NE Jiangxi Research Articles

Zircon geochronology of the deformed matrix in the NE Jiangxi ophiolitic mélange belt: Time constraints on the Neoproterozoic evolution of the Paleo–South China Ocean and assembly of the Yangtze and Cathaysia blocks

AbstractThe NE Jiangxi ophiolitic mélange belt is the key area for understanding the evolution of the Paleo‐South China Ocean and the assembly of the Yangtze and Cathaysia blocks. The age of the deformed matrix is closer to the tectonic emplacement time of the ophiolitic mélange than that of the blocks. However, the rock types, ages and structural deformation of the matrix in the NE Jiangxi ophiolitic mélange belt lack comprehensive understanding. Based on the zircon U–Pb geochronology, Hf isotope and trace element analyses of the deformed matrix in the NE Jiangxi ophiolitic mélange belt, we report the ages of meta‐rhyolite and tuffaceous phyllite to be 800–760 Ma, and the maximum depositional age of the clastic matrix is ~760 Ma. The youngest deformed matrix ages of 800–760 Ma constrain the tectonic emplacement age of the NE Jiangxi ophiolitic mélange belt to the late Qingbaikou period after 800–760 Ma and before the deposition of the Xiuning Formation (765–732 Ma). Zircon ages, trace element and Hf isotope compositions indicate that the 800–760 Ma matrix was formed in a back‐arc basin environment with obvious addition of 1000–800 Ma arc materials and recycled Paleoproterozoic crustal materials. The detrital material source areas were the 800–760 Ma arc and earlier accretionary wedge as juvenile crust. The multi‐stage arc magmatism, metamorphism and deformation in the NE Jiangxi ophiolitic mélange belt suggest that multi‐stage subduction of the Paleo‐South China Ocean and Neoproterozoic accretionary orogeny occurred during 1–0.76 Ga at the southeast margin of the Yangtze Block. Combined with the regional geological data, the Neoproterozoic back‐arc basin or the foreland basin around the Yangtze Block closed after ~760 Ma. The collision and amalgamation of the Yangtze and Cathaysia blocks resulted in the final closure of the Paleo‐South China Ocean, which have occurred in the early Paleozoic.

Early Cretaceous extension in South China: constraints from east–west-trending A-type granite belts and growth strata in terrigenous basins

ABSTRACT The late Mesozoic tectonic domain of Southeast China transitioned from compression to extension because of the Paleo-Pacific plate rollback. However, the precise timing of the initiation of extensional tectonics remains unclear. This work focuses on the E–W-trending granite belt and the growth strata of Early Cretaceous red beds in Southeast China to establish the extensional tectonic model in Southeast China. The petrogenesis of four granitic plutons from the E–W-trending granite belt in NE Jiangxi is traced on the basis of zircon U–Pb ages and whole-rock major and trace element abundances to elucidate lithosphere evolution in the study area. U–Pb zircon dating reveals the granite emplacement at 130–135 Ma. Geochemical results demonstrate that all granite intrusions can be identified as A-type granites derived from crustal and a few mantle sources respectively through lithospheric thinning and magma upwelling under the extension background. Field observation shows the enlargement of the sedimentary area of the early Cretaceous Huizhou Formation (Fm.) in the Huangshan Basin closely after (or simultaneously) 139 Ma Shiling volcanic activities. The dip angles of the lower part of Huizhou Fm. vary from 49° to 25° towards the basin centre, and the thickness exhibited a slightly increasing trend towards the lower part within the same bed. These phenomena are consistent with the shape of the growth strata. A model, in which the NE–SW-oriented structure lines transfer to the E–W-trending tectono-magmatic belts by meeting and inheriting the previous E–W-trending tectonic axis, is established in the present work. Lithospheric thinning, magma upwelling, and crustal remelting occur within an oblique extension stress field since ca. 140 Ma. Consequently, these phenomena are influenced by the Paleo-Pacific plate rollback and the inheritance of a structural framework from the regional basement series.

From arc accretion to continental collision in the eastern Jiangnan Orogen: Evidence from two phases of S-type granites

The NE Jiangxi ophiolitic mélange separates the Huaiyu and Jiuling terranes in the eastern Jiangnan Orogen of the South China Block. Two phases of S-type granites, which are geochemically and mineralogically similar to the Himalayan leucogranites, are newly recognized in mantle peridotites of the NE Jiangxi ophiolite. The first phase, a tourmaline-bearing granite porphyry, has a SIMS zircon U–Pb age of 904.4 ± 4.8 Ma and is strongly enriched in K2O (4.00–4.26 wt%) and strongly peraluminous (A/CNK = 2.94–3.24). It further shows high Rb/Sr ratios and highly fractionated REE patterns, indicating an origin due to fluid-absent melting of metapelites at relatively high temperatures. The granite porphyry exhibits highly evolved zircon Hf (−6.0 to −1.0) and whole-rock Nd (−4.2 to −5.1) isotopes, which is in contrast to juvenile-derived magmatic rocks in the Huaiyu Terrane. We thus propose that the granite porphyry was formed by melting of the metapelites sourced from the Yangtze Block subsequent to the accretion of the Huaiyu arc to the Jiuling Terrane (the continental margin of the Yangtze Block). The second phase, a muscovite-bearing quartz porphyry, is strongly peraluminous (A/CNK = 1.29–1.34) with low CaO/Na2O ratios (0.11–0.14) and low FeOt + MgO + TiO2 contents (2.39–2.75 wt%), similar to melts derived from fluid-fluxed melting of metapelites. Zircon grains from the quartz porphyry yielded U–Pb ages of ca. 803–801 Ma with mainly positive εHf(t) values (+1.9 to +8.7), which are younger than the depositional ages of the basement sequences and nearly identical to the accumulation of the overlying unmetamorphosed cover in the Jiangnan Orogen. Therefore, it is suggested that the quartz porphyry was formed in a post-collisional extensional setting subsequent to the final assembly of the Yangtze and Cathaysia blocks. In conjunction with published data, we conclude that multiple subduction-accretion events may have been involved in the formation of the South China Block during Neoproterozoic times.

Petrogenesis and Geodynamic Implications of Late Jurassic Diorite Porphyry in the Neoproterozoic Ophiolitic Mélange of NE Jiangxi (South China)

Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U–Pb dating, Hf isotopes and whole‐rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U–Pb dating by SIMS and LA‐ICP‐MS methods yielded an emplacement age of ∼160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low SiO2 (58.08wt%–59.15wt%), and high Na2O (5.00wt%–5.21wt%) and MgO (4.60wt%–5.24wt%) contents with low TFeO/MgO ratios (1.02–1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios (24.8–31.1) and high Y contents (14.6–18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions (εHf(t)=−6.1‐ −0.1; TDM2=1597–1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust (i.e., early Neoproterozoic mafic arc‐related rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal‐spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat‐slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat‐slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.

Early Neoproterozoic multiple arc–back-arc system formation during subduction–accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17–13.07 wt.%) and TiO2 (1.48–1.62 wt.%) contents and the lowest Nb/Yb (0.80–0.88) and Th/Nb (0.02–0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe–Ti basalts have the highest FeOt (15.52–16.30 wt.%) and TiO2 (3.06–3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe–Ti basalts have moderate FeOt (12.98–13.40 wt.%) and TiO2 (2.37–2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb–Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U–Pb dating on gabbros gave ages of 995±22Ma and 993±12Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon εHf(t) (+8.8 to +13.8) values for the gabbros and whole-rock εNd(t) (+5.5 to +6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe–Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990Ma. Both the present and previous studies imply that multiple arc–back-arc systems formed during long-lasting subduction–accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic.

Neoproterozoic arc-trench system and breakup of the South China Craton: Constraints from N-MORB type and arc-related mafic rocks, and anorogenic granite in the Jiangnan orogenic belt

The early Neoproterozoic Jiangnan orogenic belt, located in the southeastern margin of the Yangtze Block, is considered to mark the suture zone between the Yangtze and Cathaysia Blocks. The eastern part of this belt consists of the Huaiyu and Jiuling terranes, separated by the NE Jiangxi fault zone which represents the back-arc marginal sea. The western part of the Jiangnan orogenic belt is characterized by the Jiangnan arc and back-arc sea. Neoproterozoic mafic–ultramafic rock suites are exposed along both the Jiangshan-Shaoxing-Pingxiang-Shuangpai and the NE Jiangxi suture zones, whereas I-, S- and A-type granites occur widely in the Jiangnan region. Here, we present new LA–ICP–MS zircon U–Pb data, Hf isotopes and whole rock geochemistry for the mafic rocks and granite from the Jiangnan orogenic belt. The N-MORB (normal mid-ocean ridge basalts) type basalts from the Nanqiao of Liuyang area in the west Jiangnan orogenic belt show low K2O contents (0.74–0.16wt.%), belonging to tholeiitic series, and display LREE (light rare earth element) depleted REE (rare earth element) patterns, with positive Pb, Sr anomaly and minor negative Ti anomalies. The other mafic suites in the Daolinshan, Yiyang and Liuyang areas exposed in the eastern and western segments of the Jiangnan orogenic belt show distinctive geochemical signatures of volcanic arc, with zircon U–Pb ages of 863±6 Ma. Most of these rocks show negative Nb, Ta, Zr, Hf and Sr anomalies and have LREE enriched REE patterns, with minor negative Eu anomaly. These features coincide with those of the arc and fore-arc rock assemblages in the eastern Huaiyu Terrane and adjacent areas. Anorogenic granites of 790±5Ma are also found in the Daolinshang area. Our Hf isotopic data further reveal that the zircons from both the dolerite and A-type granite in the Daolinshan area possess positive ɛHf(t) values, yielding latest Mesoproterozoic to early Neoproterozoic model ages. Combined with previous studies, our data confirm the existence of an arc-trench system in the Jiangnan orogenic belt at ca. 863Ma. Therefore, we infer that the amalgamation of the Yangtze and Cathaysia Blocks in the eastern part occurred at some time after ca. 863 Ma. This was followed by breakup at ca. 790Ma which was triggered by mantle upwelling, as evidenced by the Daolinshan A-type granite with positive ɛHf(t) values

Petrogenesis of the Middle Jurassic Yinshan volcanic-intrusive complex, SE China: Implications for tectonic evolution and Cu-Au mineralization

Volcanic–subvolcanic rocks and associated Cu-Au deposits in Yinshan are located in the eastern part of the Neoproterozoic Jiangnan orogenic belt between the Yangtze and Cathaysia blocks, constituting a large metallogenic district in southeastern China. This paper presents new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating, element and Sr-Nd-Hf isotopic data for Yinshan igneous rocks in the Dexing region. Our results show that the Yinshan igneous rocks were produced during the Middle Jurassic (ca. 176–166Ma). Geochemically, the Yinshan igneous rocks are characterized by calc-alkaline and show some adakitic affinities with high Sr contents (up to 903ppm), Sr/Y (up to 93.2) and LaN/YbN ratios (12.4 to 65.1) and low Y (2.53 to 15.8ppm) and Yb contents (0.22 to 1.39ppm). The absence of a negative Eu anomaly, extreme depletion in Y and Yb, relatively low MgO contents (0.2 to 2.9wt.%), and relatively high 207Pb/204Pb ratio (15.555–15.575), indicate that Yinshan igneous rocks were likely derived from the partial melting of thickened lower continental crust. In addition, the Yinshan igneous rocks show similar element characteristics to arc-derived igneous rocks with bulk Earth-like εNd (t) values (−1.1 to +1.8) and initial 87Sr/86Sr (0.7036–0.7090), relatively depleted εHf (t) values (−1.5 to +5.8), and Hf model ages of 0.8–1.2Ga. However, there is no arc-related evidence in the study area in the Middle Jurassic, which indicates that these arc-related elemental and isotopic compositions are possibly inherited from Meso- to Neo-Proterozoic juvenile continental crust formed by the Neoproterozoic oceanic crust subduction along the Jiangnan Orogen. The geodynamic environment responsible for the development of the Middle Jurassic Yinshan igneous rocks is probably a localized intra-continental extensional environment along the NE Jiangxi Deep Fault, as a tectonic response to far-field stress at the margins of South China Plate during the early stage of the paleo-Pacific plate subduction. Regarding the Cu-Au mineralization, we infer that Cu-Au were released from subducted oceanic slab and reserved in the juvenile crust during Neoproterozoic subduction along the eastern Jiangnan Orogen. Partial melting of Cu-Au rich Neoproterozoic juvenile crust during the Middle Jurassic time in the Yinshan area caused the formation of adakitic rocks and large-scale Cu-Au deposits. Our model provides a new insight into genesis of adakitic rocks and related Cu-Au deposits in a non-arc setting.

Adakitic signature formed by fractional crystallization: An interpretation for the Neo-Proterozoic meta-plagiogranites of the NE Jiangxi ophiolitic mélange belt, South China

The low-temperature/high-pressure metamorphic rocks which are exposed as exotic blocks within the serpentinite matrix of the NE Jiangxi ophiolitic mélange belt consist of enclave-bearing and -free meta-plagiogranites. Both rock types are characterized by a metamorphic mineral assemblage consisting of Ab-rich plagioclase + quartz +/− clinopyroxene + amphibole while rutile, titanite and zircon occur as accessories. The former primary magmatic mineral parageneses for the meta-mafic enclaves and the meta-plagiogranites are similar and consist of An-rich-plagioclase + hornblende + ilmenite + monazite + apatite. However, the Ab-rich plagioclase abundance of the meta-mafic enclave is much lower and the hornblende and accessory mineral abundances are higher compared with those of the meta-plagiogranite host rocks. Meta-plagiogranites with mafic enclaves have a chemical composition similar to that of typical adakites, with elevated Al 2O 3 (15.86 wt.%), Mg# (53.5) and Sr contents (485 ppm) and a positive Eu anomaly. The rare earth element patterns are highly fractionated with ((La/Yb) N> 12) and low Y (< 4 ppm) and Yb (< 0.4 ppm). The mafic enclave has a basaltic composition with a negative Eu anomaly and has ca. 10 times higher HREE and Y concentrations than the plagiogranite host rocks. Enclave-free meta-plagiogranites also have high Al 2O 3 (16.08 wt.%), Na 2O (8.88 wt.%) and Mg# (52.3) values without obvious Eu enrichment or depletion. The Y and Yb concentrations (13 and 1.33 ppm, respectively) are higher than those of the enclave-bearing meta-plagiogranites and slightly lower than those of the mafic enclave. Zircon of the enclave-bearing meta-plagiogranites gives a weighted mean 206Pb/ 238U crystallization age of 970 ± 21 Ma. These meta-plagiogranites have low 87Sr/ 86Sr value of ~ 0.7026 and high εNd(970 Ma) of 6.6 to 7.4. The meta-mafic enclave has a similar isotope composition with a 87Sr/ 86Sr value of 0.7023 and a εNd( T) of 6.2. The enclave-free meta-plagiogranites have relatively higher 87Sr/ 86Sr (0.7037 to 0.7045) and lower εNd( T) (− 1.22 to − 1.28) values. Trace element data of the minerals and whole rocks suggest that Y and Yb are hosted in amphibole, apatite and titanite, Nb in rutile, titanite and amphibole and Sr mainly in plagioclase. Petrographic and geochemical evidence indicates that the meta-mafic enclaves are ‘schlieren’, which formed by the early crystallization of accessory minerals and hornblende, in the meta-plagiogranites. The adakitic signature of enclave-bearing meta-plagiogranites is interpreted to be due to fractional crystallization of hornblende and accessory minerals such as apatite and ilmenite from the parent magmas, instead of the melting of subducted, spilitized oceanic crust with residual garnet and amphibole. The plagiogranitic magma may have evolved from a basaltic parent magma which had originated in the mantle wedge beneath the Neoproterozoic ( ca. 970 Ma) island arc along the southern margin of the Yangtze block.

Obduction-type granites within the NE Jiangxi Ophiolite: Implications for the final amalgamation between the Yangtze and Cathaysia Blocks

Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al 2O 3 (14–18.33%) and Na 2O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with La N/Yb N and La N/Sm N ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with La N/Yb N and La N/Sm N ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.

Adakitic granites within the NE Jiangxi ophiolites, South China: geochemical and Nd isotopic evidence

Geochemical and Nd isotopic data are reported for the ca. 970 Ma albite granites within the Neoproterozoic NE Jiangxi ophiolite, South China. These granites are high in Al2O3 (15–17%), Na2O (7.3–8.5%), Mg# (43–62), Sr (383–920 ppm), Sr/Y (166–444) and Y/Yb (12–14), but low in K2O (0.2–0.6%), TiO2 (<0.3%), P2O5 (<0.1%). Furthermore, they display consistently high, MORB-like initial εNd values of +4.6 to +6.7 at ca. 970 Ma. These geochemcial and isotopic characters are similar to those of the modern adakites. Compared with the modern adakites, the Xiwan albite granites are depleted in middle to heavy REE with Y/Yb ratio of 12–14, which can be attributed to residual amphibole and garnet in the source. Geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted, spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole. These are the first adakitic granites identified within the NE Jiangxi ophiolite. Their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea-floor spreading, oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought, but also during the process of subduction of oceanic lithosphere. The SHRIMP U–Pb zircon age of 968±23 Ma for the Xiwan adakitic granite [Geochimica 23 (1994) 117] should be re-interpreted as the timing of subduction of oceanic crust, rather than as the age of oceanic crustal formation as previously interpreted.

Geochemistry of the Late Paleozoic radiolarian cherts within the NE Jiangxi ophiolite melange and its tectonic significance

Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO2 (78.40%-89.28%) and high Al2O3 (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al2O3 of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al2O3/(Al2O3+Fe2O3) = 0.51-0.90, shale-normalized Lan/Cen = 0.76-1.11, Ce/Ce* = 0.91-1.22, V 40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic.

Age and Origin of the Mafic Rocks and Radiolarian Cherts within the NE Jiangxi Ophiolite Melange: Implications for the Tectonic Evolution of SE China

Age and Origin of the Mafic Rocks and Radiolarian Cherts within the NE Jiangxi Ophiolite Melange: Implications for the Tectonic Evolution of SE China

Discovery of dinoflagellates from the Dengshan Group in NE Jiangxi Province and its significance

From the stratotype section of the Bazhukeng Formation, Dengshan Group (formerly assigned to the late Proterozoic), situated at Dengshan Village, Zhujia, Yiyang County, Jiangxi Province, three dinoflagellate-bearing horizons have been found, in which the lower two contain ?Conochitina sp. andCyathochitina sp., while the upper yields ?Conochitina sp. and ?Rhabdochitina sp., which testifies that the stratigraphic units concerned belong to the early Paleozoic in age. Such a discovery is significant for re-ascertaining the age of the “Banxi Group” in South China, and the tectontc namework and its evolution.

Tectono–Magmatic Evolution and Metallogenesis along the Northeast Jiangxi Deep Fault, China

Abstract: From the southernmost part of Jiangsu province to the northeastern part of Jiangxi province, China, the Northeast Jiangxi Deep Fault runs for about 400 km length with a width of 30 to 40 km. This fault marks the suture zone of two ter‐ranes of Proterozoic age. At the both sides of the fault, Yanshanian granitic activity is recognized. That is, the Dexing‐Wuyuan porphyry belt on the NW side of the fault, and the Damaoshan‐Lingshan granite belt on the SE side. The former activity is characterized by the occurrence of small stocks of granodioritic composition, rich in siderophile elements but poor in LIL elements. No distinct Eu anomaly is recognized in the REE pattern, and a low initial 87Sr/86Sr ratio is reported. Magnetite, sphene and apatite are observed as accessory minerals. On the contrary, granitic activity on the SE side of the fault is characterized by the occurrence of composite batholiths, in general of granitic to monzogranitic composition, rich in LIL and alkali elements but poor in siderophile and alkali earth elements. A strong Eu anamaly is recognized in the REE pattern, and initial 87Sr/86Sr ratios are as high as 0. 716. Fluorite, zircon and REE minerals are observed as accessory minerals. These two contrasting granitic activities are refered to as syntexis– and transformation–types, respectively, following the classification commonly used in China, and have similar petrochemical characteristics to those defined for the magnetite– and ilmenite–series, and I– and S‐type granitoids.Considering that the above igneous activity occurred far from the supposed subduction zone along the East Coast of China, intracontinental A‐type (continent to continent) subduction is proposed to have occurred northwestwards along the NE Jiangxi Deep Fault during Yanshanian time due to a strong compressional stress from SE to NW. A‐type subduction introduced the continental slab to some depth, and resulted in the production of the paired granitic activity observed on both sides of the fault. Many mineral deposits are associated with both granitic belts. In the Dexing‐Wuyuan porphyry belt, the Dexing porphyry Cu and Yinshan polymetallic deposits are representative, whereas in the Damaoshan‐Lingshan granite belt, several tens of rare metal deposits are known such as the Geyuan Nb–Ta–W–Sn deposits. Metal assemblages of those deposits reflect the source materials of magmas in both granitic belts.

SHRIMP ion microprobe zircon U-Pb Age and Sm-Nd isotopic characteristics of the NE Jiangxi ophiolite and its tectonic implications

The new SHRIMP zircon U-Pb isotopic study suggests that the crystallization age of a highly fractionated magma in the NE Jiangxi ophiolite suite is 968±23 Ma. Re-calculated Sm-Nd isochron age of 955±44 Ma is within analytical errors. consistent with the zircon U-Pb age. With the exception of two anomalous Sm-Nd data, the remaining 15 analyses so far obtained for the ophiolite gaveeNd (T) values falling into a limited range from +4.3 to +6.7, indicating that the ophiolite was derived from a relatively strongly depleted mantle source. Sm-Nd isotopic systematics in some samples may have been strongly affected by post-magmatic events, such as alteration, deformation and metamorphism, resulting in anomalouseNd (T) values. Combined with published40Ar39Ar age data, it can be concluded that the collision between the Yangtze and South China Blocks occurred during 0.97-0.80 Ga.