Back-arc Extensional Setting Research Articles

Petrogenesis and tectonic setting of the Mesozoic Huoshenmiao intrusion in the Luanchuan ore district, Henan Province, North China

The Huoshenmiao intrusion, which is located in the western domain of the Luanchuan ore district, comprises quartz diorite, monzogranite, and granite porphyry. LA–(MC)–ICPMS zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd–Hf isotopes were conducted to constrain the timing of magmatism, and to reveal the petrogenesis and tectonic setting. Zircon U–Pb dating of the quartz diorite, monzogranite, and granite porphyry yielded ages of 150.3 ± 0.6 Ma, 146.0 ± 0.6 Ma, and 145.1 ± 0.5 Ma, respectively, indicating emplacement of the intrusion took place in the Late Jurassic. The three petrofacies are characterized by arc-like trace-element distribution, enriched in large ion lithophile elements and light rare earth elements, and depleted in high field strength elements. The quartz diorite is characterized by relatively low SiO2 content (56.79–61.96%), high Mg# (0.44–0.46), and Na2O-rich (Na2O/K2O = 0.95–1.43). However, the monzogranite and granite porphyry are characterized by relatively high SiO2 content (69.70–71.96%), low Mg# (0.23–0.28), and K2O-rich (K2O/Na2O = 1.20–1.93). The εNd(t) values of the quartz diorite range from −14.6 to −14.1 and the εHf(t) values range from −19.6 to −14.8, with model ages ranging from 2.09 to 2.44 Ga. The monzogranite and granite porphyry have εNd(t) values ranging from −16.8 to −14.7 and εHf(t) values in the range of −31.9 to −16.7, with model ages ranging from 2.13 to 3.21 Ga. The differences in elemental and Sr–Nd–Hf isotopic characteristics suggest that the quartz diorite was derived from a different magma source from the monzogranite and granite porphyry. Based on these new data, we propose that the quartz diorite was derived from partial melting of an enriched lithospheric mantle that experienced metasomatism owing to dehydration of the subducted continental crust of the Yangtze Craton. The monzogranite and granite porphyry were derived from partial melting of the Taihua TTG and the enriched lithospheric mantle. Model calculations using Sr–Nd isotopic data suggest that 50–70% of enriched lithospheric mantle material would be required to generate the monzogranite and granite porphyry. We propose that the Huoshenmiao intrusion was most likely associate with two subduction events, and generated in a back-arc extensional setting related to the subduction of the Izanagi Plate beneath the Eurasian continental margin.

Cretaceous A-type volcanic–intrusive rocks and simultaneous mafic rocks along the Gan-Hang Tectonic Belt, Southeast China: petrogenesis and implications for the transition of crust–mantle interaction

ABSTRACTAbundant evidence points to the Cretaceous crust–mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125 Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 104*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (~137–125 Ma) A-type volcanic–intrusive rocks in the GHTB. Their εNd(t) and zircon εHf(t) values gradually increased through time from approximately −9.0 to −1.0 and −10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust–mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust–mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting.

Study of late-Mesozoic magmatic rocks and their related copper-gold-polymetallic deposits in the Guichi ore-cluster district, Lower Yangtze River Metallogenic Belt, East China

ABSTRACTThe Guichi ore-cluster district in the Lower Yangtze River Metallogenic Belt hosts extensive Cu–Au–Mo polymetallic deposits including the Tongshan Cu–Mo, Paodaoling Au, Matou Cu–Mo, Anzishan Cu–Mo, Guilinzheng Mo and Zhaceqiao Au deposits, mostly associated with the late Mesozoic magmatic rocks, which has been drawn to attention of study and exploration. However, the metallogenic relationship between magmatic rocks and the Cu–Au-polymetallic deposits is not well constrained. In this study, we report new zircon U–Pb ages, Hf isotopic, and geochemical data for the ore-bearing intrusions of Guichi region. LA-ICP-MS U–Pb ages for the Anzishan quartz diorite porphyrite is 143.9 ± 1.0 Ma. Integrated with previous geochronological data, these late Mesozoic magmatic rocks can be subdivided into two stages of magmatic activities. The first stage (150–132 Ma) is characterized by high-K calc-alkaline intrusions closely associated with Cu–Au polymetallic ore deposits. Whereas, the second stage (130–125 Ma) produced granites and syenites and is mainly characterized by shoshonite series that are related to Mo–Cu mineralization. The first stage of magmatic rocks is considered to be formed by partial melting of subducted Palaeo-Pacific Plate, assimilated with Yangtze lower crust and remelting Meso-Neoproterozoic crust/sediments. The second stage of magmatism is originated from partial melting of Mesoproterozoic-Neoproterozoic crust, mixed with juvenile crustal materials. The depression cross to the uplift zone of the Jiangnan Ancient Continent forms a gradual transition relation, and the hydrothermal mineralization composite with two stages have certain characteristics along the regional fault (Gaotan Fault). Guichi region results from two episodes of magmatism probably related to tectonic transition from subduction of Palaeo-Pacific Plate to back-arc extensional setting between 150 and 125 Ma, which lead to the Mesozoic large-scale polymetallic mineralization events in southeast China.

Spatial-temporal relationships of late Mesozoic granitoids in Zhejiang Province, Southeast China: constraints on tectonic evolution

ABSTRACTLate Mesozoic granitoids in South China are generally considered to have been generated under the Palaeo–Pacific tectonic regime, however, the precise subduction mechanism remains controversial. Detailed zircon U–Pb geochronological, major and trace element, and Sr–Nd–Hf isotopic data are used to document the spatiotemporal distribution of the granitoids in Zhejiang Province. Three periods of late Mesozoic magmatism, including stage 1 (170–145 Ma), stage 2 (145–125 Ma), and stage 3 (125–90 Ma), can be distinguished based on systematic zircon U–Pb ages that become progressively younger towards the SE. Stage 1 granitic rocks are predominantly I-type granitoids, but minor S- or A-type rocks also occur. Sr–Nd–Hf isotopic data suggest that these granitoids were generated from hybrid magmas that resulted from mixing between depleted mantle-derived and ancient crust-derived magmas that formed in an active continental margin environment related to the low-angle subduction of the Palaeo–Pacific plate beneath Southeast China mainland. Stage 2 granitic rocks along the Jiangshan–Shaoxing Fault are predominantly I- and A-type granitoids with high initial 87Sr/86Sr, low εNd(t), εHf(t) values and Mesoproterozoic Nd–Hf model ages. These results suggest that stage 2 granitoids were derived from mixing between enriched mantle-derived mafic magmas and ancient crust-derived magmas in an extensional back-arc setting related to rollback of the Palaeo–Pacific slab. Stage 3 granitic rocks along the Lishui–Yuyao Fault comprise mainly A- and I-type granitoids with high initial 87Sr/86Sr ratios, and low εNd(t) and εHf(t) values, again suggesting mixing of enriched mantle-derived mafic magmas with more ancient crustal magmas in an extensional back-arc setting, related in this case to the continued rollback the Palaeo–Pacific plate and the outboard retreat of its subduction zone.

Petrogenetic contrastive studies on the Mesozoic early stage ore-bearing and late stage ore-barren granites from the southern Anhui Province

Yanshanian magmatisms are intensive in the southern Anhui Province and can be divided into early (152–137 Ma) and late (136–122 Ma) stages. A Yanshanian granitic zone was found to crop out along Qingshan to Changgai areas in the Tunxi district in Field investigation which has a genetic link with molybdenum multiple metal mineralization. To be a representative syenitic granite in the southern Anhui Province, the Huangshan pluton has not been found so far to have any genetic link with mineralization. Zircon LA-ICP-MS dating indicate that the four granitic bodies from the Qingshan-Changgai zone have concurrent formed ages from 140±4 to 141±2 Ma, belonging to the Yanshanian early stage magmatism. However, the Huangshan granite is dated to be 129±2 Ma, belonging to the Yanshanian late stage magmatism. The Qingshan-Changgai granites show high SiO2 and K2O contents, low P2O5 contents and middle Al2O3 contents and are high-K calc-alkaline series metaluminum I-type granite. These rocks are characterized by enrichments in the large ion lithophile elements and light rare earth elements (REE), depletions in the high field-strength elements, and middle degree negative anomalies of Eu, geochemical features of arc or continent crustal derived magma affinities. These rocks have 87Sr/86Sr( t ) ratios from 0.7120 to 0.7125, e Nd( t ) values from ‒7.24 to ‒4.38 and zircon e Hf( t ) values of ‒4.4 to 6.7, similar to that of the coeval ore-bearing granodiorites in the southern Anhui Province. Integrated geochemical studies indicate that the Yanshanian ore-bearing granodiorites were formed by partial melting of the Meso-Neoproterozoic accreted thickened low crust. Meanwhile, the Qingshan-Changgai granites were formed through a AFC process of plagioclase+amphibole+Shangxi Group of magmas that formed the ore-bearing granodiorites. The Huangshan granites are characterized by high SiO2 and K2O contents, moderate Al2O3 contents, seagull shape REE distributed pattern and distinct Eu negative abnormities. Comparing with the Qingshan-Changgai granites, the Huangshan granites show more Ba, Sr, P, and Ti negative abnormities with no Nb and Ta depletions and are high-K calc-alkaline series metaluminum A-type granite. e Hf( t ) values of the Huangshan granites are from ‒6.6 to ‒1.2, similar to that of the early stage ore-bearing granodiorites, indicating that they were also formed by anatexis of the Meso-Neoproterozoic accreted crust, but their magma sources might be residual granulitic crust which ever underwent Yanshanian early stage I-type intermediate-acid magma extraction. Comparing studies on the two stages granites indicate that the early stage granites derived from a relative thickened low crust under a lower temperature condition. Their magma sources were Meso-Neoproterozoic accreted crust which enriched in ore-forming materials and further became more enriched through processes of magma AFC evolution. However, the late stage A-type granites originated from relative shallow crust under a higher temperature condition. Their magma source was depleted in ore-forming materials due to the early stage magma extraction and thus had weak ore-forming capacity. From early to late stage, the magmatisms tectonic setting translated from post-orogenic to anorogenic and the later corresponded to a back-arc extensional setting as increase of the slab subducted angle of the Paleo-Pacific plate.

Upper Jurassic travertine at El Macanudo, Argentine Patagonia: a fossil geothermal field modified by hydrothermal silicification and acid overprinting

AbstractThe Deseado Massif hosts numerous Late Jurassic (150 Ma) fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back-arc setting. Detailed mapping, petrography and mineralogical observations of El Macanudo outcrops verify that it represents a hot-spring-related travertine partially replaced by silica and delineated by six sedimentary facies. These are large concentric cones (F1), laminated vertical columnar structures (F2), porous layers (F3), shrubby and irregular lamination (F4), low-amplitude wavy bedding (F5) and mounds and breccias (F6). The Macanudo Norte Outcrop rocks constitute a silica-replaced travertine sequence, with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment, surrounded by a subaerial travertine apron terrace; whereas, the Macanudo Sur Outcrop is a subaerial travertine mound sequence. Structurally controlled vent areas occur in both northern (F1) and southern (F6) outcrops, mainly located along regional NNE- and ENE-trending faults. The other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas. The El Macanudo palaeo-hot spring deposit is situated in an eroded Jurassic volcanic centre, and records a complex evolutionary-fluid history. The sediments archived three different Jurassic events, when large and long-lasting hydrothermal systems were active across the region. This relative temporal sequence was formed by: (1) travertine precipitation; (2) development of a silica cap, where early silicification was responsible for exceptional preservation of some stromatolitic fabrics; and (3) acid alteration, recorded by dissolution textures and clay formation, and caused by a palaeo-phreatic water-level drop.

Mineralogical and isotopic constraints on the genesis of the Jingchong Co–Cu polymetallic ore deposit in northeastern Hunan Province, South China

The Jingchong Co–Cu polymetallic deposit and a few other Co (–polymetallic) ore deposits are located in northeastern Hunan Province, in the central segment of the Jiangnan Orogen, South China. These deposits occur along the NE-trending Changping (Changsha–Pingjiang) deep fault zone and show a spatial association with the Late Jurassic–Early Cretaceous Lianyunshan granitoids, which intruded the Archean to Paleoproterozoic Lianyunshan Group comprising metaclastic-sedimentary rocks interlayered with mafic-intermediate igneous rocks. The Jingchong deposit is hosted within an altered fracture zone at the footwall of the Changping fault zone, and comprises an upper Pb–Zn ore zone and a lower Co–Cu ore zone. Wall rock alteration is dominated by silicification and chloritic alteration. Three types of ores were identified, i.e., quartz–sulfide veins, altered structural breccias and altered fractured rocks. Integrated field work and microscopy observations reveal a three-stage hydrothermal process related to mineralization: 1) quartz+chlorite (ChlI)+pyrite, 2) quartz+chlorite (ChlII)+pyrite+chalcopyrite+cobaltite, and 3) quartz+carbonate+galena+sphalerite±pyrite±chalcopyrite. LA-ICPMS analyses reveal that the Co contents are 1315–6473ppm, 76–3866ppm, and 5–435ppm for pyrite from the first-, the second-, and the third stages, respectively. EPMA analyses of chlorite show that ChlI and ChlII have Fe/(Fe+Mg) ratios of 0.65–0.80 and 0.76–0.92, and AlIV contents of 1.30–1.65 apfu (atom per formula unit) and 1.18–1.48 apfu, respectively. Empirical geothermometry of chlorite yielded temperatures of 256–318°C (av. 281°C) and 239–290°C (av. 256°C) for first- and second-stage chlorite, respectively. The δ34SVCDT values of sulfides (−4.9‰ to 0.2‰, av. −1.6‰) suggest a magmatic source for the sulfur, and the 3He/4He (0.017–0.30Ra) and 40Ar/36Ar (285–306) ratios of fluid inclusions in pyrite suggest that the ore-forming fluids were mainly crust-derived magmatic hydrothermal fluids with possible meteoric water inputs. The 206Pb/204Pb (18.042–18.746), 207Pb/204Pb (15.592–15.741) and 208Pb/204Pb (38.237–39.150) ratios of ore sulfides are similar to those of the Lianyunshan Group, suggesting that the metals may have been derived from the latter. Integrating the geological, mineralogical and geochemical data documented in this study, we propose that the Jingchong Co–Cu polymetallic deposit formed from a hydrothermal system related to the Lianyunshan pluton, which was derived from crustal anataxis of the Lianyunshan Group in a back-arc extensional setting in the Late Mesozoic.

Tectonic Evolution of the Eastern Black Sea and Caucasus: an introduction

Abstract This Special Publication presents the results of 15 different studies in the Black Sea–Caucasus segment of the Alpine–Tethys orogenic realm. The main focus of these studies is the style and timing of key tectonic events occurring primarily during the area's post-Pangaean evolution. The methodologies encompass: geophysics, including active and passive crustal-scale seismology and common depth point reflection seismic profiling (both onshore and marine), palaeomagnetism and magnetostratigraphy; field geology, including biostratigraphic recorrelation; radiochronology; igneous rock geochemistry, including analyses of the obducted ophiolites; and low-temperature thermochronology. The geological record of the area is essentially one of sedimentary basins formed in an extensional back-arc setting and their subsequent compressional deformation during the closure of at least two branches of the Neotethys Ocean system.

Late Jurassic Sn metallogeny in eastern Guangdong, SE China coast: Evidence from geochronology, geochemistry and Sr–Nd–Hf–S isotopes of the Dadaoshan Sn deposit

The newly discovered Dadaoshan Sn deposit is located in the eastern Guangdong Sn–W province, coastal SE China. The Sn mineralization, hosted in Jurassic porphyritic granite and the Lower Jurassic Jinji Formation sedimentary wall rocks, is considered to be granite-related. In this study, the porphyritic granite was LA–ICP–MS zircon U–Pb dated to be 153.2±1.2Ma, consistent with the syn-mineralization molybdenite Re–Os age of 152.6±1.8Ma. The porphyritic granite samples are weakly peraluminous (A/CNK=1.0–1.1) and high-K calc-alkaline. The rocks contain high SiO2 (72.9–75.6wt%), moderate Rb/Sr (5–9) and low ΣREE (136–223ppm). They are enriched in F, Li, Rb and Sn, depleted in Ba, Sr, P, Zr, Th, Nb and Y, and have distinct negative Eu anomalies (δEu=0.09–0.18), suggesting that the porphyritic granite is highly fractionated I-type granite. The calculated initial 87Sr/86Sr (0.711582–0.715173), relatively low ɛNd(t) (−9.48 to −8.54; TDM2=1638–1814Ma), and the zircon εHf(t) (−14.2 to −5.1; two-stage model ages=1528–2103Ma) all suggest that the granite was mainly crustal-derived with little mantle input. Sulfur isotopic compositions for the sulfides (arsenopyrite and chalcopyrite: δ34S=−1.1 to 1.4‰, average=−0.1) imply a dominantly magmatic sulfur source. The calculated zircon Ce4+/Ce3+ and EuN/EuN∗ ratios of the Dadaoshan granite range from 1.0 to 112 (mean=31.7) and from 0.04 to 0.37 (mean=0.14), respectively, indicating a low oxygen fugacity for the magma. The reducing and highly fractionated nature of the Dadaoshan granitic magma may have played a key role in the Sn mineralization.It was previously argued that the Jurassic Sn–W mineralization and its causative magmatism were largely confined in the South China interior, e.g., the Nanling Range. Our new data suggest that the Late Jurassic Sn–W mineralization and its causative magmatism actually extended to the SE China coastal area. The Dadaoshan granite may have been generated from partial crustal melting led by underplating of mantle-derived magmas in an extensional environment. Regional extension may have been related to the west-directed, flat-slab subduction and delamination of the Paleo-Pacific (Izanagi) plate beneath the South China block. Another suite of Early Cretaceous Sn–W-bearing granitic rocks in eastern Guangdong may have mainly been crustal-derived with minor mantle input, and likely occurred under back-arc extensional setting led by the Paleo-Pacific subduction rollback.

Petrogenesis of the Dalongkai ultramafic-mafic intrusion and its tectonic implication for the Paleotethyan evolution along the Ailaoshan tectonic zone (SW China)

Layered ultramafic-mafic intrusions are usually formed in an arc/back-arc or intra-plate tectonic environment, or genetically related to a mantle plume. In this paper, we report on an ultramafic-mafic intrusion, the Dalongkai intrusion in the Ailaoshan tectonic zone (SW China), whose occurrence is closely associated with arc/back-arc magmatic rocks. The Dalongkai intrusion is composed of plagioclase-lherzolite, hornblende-peridotite, lherzolite and wehrlite at the bottom, cumulate plagioclase-pyroxenite at the middle part, changing to fine-grained gabbro towards the upper part of the intrusion, forming layering structure. Zircons from the plagioclase-pyroxenites and gabbros yielded U-Pb ages of 272.1±1.7Ma and 266.4±5.8Ma, respectively. The plagioclase-pyroxenites show cumulate textures, and are characterized by high MgO (25.0–28.0wt.%; mg#=80.6–82.3), Cr (1606–2089ppm) and Ni (893–1203ppm) contents, interpreted as early cumulate phases. By contrast, the gabbros have relatively lower mg# values (56.3–62.7), and Cr (157–218ppm) and Ni (73–114ppm) concentrations, and may represent frozen liquids. The plagioclase-pyroxenites and gabbros share similar chondrite-normalized REE patterns and primitive mantle-normalized trace element profiles which are analogous to those of typical back-arc basin basalts. The εNd(t) values for both rock types range from +2.20 to +4.22. These geochemical and isotopic signatures suggest that the Dalongkai ultramafic-mafic rocks originated from a MORB-like mantle source metasomatized by subduction-related, sediment-derived fluids. Our data, together with other geological evidence, indicate that the emplacement of the Dalongkai ultramafic-mafic intrusion most likely occurred in a back-arc extensional setting associated with subduction of the Ailaoshan Paleotethyan branch ocean during the Middle Permian, thus ruling out the previously speculated linkage to the Emeishan mantle plume, or to an intra-continental rift.

Cretaceous magmatism and Mo mineralization in the South China Mo Province: U–Pb and Re–Os geochronology constraints from the Sanzhishu porphyry Mo deposit

Compared with the Mesozoic W–Sn–Cu–Au (Pb–Zn–Ag) mineralization in the South China Mo Province (SCMP), detailed studies on Mo deposits, especially Cretaceous Mo deposits are rare due to limited exposures. The Sanzhishu porphyry Mo deposit, located in the SCMP, is characterized by disseminated and veinlet-type molybdenite–quartz veins within monzogranite. Zircon SIMS U–Pb analysis of the monzogranite yields a concordia age of 112.3Ma, indicating that the emplacement of the Sanzhishu monzogranite was in the Early Cretaceous. Six molybdenite samples yield Re–Os ages between 111 and 112Ma, with an isochron age of 111±6.4Ma, which is consistent with the zircon age of the ore-bearing monzogranite.A new compilation of reliable Re–Os isotope age data indicate that Cretaceous Mo mineralization in the SCMP, occurred during two major episodes at 130–145Ma and 90–120Ma, with most deposits forming in the later episode. During the Cretaceous, a back-arc extensional setting in SE China is evidenced by the occurrence of numerous high-F Mo deposits, widespread A-type granites and the development of pull-apart basins. The increasing trend in Re contents of molybdenite from the early Mo mineralization period to the late Mo mineralization period in the SCMP indicates that the area experienced stronger crust–mantle interaction in the later mineralization episode. Based on geological evidence, we propose that the early episode of Mo mineralization formed in a tectonic setting associated with a metamorphosed subducted slab that experienced an increase in density and an increase in the gravitational pull on the slab itself, which induced rollback of the subducting Paleo-Pacific slab. The late episode of Mo mineralization developed in an extensional tectonic setting caused by the continued rollback and the eventual slab break-off.

Zircon age and geochemistry of the Tost bimodal volcanic rocks: Constraints on the Early Carboniferous tectonic evolution of the South Mongolia

SIMS zircon U–Pb dating, geochemical and Sr–Nd isotopic data are presented for the Late Paleozoic volcanic rocks from Tost area in Mongolia, the southern portion of the Central Asian Orogenic Belt (CAOB). The Tost volcanic rocks show a bimodal feature characterized by a mafic member of basalt and a felsic component of rhyolite, which are temporally and spatially related each other, implying a genetic relationship. Zircon U–Pb isotopic data of the rhyolite constrain the Tost bimodal magmatism occurring from 355Ma to 320Ma. The Tost basalt is characterized by high abundances in Th, U and Pb, slightly enriched LREE patterns and low HFSE/LREE ratios. These features, together with their OIB-like isotopic signature ((87Sr/86Sr)i=0.7039378–0.704397, εNd(t)=3.55–5.02), suggest that they were likely derived from low-degree partial melting of a metasomatized asthenospheric mantle source with subordinate input of subduction components. The Tost rhyolite, which displays an intimate affinity to Tost basalt, with enrichment in Th, U and Pb, depleted in Nb, Ta and Ti, and gently right-tilted REE patterns, is inferred to be generated by partial melting of a juvenile lower crustal source heated by underplating mafic magmas which rise from asthenosphere during continued rifting. The Tost bimodal volcanic rocks are comparable both in age and composition with those in the East Tianshan, which together constitute an E–W-oriented belt of bimodal volcanic rocks, marking an Early Carboniferous rifting event. Considering regional geology, we propose that the rifting took place in a back-arc extensional setting, probably induced by the subduction of the Dzungaria Ocean between the East Tianshan and Junggar-Kazakhstan plate during the Early Carboniferous.

Mineral Chemistry and Petrogenesis of the Puesto La Peña Undersaturated Alkaline Potassic Complex, Mendoza, Argentina

The late Early Miocene Puesto La Pena alkaline potassic complex, Mendoza, Argentina, is a multiphase intrusion with a large variety of rock types (including pyroxenite, malignite, borolanite, syenite, mafic to felsic dikes, and volcanic necks). This study focuses on the mineralogy of the various lithologies and the mineral chemical data in order to define a petrogenetic model that explains the fractionating phases of the complex. Geothermobarometric data and characteristics of and variations in mineral composition are in agreement with a parental magma that began the crystallization process at around 35–40 km depth with initial temperatures of around 1350 °C. After cooling to about 900 °C, a fractional crystallization process was dominant in a crustal environment at moderate to shallow depths, the ascent of magma being related to crustal extension. Crustal contamination seems to have been limited. In the proposed model, pyroxenite is a cumulate phase of the original intrusion, being comagmatic with the malignite-syenite. The late-stage evolution of the complex involves evolution towards the calcite liquidus field, which explains the presence of clusters of calcite-dolomite crystals and carbonatitic dikelets associated with late ouachititic dikes. The mineral composition reflects an environment of crystallization at moderate to shallow depths, low pressure, low silica, and high temperature, which is consistent with the model for the complex, considered to represent the root zone of an alkaline caldera-style volcano emplaced in an extensional back-arc setting.

In situ zircon Hf–O isotopic analyses of late Mesozoic magmatic rocks in the Lower Yangtze River Belt, central eastern China: Implications for petrogenesis and geodynamic evolution

A combined study of whole-rock major and trace elements, Sr–Nd isotopes, zircon U–Pb dating, and in situ zircon Hf–O isotopes has been carried out for late Mesozoic magmatic rocks in the Lower Yangtze River Belt. The results provide insights into the origin of mantle sources of magma above a subduction zone, and thus into the petrogenesis of high-K calc-alkaline rocks, shoshonites, and A-type granites on continental margins, and the associated tectonic transformation from a continental arc to a back-arc extensional setting. The late Mesozoic magmatism can be subdivided into three stages: high-K calc-alkaline intrusions (148–133Ma), shoshonitic rocks (133–127Ma), and A-type granitoids (127–123Ma). All the rocks have consistent arc-like trace element characteristics with positive anomalies of Rb, Th, U, Pb, and LREE, negative anomalies of Nb, Ta, and Ti, and enriched Sr–Nd–Hf isotopic signatures. The first-stage intrusions in the Tongling area usually host dark enclaves of diorite, have high Sr/Y ratios, and low Y contents, and contain zircons with relatively low εHf(t) values (−38.6 to −6.6) and high δ18O values (5.7‰ to 10.1‰). A few inherited zircons with Neoarchean to Paleoproterozoic ages and highly enriched Hf isotopic compositions were detected in both the host intrusive rocks and the enclaves. The second-stage Ningwu volcanics contain zircons with moderate εHf(t) values (−13.3 to −3.8) and elevated δ18O values (5.4‰ to 7.6‰). The third-stage intrusions can be divided into A1- and A2-type granitoids, and their zircons have relatively high δ18O values of 6.7‰ to 10.3‰ and high εHf(t) values of 0 to −7.9. Based on these geochemical data we drew the following conclusions. Before 148Ma, following metasomatism by slab-derived fluid/melts, partial melting of the lithospheric mantle produced basaltic magma in the context of a subducting paleo-Pacific plate. This basaltic magma mixed with magma derived from the Archean lower crust, and the underplated and thickened juvenile lower crust. From 148 to 133Ma, continuous production of these mixed magma resulted in their intrusions as basic rocks at shallow levels. Meanwhile, partial melting of the thickened juvenile crust formed the intermediate-acid rocks of the first stage. As subduction continued, and the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to one of back-arc extensions. Metasomatism and decompression induced intensive partial melting of the lithospheric mantle, and these magmas, together with a limited amount of assimilated crustal materials, formed the second-stage volcanics. Roll-back of the subducted slab resulted in extension, causing disassembling of the lower lithosphere and lithospheric thinning, and the upwelling of hot asthenosphere. The A2-type granites were the result of the reworking of the Mesoproterozoic juvenile crust in this tectonic setting of extension. On the other hand, the A1-type granitoids were formed from magmas that were derived both from the matasomatized mantle and from the A2-type granitic material that had its origins in the Mesoproterozoic juvenile crust. We suggest that an integrated and comparative study of the multiple stages of development of these magmatic rocks is the key to understanding the tectonic evolution and associated magmatic activities in this continental intraplate setting.

The Gaosongshan epithermal gold deposit in the Lesser Hinggan Range of the Heilongjiang Province, NE China: Implications for Early Cretaceous mineralization

The newly discovered and large ca. 99Ma Gaosongshan gold deposit is located in the Lesser Hinggan (or Xinggan) Range to the west of the Jiamusi Massif. The deposit is located at the border of the Great Hinggan Range–Mongolian Orogen with the Circum-Pacific tectonic belt and has a resource of ~22t @ 6.3g/t Au. Gaosongshan is classified as an adularia–sericite epithermal deposit associated with the large tensional Shaqihe Fault and has a low-sulfide content with a high concentration of gold hosted by pure chalcedonic quartz, which makes the deposit both unique and economically important in the area. Early Cretaceous volcanic rocks host the deposit as well as other gold deposits in the region. The mineralization at Gaosongshan includes Au, Ag, Mo, Sb, As, Pb and Hg. Stable isotope studies of fluid inclusions associated with the mineralization suggest that the mineralizing fluid had a meteoric origin (DV-SMOW: −129 to −111‰; 18Ofluid: 7.9 to 13.8‰). Fluid inclusion thermography indicates that the mineralized fluid was between 150° and 310°C and had a low salinity (0.7–3.71wt.% NaCl), low density (0.48–0.94g/cm3), shallow metallogenic depth (430–1270m), and a large amount of reducing gaseous components (CH4, C2H6, CO, N2, and CO2). The presence of large quantities of flaky quartz, adularia, gas-rich fluid inclusions, low-sulfide minerals and pure gold-bearing chalcedonic quartz suggests that fluid boiling was the principal mechanism for the gold precipitation. Sulfur isotopic data (δ34S: ~ −2.4 to 2.9‰) indicate a deep magmatic origin for the mineralization, and the Pb isotopes (206Pb/204Pb: 18.14–18.46; 207Pb/204Pb: 15.51–15.57; 208Pb/204Pb: 38.01–38.40) also indicate that the metallogenic source of the deposit contained a significant mantle component. The mineralizing fluid interacted with a Late Paleozoic (ca. 260–253Ma) substrate and ca. 141–91Ma volcanic host rocks. The geological and geochemical characteristics of the deposit, and the Tuanjiegou, Dongan, Sandaowanzi and other large gold deposits represent an Early Cretaceous (114–80Ma) epithermal mineralizing events in the Lesser Hinggan Range. The belt is controlled by the volcanic Sunwu–Jiayin Basin formed in a back-arc extensional setting along an active continental margin in East China, and the gold deposits in the belt have the same genesis as porphyritic Au(–Cu) deposits along the continental margin in eastern Jilin and Heilongjiang provinces. It is here proposed that the mineralization in both regions is related to the subduction of the Paleo-Pacific plate.

Metallogenesis and the minerogenetic series in the Gangdese polymetallic copper belt

The Gangdese is a newly explored porphyry Cu ore belt in China. In fact, except for the porphyry Cu deposits there are a lot of other deposits, forming a polymetallic copper belt in the southern Gangdese (SG, i.e., southern Lhasa terrane) and Gangdese back–arc fault uplift belt (GBAFUB, also called Lungar–Nyainqêntanglha arc). The Gangdese polymetallic copper belt contains porphyry Cu(–Mo–Au), Cu–Au and Mo(–Cu), skarn Cu(–Mo–Au–W), Pb–Zn(–Ag), Fe(–Cu) and Mo–W, breccia-type and hydrothermal vein-type Pb–Zn(–Ag) deposits. The porphyry Cu–Au mineralization occurred at the Xiongcun area of SG during the Middle Jurassic (175–160Ma) and was associated with island arc magmatism related to the northward subduction of the Neo–Tethyan oceanic slab. The Cretaceous (ca. 112Ma) skarn Fe(–Cu) mineralization appeared at the Nixiong area of GBAFUB and was generated in a back arc extensional setting related to the Neo–Tethyan oceanic subduction beneath the Lhasa terrane. The Paleocene–Eocene (65–51Ma) porphyry Mo(–Cu), skarn Mo–W, and skarn Pb–Zn(–Ag) deposits are located in the eastern part of GBAFUB and related to granitoids originating from the partial melting of the old crust with contributions from the mantle. In contrast, the Paleocene–Eocene (60–45Ma) porphyry Cu, skarn-, breccia-, and hydrothermal-vein-type Pb–Zn(–Ag), and skarn Fe–Cu deposits at the Namling area were associated with granitoids derived from the partial melting of the mantle with some input of the old crust. The Oligocene (30–23Ma) porphyry–skarn Cu–Mo–W–Au mineralization occurred at the Zedong area adjacent to north of the Indus–Yarlung–Zangbo suture zone in a post collisional setting. The intensified Miocene (21–13Ma) porphyry–skarn Cu(–Mo–Au), and skarn–hydrothermal-vein-type Pb–Zn(–Ag) deposits formed in the SG, whereas the porphyry Mo(–Cu) and skarn–hydrothermal-vein-type Pb–Zn(–Ag) deposits formed in the GBAFUB. The porphyry Mo(–Cu) mineralization have more substantial contributions from the old crust, in addition to the mantle, than those with porphyry Cu(–Mo–Au) mineralization. The source region of the high Sr/Y intrusions associated with post collisional porphyry Cu(–Mo–Au) mineralization is probably the arc magmas that were generated at the subduction stage and have remained stalled at the boundary between the crust and mantle since the Neo–Tethyan Ocean closed. The early subduction of the Neo–Tethyan oceanic lithosphere played a crucial role in the generation of ore deposits formed in collisional and post collisional settings. Six minerogenic series (or groups) can be recognized in the Gangdese polymetallic copper belt: (1) Jurassic porphyry Cu–Au minerogenic series related to island arc magmatism, (2) Cretaceous skarn Fe(–Cu) minerogenic series in back arc extensional setting, (3) Paleocene–Eocene granitoids-related polymetallic minerogenic series in collisional setting, (4) Oligocene porphyry–skarn Cu–W–Mo–Au minerogenic series in post collisional setting, (5) Miocene Cu–Mo–Au–Pb–Zn–Ag minerogenic series related to granitoids with substantial contributions from the mantle in post collisional setting, and (6) Miocene Mo–Cu–Pb–Zn minerogenic series related to granitoids with significant contributions from the old crust in post collisional setting. During mineralization, some deposits underwent superimposed mineralization, such as the Jiru porphyry Cu deposit where the Eocene and Miocene porphyry-type Cu mineralization occurred. The minerogenic series can be applied to ore prospecting. For example, when either Pb–Zn(–Ag) veins or porphyry–skarn Mo(–W–Cu) orebodies are known in an area, one of them could provide a useful exploration vector for the other.

Inferring a deep-crustal source terrane from a high-level granitic pluton: the Strathbogie Batholith, Australia

The Strathbogie Igneous Complex is comprised of the ignimbritic rocks of the Violet Town Volcanics and the granitic rocks of the Strathbogie batholith. It is Late Devonian in age and postorogenic-extensional in tectonic setting. The batholith was constructed from peraluminous, metasediment-derived magmas emplaced as several internally heterogeneous plutons. Chemical variation in the magmas was largely inherited from the protolith rather than having been produced by differentiation (crystal–liquid separation) or magma mixing. The Strathbogie magmas formed during a granulite-facies metamorphic event that caused partial melting of the rocks of the Proterozoic Selwyn Block, which forms the basement in this region. The chemistry of the Strathbogie batholith, the Violet Town Volcanics and various other felsic complexes of similar age, implies that the Selwyn Block here originally consisted of andesite, dacite, greywacke and pelite, probably deposited in a back-arc extensional setting. The sedimentary components of this protolith may have been deposited in a basin that was extending and deepening with time, so that the sediments contained progressively higher ratios of clay to volcanic materials. Much later, in the Late Devonian, extensional tectonics allowed the emplacement of mantle magmas into the deep and middle crust, causing the low-pressure granulite-facies metamorphic event that was responsible for the production of the crustal components in the granitic magmas of Central Victoria.

Geochronology and geochemistry constraints of the Early Cretaceous Taibudai porphyry Cu deposit, northeast China, and its tectonic significance

The southern Great Xing’an Range (SGXR), located in the southeastern part of Inner Mongolia, China, shows intense Mesozoic tectono-magmatic activity and hosts economically important polymetallic (Cu–Pb–Zn–Sn–Fe–Ag–Au–Mo) mineralization. Here, we present new zircon U–Pb ages, whole-rock geochemical data, Nd–Sr–Hf isotopic data and Re–Os ages for the Taibudai deposit in the SGXR. The Taibudai granitoids show high SiO2 (70.62–72.13wt.%) and alkali (Na2O+K2O=7.04–8.60wt.%) concentrations, low MgO (0.89–1.37wt.%) and Al2O3 (∼14wt.%), ASI ratios <1.1 (0.94–0.97), LILEs (e.g., Rb) enriched, HFSEs (e.g., Nb, Ta, Ti, and P) depleted, and have low Sr and Yb concentrations, classifying these rocks as fractionated I-type granites. The Taibudai granitoids have negative εNd (t) values ranging from −2.2 to −1.6 and relatively low initial 87Sr/86Sr ratios from 0.70536 to 0.70581. In situ Hf isotopic analyses on zircons using LA–MC–ICP–MS show variable positive εHf (t) values ranging from +0.80 to +13.55, corresponding to relatively young two-stage Hf model ages from 801 to 942Ma (excluding one spot). These mineralogical, geochemical, and isotopic features strongly suggest that the primary magmas of the Taibudai granitoids were derived mainly from the partial remelting of Neoproterozoic juvenile crustal material, with no remarkable modification through incorporation of continental or subduction-related material. Re–Os isotope analyses of molybdenite from the deposit yield an ore-forming age of 137.1±1.4Ma. Re contents range from 4.37 to 41.77ppm, implying ore material components have a mixed crust–mantle origin. SHRIMP analysis of zircons show that the monzogranitic porphyry and biotite granite in the Taibudai deposit were formed at 137.0±0.9Ma and 138.3±0.9Ma, respectively, indicating a temporal link between granitic magmatism and Cu mineralization. This result, combined with the regional geology, tectonic evolution, and age data from the literature, suggests that the Early Cretaceous (∼140Ma) was the peak metallogenic epoch for the Great Xing’an Range, and the mineralization in this period generally takes the form of porphyry, skarn, or hydrothermal polymetallic ore deposits in an active extensional continental margin environment. The Taibudai porphyry and associated mineralization provides a typical example of magmatism and metallogeny associated with a Paleo-Pacific plate subduction, continental margin, back-arc extensional setting.

Structural evolution of the early Permian Nambucca Block (New England Orogen, eastern Australia) and implications for oroclinal bending

The Paleozoic to early Mesozoic southern New England Orogen of eastern Australia exhibits a remarkable ear-shaped curvature (orocline), but the geodynamic processes responsible for its formation are unclear. Oroclinal bending took place during the early Permian, simultaneously with the deposition of the rift-related Sydney, Gunnedah, and Bowen basins, which bound the oroclines to the west. The Nambucca Block is another early Permian rift basin, but it is situated in the core of the oroclinal structure. Here we present new stratigraphic, structural, and geochronological data from the Nambucca Block in an attempt to better understand its tectonic history and relationships to the formation of the oroclines. We recognized four phases of folding and associated structural fabrics (S1–4), with the second phase (S2) dated at 275–265 Ma by 40Ar/39Ar geochronology of muscovite. This age overlaps with independent constraints on the timing of oroclinal bending, suggesting that the earlier two phases of deformation in the Nambucca Block (F1 and F2) were associated with orocline formation. We propose that oroclinal bending involved three major stages. The first stage (<300 Ma) was associated with variations in rates of trench rollback and formation of rift basins in a hot extensional back-arc setting. This was followed by a second stage of oroclinal bending, possibly linked to dextral wrench faulting, which involved ~ N-S contraction (F1). Subsequent deformation at 275–265 Ma involved formation of nappe-style structures (F2). This phase of contractional deformation may have resulted from an increased plate coupling that was possibly linked to flat-slab subduction.

Generation of calc-alkaline andesite of the Tatun volcanic group (Taiwan) within an extensional environment by crystal fractionation

The Pliocene–Pleistocene northern Taiwan volcanic zone (NTVZ) is located within a trench-arc–back-arc basin and oblique arc–continent collision zone. Consequently the origin and tectonic setting of the andesitic rocks within the NTVZ and their relation to other circum-Pacific volcanic island-arc systems is uncertain. Rocks collected from the Tatun volcanic group (TTVG) include basaltic to andesitic rocks. The basalt is compositionally similar to within-plate continental tholeiites whereas the basaltic andesite and andesite are calc-alkaline; however, all rocks show a distinct depletion of Nb-Ta in their normalized incompatible element diagrams. The Sr-Nd isotope compositions of the TTVG rocks are very similar and have a relatively restricted range (i.e. ISr = 0.70417–0.70488; εNd(T) = +2.2 to +3.1), suggesting that they are derived directly or indirectly from the same mantle source. The basalts are likely derived by mixing between melts from the asthenosphere and a subduction-modified subcontinental lithospheric mantle (SCLM) source, whereas the basaltic andesites may be derived by partial melting of pyroxenitic lenses within the SCLM and mixing with asthenospheric melts. MELTS modelling using a starting composition equal to the most primitive basaltic andesite, shallow-pressure (i.e. ≤1 kbar), oxidizing conditions (i.e. FMQ +1), and near water saturation will produce compositions similar to the andesites observed in this study. Petrological modelling and the Sr-Nd isotope results indicate that the volcanic rocks from TTVG, including the spatially and temporally associated Kuanyinshan volcanic rocks, are derived from the same mantle source and that the andesites are the product of fractional crystallization of a parental magma similar in composition to the basaltic andesites. Furthermore, our results indicate that, in some cases, calc-alkaline andesites may be generated by crystal fractionation of mafic magmas derived in an extensional back-arc setting rather than a subduction zone setting.