Subduction-related Geochemical Signatures Research Articles

Petrogenesis of Early Cenozoic Sarıcakaya–Nallıhan Volcanism in NW Turkey: Implications for the Geodynamic Setting and Source Characterization of the Balkanatolia Magmatic Realm

Sarıcakaya–Nallıhan Volcanism was generated within the Balkanatolia Magmatic Realm between 48 and 44 Ma (by 40Ar–39Ar age determination) and is represented by three different volcanic units all displaying subduction-related geochemical signatures, such as depletion in HFSE and enrichment in LREE and LILE. The first unit (V1) consists of nepheline-normative, olivine basalts with OIB-like affinity. The second (V2) and third (V3) units are represented by more evolved compositions such as basaltic-andesitic, andesitic, and dacitic-rhyolitic lavas. Even the most basic lavas have elevated Mg# values (62–69), and they are far from representing the true mantle melts. Source characterization of Sarıcakaya–Nallıhan Volcanism reveals that there might be two possible mantle sources for the primary melts of the lavas: (i) metasomatized peridotitic mantle fluxed by sedimentary melts, or (ii) accreted mélange. The direct melting of the mélange-like lithologies is a more favorable mechanism for the Middle Eocene (44–40 Ma) magmatism in Balkanatolia since the Hf–Nd trace element, Nd isotopic systematics and petrological modelling efforts supported the latter. Overall, Early Cenozoic magmatism within this realm was characterized, first (58–44 Ma) by contractional and later (44–40 Ma) by extensional tectonics and the late-stage magmatic phase in the area was possibly controlled by melting of accreted mélange-like lithologies. The presented data indicate that mélange melting might be much more common than envisaged for the magmatism in the Alpine–Himalayan orogenic belt.

Petrogenesis of Bonai volcanic rocks from Singhbhum Craton (Eastern India): Geophysical and geodynamic implications for pervasive plume-lithospheric interaction

• Petrogenesis and geodynamic evolution of Bonai volcanic, Singhbhum Craton. • Volcanic rocks generated by mantle plume within spinel lherzolite stability field. • Mantle potential temperature of 1300-1700°C and pressure 0.5-3.5 GPa. • Lower crustal exhumation, highly denudated lithospheric keel and magma underplating. • Crust-mantle thermal interaction, contamination and fractional crystallization. Singhbhum Craton of the Indian Shield is one of the oldest Eoarchean segments on the Earth, which contains several well-preserved Mesoarchean-Paleoproterozoic volcanic suites. In this study, we investigate petrogenesis of the Bonai suite of volcanic rocks located in the western part of the craton on the basis of detailed geological, geochemical and geophysical data, which provide a new understanding of its regional geodynamic evolution. These volcanics are generally fine to medium-grained, exhibit porphyritic texture and contain plagioclase and pyroxene as primary minerals and amphiboles, epidote, sericite and chlorite as secondary minerals. The accessory phases are mainly zircon, ilmenite, pyrite and chalcopyrite. They can be petrographically and geochemically classified as basalt and basaltic andesite of tholeiitic nature. They are characterized by moderate to high contents of SiO 2 (49.35-57.81 wt%), MgO (7.14-11.39 wt%), Al 2 O 3 (10.21-14.68 wt%), CaO (5.14-11.15 wt%), TiO 2 (0.48-2.03 wt%), Cr (32-483 ppm) and Ni (34-175 ppm). Their HFSE/REE ratios and REE patterns indicate their derivation from a relatively low melting of a similar mantle source within the spinel lherzolite stability field. They are formed as a result of decompressive melting of the mantle plume near the crust-mantle boundary, which underwent fractional crystallization and crustal contamination. The plume had an initial mantle potential temperature between 1300°C and 1700°C with corresponding pressure of 0.5 to 3.5 GPa. In Nb/Y-Zr/Y plot, almost all the samples fall in the field of oceanic plateau basalt with mantle plume affinity. This region is underplated by about 14 km thick magmatic rocks above the Moho, over which gravity field and regional elevation are consistently high and the lithosphere is extremely thin. These characteristics corroborate well with the mantle plume signatures. Subduction related geochemical signatures appear absent.

Relicts of a Cambrian oceanic arc in the Lajishan suture, NE Tibetan Plateau: Evidence for early-stage subduction within the Proto-Tethyan Ocean

Lajishan suture in the Qilian Orogen records subduction process of the southern branch of the Proto-Tethyan Ocean. However, when and where the subduction started remain controversial issues. We present a study of the Cambrian volcanic rocks from the Lajishan suture. These volcanic rocks are coherent and composed of basalt, andesite, and volcanic breccia with minor interlayers or veins of dacite and rhyolite. Zircons from the dacite and rhyolite yield concordant Early-Middle Cambrian ages ranging from 521 to 510 Ma. Basalt and andesite exhibit subduction-related geochemical signatures with enrichment in large ion lithophile elements and depletion in high field strength elements. They have evolved REE, LREE, ε Nd (t) (+2.6 to +8.2), and (La/Sm) N ratios, suggesting derivation from depleted mantle wedge with addition of hydrous fluid or various amounts of sediment-derived melts. Dacite and rhyolite have positive zircon ε Hf (t) values (+6.1 to +11.6) and whole-rock ε Nd (t) values (+1.7 to +4.2). They were generated by partial melting of juvenile mafic crust resulting from magma underplating. These Cambrian volcanic rocks in the Lajishan suture are exposed to the south of the accretionary complex and therefore are likely to be relicts of Cambrian oceanic arc built on the oceanic crust during southward subduction of the South Qilian Ocean. Considering the Cambrian-Ordovician intra-oceanic trench-arc-basin system reconstructed in the North Qilian suture, it is suggested that both southern and northern branches of the Proto-Tethyan Ocean have experienced intra-oceanic subduction process which initiated at ca. 530 Ma. • Cambrian volcanic rocks are coherent and widely exposed in the Lajishan suture. • They represent the relicts of an oceanic arc within the South Qilian Ocean. • Early-stage subduction of the Proto-Tethyan Ocean initiated at ca. 530 Ma.

First identification of a Cathaysian continental fragment beneath the Gagua Ridge, Philippine Sea, and its tectonic implications

AbstractThe tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized.

Cu-Ni mineralization in Early Permian mafic complexes in the Kalatage area of eastern Tianshan (NW China): Petrogenetic constraints from geochronology, geochemistry, and Hf-Sr-Nd-Os isotopes

The Early Permian was a critical period in the long history of accretion and collision of the southern Altaid orogen. Early Permian mafic-(ultramafic) intrusions and mafic–ultramafic-related sulfide Cu-Ni deposits are common in the eastern Tianshan, but the sulfide Cu-Ni deposits related to the mafic intrusions are still poorly studied, and yet provide key information on the ambient tectonic evolution. In order to help understand their petrogenesis, we report new data on the Early Permian mafic intrusions and related sulfide Cu-Ni deposits in the Dananhu arc of the eastern Tianshan in NW China. The ore-bearing Yueyawan complex is a well-differentiated, zoned mafic intrusion consisting of troctolite, olivine gabbro, layered gabbro, gabbro, and leucogabbro from the bottom upwards. The ore bodies are primarily strata-bound within olivine gabbros and troctolites at the bottom of the complex. The ores are more enriched in Cu than Ni than the more enriched, disseminated, veined and massive equivalents. Gabbros from the mafic complex yield zircon crystallization ages that range from 265 Ma to 273 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb, and Sr, depletion in Nb and Ta, positive Eu and Ti anomalies, high εNd(t) values of +7.44 to +8.06, zircon εHf(t) values of +11.68 to +20.58, and low (87Sr/86Sr)i (0.703322–0.703416). The gabbroic rocks have positive correlations between SiO2 and TiO2, Al2O3, CaO, Nb, and Y, and negative correlations between SiO2 and MgO, Ni, and Co. The (187Os/188Os)i and γOs values of sulfides from the Yueyawan complex that contains Cu-Ni deposits range from 0.266 to 0.372 and 112 to 197, respectively. These geological, petrological, and geochemical characters indicate that the gabbros were mantle-derived, and that fractionation and crustal contamination were essential for the genesis of the magmatic Cu-Ni sulfide deposits. After integrating our data with those of previous investigations, we conclude that the protoliths of the sulfide deposits were probably derived from an asthenospheric garnet-free mantle in a subduction setting in the Early Permian. Oblique ocean-ridge subduction and a resultant slab window plausibly induced asthenospheric upwelling that gave rise to the generation of the mantle-derived melts, which likely moved into strike-slip faults through a slab window in the supra-subduction zone. Strong fractionation, crustal contamination, and multi-stage eruptions led to formation of the Cu-Ni sulfide deposits.

Zircon Chemistry and Oxidation State of Magmas for the Duobaoshan-Tongshan Ore-Bearing Intrusions in the Northeastern Central Asian Orogenic Belt, NE China

The Duobaoshan (DBS)-Tongshan (TS) porphyry Cu–(Mo) deposit (4.4 Mt Cu, 0.15 Mt Mo) is located in the northeastern part of the central Asian orogenic belt (CAOB) in northeastern China. It is hosted by early Ordovician dioritic to granodioritic intrusions which are characterized by the subduction-related geochemical signatures including high concentrations of large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and low concentrations of heavy REEs (HREEs) and high-field -strength elements (HFSEs), such as Nb, Ta, Zr and Ti in bulk rock compositions. Furthermore, they show adakitic geochemical signatures of high Sr/Y ratios (29~55) due to high Sr (290~750 ppm) and low Y (<18 ppm). Zircon trace element abundances and published Sr-Nd-Hf isotope data of these rocks suggest that the parental magmas for these ore-bearing intrusions were rich in H2O and formed by partial melting of a juvenile lower crust/lithospheric mantle or metasomatized mantle wedge during the northwestward subduction of the Paleo-Asian Ocean before the collision of the Songnen block with the Erguna-Xing’an amalgamated block in the early Carboniferous. Values of Ce4+/Ce3+ and Ce/Nd in zircons are 307~461 and 14.1~20.3 for mineralized granodiorites, and 231~350 and 12.4~18.2 for variably altered diorite and granodiorites in DBS, whereas those for DBS-TS microgabbros are 174~357 and 7.4~22, and 45.9~62.6 and 5.0~5.8 for the early Mosozoic Qz-monzonites, respectively. Zircon Eu/Eu* values are high and similar among mineralized granodiorites (~0.6), altered diorite and granodiorites (~0.6) and the Mesozoic Qz-monzonites (~0.8), whereas the values are low and variable for the DBS-TS microgabbros (0.3~0.6). The magma oxidation state calculated from zircon chemistry and whole rock compositions are FMQ +1.0 to +1.5 in mineralized samples, and FMQ +2.4 to +4.2 in altered samples. The values are comparable to those for the fertile intrusions hosting porphyry Cu-Mo-(Au) deposits in the central and western CAOB and elsewhere in the world. Elevated oxidation state is also observed in the TS microgabbros, FMQ +1.4 to +1.9, and the early Mesozoic Qz-monzonites, FMQ +2.4 to +2.5. Comparison of zircon geochemistry data from porphyry deposists elsewhere suggests that positive Ce anomalies are generally associated with fertile intrusions, but not all igneous rocks with high Ce anomalies are Cu fertile. The findings in this study are useful in exploration work and evaluating oxidation state of magmas for porphyry Cu-(Mo) deposits in the region and elsewhere.

Petrogenesis of Late Carboniferous-Early Permian mafic-ultramafic-felsic complexes in the eastern Central Tianshan, NW China: The result of subduction-related transtension?

Knowledge of the Late Paleozoic accretionary processes responsible for the Altaids (or southern Central Asian Orogenic Belt) is critical for a better understanding of continental growth in Central Asia. Here, we systematic study the Late Carboniferous mafic-ultramafic-felsic complexes in eastern Central Tianshan to understand the tectonic evolution of the southern Altaids. Two gabbros yield zircon crystallization ages of 301.5 ± 2.3 Ma and 300.7 ± 2.6 Ma and one granite is dated as 301.6 ± 1.5 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb and Sr, negative Nb and Ta anomalies, and negative to positive Eu and Ti anomalies. The granites are the A2-type granites with high contents of SiO2, K2O + Na2O, Ga, Zr, Nb, Y, HFSE, REE, and high ratios of Fe/Mg and Y/Nb (>1.2). The gabbros and granites have high εNd(t) values of +1.64 to +5.03, zircon εHf(t) values of −2.8 to +9.1, and low (87Sr/86Sr)i (0.704007–0.705437) values. These geochemical characters demonstrate that the rocks in the complexes are fractionation genesis and that the A-type granitic rocks are the highest fractionation facies of tholeiitic mafic rocks. These results suggest that they were originated from the asthenospheric garnet-free mantle in a supra-subduction zone. Combined with regional data, we suggest that the oblique subduction with slab break-off of the Southern Tianshan oceanic plate plausibly induced asthenospheric upwelling to form the tholeiitic magma, and which intrude along the strike-faults the Central Tianshan-Yamansu arc. Accordingly, these results reveal the Paleo-Asian Ocean continued to subduct at the southern Altaids in the Late Carboniferous.

Early Devonian tectonic conversion from contraction to extension in the Chinese Western Tianshan: A response to slab rollback

Abstract Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532–407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407–397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530–410 Ma) and late Paleozoic (ca. 410–320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan.

Miocene syn-rift evolution of the North Croatian Basin (Carpathian–Pannonian Region): new constraints from Mts. Kalnik and Požeška gora volcaniclastic record with regional implications

Mts. Kalnik and Požeska gora volcaniclastic sequences hold valuable information concerning the Miocene syn-rift evolution of the North Croatian Basin, and the evolution of the Carpathian–Pannonian Region and the Central Paratethys. We present volcanological, high-precision geochronological, and compositional data of volcanic glass to constrain their tephrochronology, magmatic provenance, and timing of the initial Central Paratethys flooding of the North Croatian Basin. Based on CA-ID-TIMS U–Pb zircon ages (18.060 ± 0.023 Ma for Mt. Kalnik and 15.345 ± 0.020 Ma for Mt. Požeska gora) and coeval 40Ar/39Ar sanidine ages (18.14 ± 0.38 Ma and 18.25 ± 0.38 Ma for Mt. Kalnik and 15.34 ± 0.32 Ma and 15.43 ± 0.32 Ma for Mt. Požeska gora), Mt. Kalnik rhyolitic massive ignimbrites and Mt. Požeska gora rhyolitic primary volcaniclastic turbidites are coeval with Carpathian–Pannonian Region Miocene post-collisional silicic volcanism, which was caused by lithospheric thinning of the Pannonian Basin. Their affiliation to Carpathian–Pannonian Region magmatic activity is supported by their subduction-related geochemical signatures. Although Mts. Kalnik and Požeska gora volcaniclastics are coeval with the Bukkalja Volcanic Field Csv-2 rhyolitic ignimbrites, North Alpine Foreland Basin, Styrian Basin, Vienna Basin, and Dinaride Lake System bentonites and volcaniclastic deposits, reliable tephrochronological interpretations based on comparison of volcanic glass geochemical composition are not possible due to a lack of data and/or methodological discrepancies. Our new high-precision geochronology data prove that the initial Middle Miocene (Badenian) marine flooding of parts of the North Croatian Basin occurred at least ~ 0.35 Ma (during the NN4 Zone) before the generally accepted ~ 15 Ma maximum flooding age at the basin scale, calibrating the timing of the onset of the widespread “mid-Langhian” Central Paratethys flooding.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen, of which the Solonker suture, as a major regional suture, coincides closely with an early Permian paleobiogeographical boundary. This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane. Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian–Early Triassic, uncertainty remains because of a lack of typical collision-related features (e.g., high-grade regional metamorphism and well-developed fold–thrust structures) and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes (288–275 Ma) and Middle-Late Permian undeformed layered gabbros, strongly peraluminous granites, and I-type granites (265–254 Ma) in the Xinhure area along the northern margin of the North China Craton. The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton. In contrast, the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle–upper crust, respectively, triggered by asthenospheric upwelling. Therefore, a transition from an end-compressional to an extensional environment according to a transition from collision termination to post-collision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma. This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.

The role of post-collisional mafic magmatism in the late stages of evolution of the northernmost Arabian-Nubian Shield (ANS): A case study from Isla Complex, Wadi Isla, South Sinai, Egypt

Isla complex is one of the late Neoproterozoic igneous complexes of the Arabian-Nubian Shield in southern Sinai. This research aims to examine the petrogenesis and tectonic setting of Isla complex, as well as inferring the P-T conditions of the parent magma and the depth of intrusion. A detailed field studies were carried out and representative samples for petrographic and geochemical studies were collected. Chemical analyses of major and trace elements for 18 samples and microprobe analyses for different mineral phases in olivine gabbro were performed. The Isla complex is distinguished into olivine gabbro, diorite and monzonite. The interrelationships between certain oxides and/or oxide ratios point to the important role of fractional crystallization process during the formation of the studied complex and the evolution of its mafic-intermediate rock units. The major fractionated phases were clinopyroxene, amphibole and plagioclase. The analyzed rocks show a gradual decrease in concentration of HFSE from olivine gabbro to monzonite via diorite suggesting overall co-magmatic origin. The low K content in plagioclase and low and constant Y/Nb, Nb/Zr, and K/Nb ratios in gabbroic rock relative diorites and monzonites suggest that during the formation of more evolved rock units, the crustal contamination process became more important. The rock varieties of the Isla complex are fresh and not deformed indicating post-collisional emplacement. The geochemical affinity of Isla complex is analogous to other within-plate post-collisional mafic magmatism of south Sinai. The subduction-related geochemical signature of the examined rocks can be interpreted by partial melting of an enriched lithospheric mantle source following an earlier subduction stage in the ANS. Based on hornblende-plagioclase thermometry, the gabbroic rocks of the Isla complex crystallized at a pressure of 5.5 kbar (~20 km depth), the crystallization temperature was estimated at 823 to 967 °C. The model of lithospheric delamination is supposed to form the studied melt since the delamination of the lithospheric mantle caused the asthenosphere to rise rapidly initiating partial melting and creating the studied gabbroic melt, followed by the fractionation of this melt to form different rock varieties of the Isla complex. The melts then penetrated the crust's base to create the Isla complex in southern Sinai. The estimated age of crystallization and emplacement of the examined complex is < 590–610 Ma, as it invades the old calc-alkaline granites.

Geochemistry and Sr–Nd–Hf isotopes of granitoids from the Xiaotuergen porphyry Cu deposit, Altay orogenic Belt, Xinjiang, Northwest China: Implications for petrogenesis, tectonic setting, and mineralization

The Xiaotuergen deposit is the first porphyry Cu deposit to be identified in the northern Xinjiang Altay, Northwest China. Voluminous granitoids comprising granite porphyry, granodiorite porphyry, biotite monzogranite, and quartz porphyry are present in the ore district. Granodiorite porphyry hosts the main Cu mineralization, in disseminated, veinlet, veinlet-disseminated, and stockwork forms. The Xiaotuergen intrusions are weakly metaluminous to peraluminous calc-alkaline to alkaline granitic rocks, and exhibit typical subduction-related geochemical signatures comprising light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion, and low Sr/Y and La/Yb ratios. They are depleted in Nb–Ta–Ti, P, and Eu–Sr, yield moderate Mg# values (41.10–55.38), and show negative but near-zero εNd(t) values (−2.0 to −0.1) and variable positive εHf(t) values (+1.52 to +11.08). Geochemical constraints indicate that the Xiaotuergen granitoids are highly evolved I-type granites which were derived from strong fractional crystallization of a parental magma, mixed with materials derived from partial melting of both subducting oceanic sediments and the overlying mantle wedge. The Xiaotuergen granitoids are products of the same tectono-magmatic activity as that which produced the synchronous magmatic rocks of the Xinjiang Altay, having been formed in a continental arc setting during the early Devonian. The Xiaotuergen ore-bearing porphyries are identified as the shoshonite series, rather than the adakite series. Mantle-derived contributions played a crucial role in the formation of magmas with Cu and S contents that were high enough to produce economic mineralization. The mature arc environment and long-lived magmatism provided favorable conditions for the development of porphyry Cu mineralization.

Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U–Pb dating, geochemistry and Sr–Nd–Hf isotopes was carried out for three types of intrusive rocks from the Hannan region, including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the Tiechuanshan and Huangguan A-type granites. LA-ICP-MS zircon U–Pb dating results show that the studied intrusions have crystallization ages ranging from ca. 880 to 770Ma. The ca. 880Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low high field strength elements (HFSE) concentrations that indicate a cumulate origin. These gabbros display subduction-related geochemical signatures with whole-rock initial 87Sr/86Sr (ISr)=0.704–0.705, εNd(t)=−0.4 to +0.6 and zircon εHf(t)=+6.9 to +8.4, suggesting that they were derived from the partial melting of an enriched lithospheric mantle that had been modified by slab-derived materials. The ca. 860Ma Tianpinghe I-type granites have high ISr (> 0.704) and negative εNd(t) (−5.6 to −3.5) values and zircon εHf(t)=−1.9 to +1.6, indicating their sources were mixtures of Neoproterozoic juvenile mafic crust and minor Paleoproterozoic crustal components, whereas the ca. 770Ma Taojiaba I-type granites have relatively low ISr (<0.704) and positive εNd(t) (+1.5 to +2.4) with zircon εHf(t)=+6.2 to +9.5, implying that they were dominantly sourced from the Neoproterozoic juvenile crustal materials. The geochemical and Sr–Nd–Hf isotopic data imply that the ca. 780Ma Tiechuanshan A-type granites were crystallized from a reduced and peralkaline magma, which was generated by partial melting of a heterogeneous source involving Neoproterozoic juvenile and ancient mafic crust under dry and reducing conditions. In contrast, the parental magma of the ca. 774Ma Huangguan A-type granites was oxidized, aluminous and calc-alkalic and resulted from the dehydration melting of tonalitic to granodioritic rocks under water-subsaturated and oxidizing environments. In combination with previous work, the ca. 880–860Ma Beiba gabbros and Tianpinghe I-type granites are interpreted to be formed during the stage of initial back-arc extension that was caused by subduction-induced convection in the mantle wedge. Subsequently, the ca. 780–770Ma Tiechuanshan and Huangguan A-type granites and Taojiaba granodiorites were formed in a continental back-arc rift setting related to the rollback of subducted oceanic slab. Our new results support that an arc-back-arc system developed in the northwestern Yangtze Block during the Neoproterozoic.

The South Um Mongul Cu-Mo-Au prospect in the Eastern Desert of Egypt: From a mid-Cryogenian continental arc to Ediacaran post-collisional appinite-high Ba-Sr monzogranite

The South Um Mongul prospect is a Cu-Mo-Au porphyry system. It is covered by porphyritic dacite and hornblende gabbro. Both units are intruded by monzogranite, which encloses xenoliths of both units. Using LA-ICP-MS U-Pb zircon method, the dacite is dated at ca. 773±6.9Ma, while the gabbro and the monzogranite are dated at 603±3.5 and 558±4.6Ma, respectively. The dacite age is consistent with the mid-Cryogenian subduction-related magmatic stage and the gabbro-monzogranite age is comparable to the Ediacaran post-collisional magmatic stage during the evolution of the Arabian-Nubian Shield. The dacite is akin to high-K I-type granitoids and its primitive mantle-normalized trace element patterns show negative Nb anomalies and enrichment in LILE (large ion lithophile elements), Th and U over HFSE. These geochemical characteristics are similar to those of felsic magma formed in a subduction-related tectonic setting. The high La/Ybcn (7.2–30.9), Nb/Yb (2.63–4.41) and Th/Yb (2.07–3.04) ratios of the dacite are comparable to continental rather than oceanic arc systems. Its low Sm/Yb ratios (1.84–3.13) support the primitive nature of the crust beneath the continental arc and derivation from a garnet-free lower crustal source. The dacite has low Sr/Y ratios (5–9) and its Eu/Eu⁎ ratios range from 0.66 to 0.83. Similar to dacite, the primitive mantle-normalized trace element patterns of the post-collisional suite show a subduction-related geochemical signature. However, the gabbro is characterized by Th/Ta ratios (3.4–14.8), which are comparable with the within-plate tectonic setting. The subduction-related geochemical signature is inherited from long subduction history beneath the Arabian-Nubian Shield. Both the gabbro and monzogranite are characterized by high Ba (404–590ppm and 936–1590ppm, respectively) and Sr (611–708ppm and 624–793ppm, respectively) contents, which make them analogous to the Caledonian appinite-high Ba-Sr granite assemblage. The formation of these rocks is related to the Ediacaran lithospheric erosion accompanying slab break-off. This process induced asthenospheric upwelling, which led to partial melting of the lithosphere previously metasomatised by subducted sediments involving carbonates impregnated by hydrothermal barite. Melting of this lithosphere led to the formation of the hornblende gabbro. Underplating by the mafic magma led to melting of the lower crust and the formation of high Ba-Sr monzogranite in the area. The high Sm/Yb (2.94–4.19) and Sr/Y (52–74) ratios of the monzogranite may indicate the presence of garnet in the melted amphibolitic lower crust. The higher Sr/Y ratios, lower HFSE (high field strength elements) contents and the absence of pronounced Eu anomalies in monzogranite relative to dacite suggest the productive nature of the post-collisional magma relative to the continental arc magma in this prospect.

Geochemistry, petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen, South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834Ma) and the late Neoproterozoic (770–750Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive εNd(t) values (+2.7 to +6.6) whereas mafic rocks exhibit negative εNd(t) values (−5.8 to −0.9). The basaltic rocks show TiO2 contents of 0.62–0.69wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show εNd(t) values (−1.23 to +3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO2 contents of 1.01–1.33wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750Ma.

10Be, 18O and radiogenic isotopic constraints on the origin of adakitic signatures: a case study from Solander and Little Solander Islands, New Zealand

Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands, south of mainland New Zealand, are porphyritic trachyandesites and andesites (58.20–62.19 wt% SiO2) with phenocrysts of amphibole, plagioclase and biotite. The Solander and Little Solander rocks are incompatible element enriched (e.g. Sr ~931–2,270 ppm, Ba ~619–798 ppm, Th ~8.7–21.4 ppm and La ~24.3–97.2 ppm) with MORB-like Sr and Nd isotopic signatures. Isotopically similar quench-textured enclaves reflect mixing with intermediate (basaltic-andesite) magmas. The Solander rocks have geochemical affinities with adakites (e.g. high Sr/Y and low Y), whose origin is often attributed to partial melting of subducted oceanic crust. Solander sits on isotopically distinct continental crust, thus excluding partial melting of the lower crust in the genesis of the magmas. Furthermore, the incompatible element enrichments of the Solander rocks are inconsistent with partial melting of newly underplated mafic lower crust; reproduction of their major element compositions would require unrealistically high degrees of partial melting. A similar argument precludes partial melting of the subducting oceanic crust and the inability to match the observed trace element patterns in the presence of residual garnet or plagioclase. Alternatively, an enriched end member of depleted MORB mantle source is inferred from Sr, Nd and Pb isotopic compositions, trace element enrichments and eHf ≫ 0 CHUR in detrital zircons, sourced from the volcanics. 10Be and Sr, Nd and Pb isotopic systematics are inconsistent with significant sediment involvement in the source region. The trace element enrichments and MORB-like Sr and Nd isotopic characteristics of the Solander rocks require a strong fractionation mechanism to impart the high incompatible element concentrations and subduction-related (e.g. high LILE/HFSE) geochemical signatures of the Solander magmas. Trace element modelling shows that this can be achieved by very low degrees of melting of a peridotitic source enriched by the addition of a slab-derived melt. Subsequent open-system fractionation, involving a key role for mafic magma recharge, resulted in the evolved andesitic adakites.

Subduction-related enrichment of the Neapolitan volcanoes (Southern Italy) mantle source: New constraints on the characteristics of the slab-derived components

The Neapolitan volcanic area (Southern Italy), which includes the Phlegrean Volcanic District and the Somma–Vesuvius complex, has been the site of intense Plio-Quaternary magmatic activity and has produced volcanic rocks with a subduction-related geochemical and isotopic signature. High-Mg, K-basaltic lithic lava fragments dispersed within hydromagmatic tuff of the Solchiaro eruption (Procida Island) provide constraints on the nature and role of both the mantle source prior to enrichment and the subduction-related components. The geochemical data (Nb/Yb, Nb/Y, Zr/Hf) indicate a pre-enrichment source similar to that of enriched MORB mantle. In order to constrain the characteristics of subducted slab-derived components added to this mantle sector, new geochemical and Sr–Nd-isotopic data have been acquired on meta-sediments and pillow lavas from Timpa delle Murge ophiolites. These represent fragments of Tethyan oceanic crust (basalts and sediments) obducted during the Apennine orogeny, and may be similar to sediments subducted during the closure of the Tethys Ocean. Based on trace element compositions (e.g., Th/Nd, Nb/Th, Yb/Th and Ba/Th) and Nd-isotopic ratio, we hypothesize the addition of several distinct subducted slab-derived components to the mantle wedge: partial melts from shales and limestones, and aqueous fluids from shales, but the most important contribution is provided by melts from pelitic sediments. Also, trace elements and Sr–Nd-isotopic ratios seem to rule out a significant role for altered oceanic crust. Modeling based on variations of trace elements and isotopic ratios indicates that the pre-subduction mantle source of the Phlegrean Volcanic District and Somma–Vesuvius was enriched by 2–4% of subducted slab-derived components. This enrichment event might have stabilized amphibole and/or phlogopite in the mantle source. 6% degree of partial melting of a phlogopite-bearing enriched source, occurring initially in the garnet stability field and then in the spinel stability field can generate a melt with trace elements and Sr–Nd-isotopic features matching those of high-Mg, K-basalts of Procida Island. Furthermore, 2% partial melting of the same enriched source can reproduce the trace elements and isotopic features of the most primitive magmas of Somma–Vesuvius, subsequently modified by assimilation of continental crust during fractional crystallization processes at mid-lower depth. Combined trace element and Sr–Nd isotope modeling constrains the age of the enrichment event to 45Ma ago, suggesting that the Plio-Quaternary magmatism of the Neapolitan area is post-orogenic, and related to the subduction of oceanic crust belonging to the Tethys Ocean.

Geochemistry of Silurian-Devonian volcanic rocks in the Coastal Volcanic belt, Machias-Eastport area, Maine: Evidence for a pre-Acadian arc

The Coastal Volcanic belt in easternmost Maine consists of Silurian–Devonian volcanic rocks related to the convergence of Avalonia with the leading edge of composite Laurentia just before and during early stages of the Acadian orogeny. New major- and trace-element analyses from the Machias-Eastport area indicate that the Early to Late Silurian volcanic rocks in the Dennys, Edmunds, and Leighton Formations formed in a continental arc environment, in contrast to some earlier interpretations. These rocks have a continuous compositional range from basalt, through andesite, to rhyolite. The mafic rocks in these formations are calc-alkaline and show subduction-related geochemical signatures on various discrimination diagrams, whereas the felsic rocks have geochemical characteristics of volcanic arc granites. In contrast, the thick volcanic deposits of the younger Late Silurian Eastport Formation are bimodal and tholeiitic and have transitional to within-plate geochemical signatures. Thus, the transition over time, as the Acadian seaway closed, from subduction and arc settings to continental extension can been seen in this rock sequence, and it constrains the time of Avalonia’s docking. Regionally, volcanic rocks in the Kingston terrane of southern New Brunswick are likely correlatives of the earlier arc-volcanic rocks (Dennys, Edmunds, and Leighton Formations), while Eastport Formation analogues are found in the coastal Maine magmatic province, Cranberry Island series, and probably the Passamaquoddy Bay volcanic series, which similarly appear to have formed after the transition to an extensional environment occurred, although other extensional settings cannot be entirely ruled out. Basalts in the post-Acadian Perry Formation indicate continental extension was also active in the Late Devonian.

Pulling apart the Mid to Late Cenozoic magmatic record of the Gulf of California: is there a Comondú Arc?

AbstractThe composition of the lithosphere can be fundamentally altered by long-lived subduction processes such that subduction-modified lithosphere can survive for hundreds of millions of years. Incorrect petrotectonic interpretations result when spatial–temporal–compositional trends of, and source contributions to, magmatism are not properly considered. Western Mexico has had protracted Cenozoic magmatism developed mostly in-board of active oceanic plate subduction beneath western North America. A broad range of igneous compositions from basalt to high-silica rhyolite were erupted with intermediate to silicic compositions in particular, showing calc-alkaline and other typical subduction-related geochemical signatures. A major Oligocene rhyolitic ignimbrite ‘flare-up’ (&gt;300 000 km3) switched to a bimodal volcanic phase in the Early Miocene (c.100 000 km3), associated with distributed extension and opening of numerous graben. Extension became more focusedc.18 Ma resulting in localized volcanic activity along the future site of the Gulf of California. This localized volcanism (known as the Comondú ‘arc’) was dominantly effusive and andesite–dacite in composition. Past tectonic interpretations of Comondú-age volcanism may have been incorrect as these regional temporal–compositional changes are alternatively interpreted as a result of increased mixing of mantle-derived basaltic and crust-derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting Guadalupe Plate.Supplementary material:References from which whole-rock geochemical and radiometric age data have been compiled in this paper are available athttp://www.geolsoc.org.uk/SUP18645