibly at 400–430 Ma also occurred in an intracontinental setting probably in response to the interaction of the South China Block with the Australian‐Indian margin of east Gondwana.
To constrain temporal–spatial patterns of the Kwangsian and Indosinian orogenic events and the Phanerozoic tectonic evolution of the eastern South China Block, this paper documents the zircon U–Pb ...geochronology of the high-grade metamorphic rocks for key outcrops in the eastern SCB and the P–T path for the Indosinian metamorphic amphibolites. Our data show that the representative amphibolites from the originally-defined Badou, Mayuan, Chencai, Zhoutan and Yunkai Groups and associated granulites yielded zircon U–Pb metamorphic ages of 428–468Ma. The weighted mean ages of 221–246Ma are also given by the metamorphic zircons from amphibolites and granulites in the Yunkai, Shiwandashan, northern Wuyi and southern Hunan regions. The two age groups are followed by the 40Ar/39Ar plateau ages of 406–438Ma and 195–217Ma for synkinematic minerals from major shear zones in the eastern South China Block, respectively. The age-spans of 406–468Ma and 195–246Ma roughly coincide with the formation time of the Kwangsian (396–462Ma) and Indosinian (202–248Ma) granites of the eastern South China Block, respectively. The strongly compressive deformation, metamorphism and granitic magmatism are roughly contemporaneous for the Kwangsian and Indosinian tectonothermal events in the eastern South China Block. They occurred over a similar spatial extent, only reaching westward to the Jiangnan–Xuefeng Domains across the Jiangshan–Shaoxing Fault. The amphibolites from Zhouyuan (SW Zhejiang Province) of the Cathaysia Block, which have the Indosinian zircon U–Pb metamorphic ages of 243–246Ma, underwent three episodes of metamorphism (M1–M3) and yielded a near-isothermal decompression clockwise P–T path. In combination with other geological data, it is proposed that the ages of ca. 430Ma and ca.230Ma might be interpreted as the transformational time from the prograde- to retro-metamorphism for the Kwangsian and Indosinian events, respectively. The Kwangsian and Indosinian orogenic events occurred in a similar tectonic setting where the Cathaysia and Yangtze Blocks remained contiguous, and both involved initial crustal thickening followed by rapid exhumation and final cooling, respectively. Their occurrence can be attributed to intra-plate deformation and orogenesis without the Phanerozoic subduction of the Cathaysia Block underneath the Yangtze Block, in response to the far-field stress fields for the subduction/collision beyond the southern margin of the South China Block. Our geochronological data for the amphibolites and granulites also indicate the development of abundant Neoproterozoic (ca. 800Ma) mafic magmatism in the Cathaysia interior.
► The amphibolite and granulite yielded zircon U–Pb metamorphic ages of 428–468Ma and 221–246Ma. ► The Kwangsian and Indosinian events of the SCB are limited at 396–462 and 202–248Ma, respectively. ► The deformation, metamorphism and magmatism of two events occurred over a similar spatial extent. ► The Cathaysia and Yangtze Blocks remain contiguous during the Phanerozoic period.
To achieve a better understanding of the Kwangsian orogenic event of the eastern South China Block, this paper documents a set of new zircon U–Pb geochronological and Hf isotopic data and whole-rock ...elemental and Sr–Nd isotopic analytical results for the representative massive granite intrusions across the Jiangshan-Shaoxing fault. The studied samples are classified into two groups, representing the rocks from the Cathaysia Block to the east of the Jiangshan-Shaoxing Fault (Group 1) and those from the eastern Yangtze Block between the Anhua-Luocheng and Jiangshan-Shaoxing faults (Group 2). The Group 1 samples gave the zircon U–Pb ages of 405–454Ma and εHf(t) values of −3.6 to −15.2 with Hf model ages of 1.6–2.4Ga. Group 2 yielded the zircon U–Pb ages of 400–432Ma and εHf(t) values of −0.2 to −12.7 with Hf model ages of 1.3–2.2Ga. Geochemically, the Group 1 samples (A/CNK=1.02–1.43) have relatively lower Al2O3, MgO, CaO, P2O5 and εNd(t) but higher K2O+Na2O than those of Group 2 (A/CNK=0.93–1.44). Both groups show similar chondrite-normalized patterns of rare-earth elements with Eu/Eu* values of 0.15–0.92 and strongly negative Ba, Sr, Nb, P and Ti anomalies in primitive mantle-normalized spider diagrams. Their εNd(t) values range from −11.1 to −8.0 for Group 1, and −8.9 to −5.0 for Group 2, generally similar to those of Precambrian paragneiss and contemporaneous gneissoid granites in the eastern South China Block. Our geochronological results indicate that the Kwangsian massive granites in the eastern South China Block were crystallized between 400Ma and 454Ma, synchronous to the Kwangsian gneissoid granites along the Wugong and Wuyi-Baiyun-Yunkai domains in the eastern South China Block. The synthesis of these whole-rock geochemical and in-situ zircon Hf isotopic data suggests that both the Group 1 and 2 granites across the Jiangshan-Shaoxing Fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the Precambrian basement. The input of juvenile mantle-derived magma is insignificant. Taking into account other available evidences, it is proposed that the Jiangshan-Shaoxing Fault might be a pre-existing boundary between the Yangtze and Cathaysia Blocks, which was reactivated during the middle Paleozoic (Kwangsian) orogenic event. The massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime.
► The Kwangsian massive granites occurred in the eastern South China Block and are emplaced at 400–454Ma. ► The granites are derived from Precambrian SCB basement with poor input of the juvenile component. ► The Jiangshan-Shaoxing fault is a reactivated boundary during the Kwangsian intracontinental reworking.
•The Linjiawan volcanic gravels and Yinzhuba high-mg agglomerate erupted at 834–835Ma and 824Ma, respectively.•The Cangshuipu Group is constituted by the Linjiawan conglomerate and the overlying ...Yinzhuba agglomerate.•The Lengjiaxi and Banxi Groups were likely deposited at ∼860–835Ma and ∼810–720Ma, respectively.•The assemblage of the Yangtze with Cathaysia initiated at ∼830Ma and ended at ∼810Ma along the central Jiangnan orogen.•An early neoproterozoic arc-continent assembly model is herein proposed for the Jiangnan orogen.
The early Neoproterozoic amalgamation process of the Yangtze with Cathaysia blocks in South China is an important geoscience issue and its collisional time has attracted extensive attention during the past decades. The Linjiawan conglomerate and Yinzhuba agglomerate of the Cangshuipu Group and associated rocks along the Central Jiangnan Orogen preserved key records for better constraining the assembly process of the Yangtze with Cathaysia blocks. This study presents a set of new geochronological and geochemical data of the volcanic gravels and the associated rocks from the Lengjiaxi, Cangshuipu and Banxi groups. Our data show that the dacitic and rhyolitic gravels in the Linjiawan conglomerate give zircon U–Pb ages of 834±2Ma and 835±6Ma, respectively, significantly older than that (814–824Ma) of the Yinzhuba agglomerate. The volcanic gravels in the Linjiawan conglomerate are characterized by subalkaline dacite and rhyolite, geochemically distinct from the high-mg andesite of the Yinzhuba agglomerate. Such characteristics, along with our field observations, suggest that the Yinzhuba agglomerate is conformably underlain by the Linjiawan conglomerate. The detrital zircons from the sandstones in the Lengjiaxi Group and the lower and upper parts of the Banxi Group yield the predominant early Neoproterozoic age-peaks at 863Ma, 810Ma and 795Ma, respectively. The sandy gravel from the Hengluchong conglomerate at the basal Banxi Group give the youngest age-peak of 810Ma. These data suggest that the Linjiawan and Hengluchong conglomerates represent two angular unconformities, in turn separating the Cangshuipu from Lengjiaxi (∼860–835Ma) and Banxi (∼810–720Ma) Groups. The unconformities were deposited at ∼830Ma and ∼810Ma, respectively, reflecting the initial collision and final assemblage of the Yangtze with Cathaysia blocks along the Central Jiangnan Orogen. The synthesis of all the available data indicates that the assemblage process of the Yangtze with Cathaysia blocks might be episodic with the collisional time of the Eastern Jiangnan Orogen being significantly earlier than that of the Central Jiangnan Orogen. An early Neoproterozoic arc-continent assembly model is herein proposed for the Jiangnan Orogen.
•Dioritic rocks in the Ailaoshan zone yield U–Pb ages of ∼800Ma.•They derived from partial melting of mafic lower crust.•They have been produced in a subduction-related tectonic setting.•The igneous ...activity records the Neoproterozoic continental growth of SW Yangtze.
Neoproterozoic felsic igneous rocks associated with mafic-ultramafic bodies along the margins of the Yangtze Block, South China, can be used to constrain the continental crustal growth and secular evolution of the region. LA-ICPMS zircon U–Pb dating of the Adebo quartz diorite pluton in the Ailaoshan tectonic zone on the southern margin of the Yangtze Block gives the Neoproterozoic age of 800±7Ma and ɛHf(t) values in the range of −1.03 to +3.75 with two-stage model age of 1.3–1.6Ga. The pluton is characterized by relatively low SiO2 (60.97–64.41wt.%) and total alkalis (K2O+Na2O, 7.35–9.14wt.%) and high Al2O3 content (16.98–18.21wt.%) with mg-number of 36–39. REE-normalized patterns show enrichment in LREE with (La/Yb)cn of 11.36–19.77 and Europium negative anomalies with Eu/Eu*=0.61–0.74. The samples are characterized by negative Nb-Ta ((Nb/La) n=0.18–0.35) and P, Ti, Sr anomalies and high Y concentrations (33.79–41.04ppm) and low Sr/Y ratios (5.65–10.16). Their isotopic composition are similar to those of the Neoproterozoic mafic igneous rocks in the Ailaoshan zone and the southwestern Yangtze Block, indicating that the quartz diorite was produced by partial melting of mafic lower crust. The diorite also shows the similar geochemical characteristics with adakitic rocks from thickened lower crust or amphibolite and eclogite experimental melts. In combination with their arc-related geochemical signatures and synchronous developed adakitic rocks in the region, the Adebo quartz diorite pluton might be produced in a subduction-related tectonic setting during Neoproterozoic crustal growth along the margins of Yangtze Block.
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