Fractionation Of Hornblende Research Articles

Diverse apatite geochemical compositions in early Paleozoic granitoids of the North Qinling orogen, China: Insights into their petrogenesis and magma sources

Apatite geochemistry in granitoids is controlled primarily by the original melt compositions and partition coefficients. However, additional factors (e.g., post-crystallization effects, mineral crystallization sequences, and crustal assimilation) can also contribute to variations in apatite compositions. In this study, we collected three groups of early Paleozoic granitoids from the North Qinling Orogen and obtained in situ geochemical data for apatite. The granitoids were formed by partial melting of oceanic arc crust (group A), recycling of ancient crust (group B), and melting of the lithospheric mantle and extensive fractionation (group C). Trace element and Nd isotope data show the apatite from the Taiping and Manziying plutons (group A) were affected by post-crystallization processes and crustal assimilation, respectively. In addition, crystallization of plagioclase/allanite and apatite resulted in the variable Eu anomalies and rare earth element contents of apatite in the group A granitoids. In contrast, apatite compositions in group C were controlled primarily by the fractionation of hornblende. However, the decoupling of Eu anomalies of apatite and their host rocks, particularly for group B, cannot be easily explained by the aforementioned factors. As such, the redox state may also have been an important factor. We used a ternary diagram (10 × [Eu/Eu*]N–V–Ga) to constrain the redox state from the apatite data, and provide insights into the types and petrogenesis of the host granitoids. Combined with the whole-rock and zircon geochemical data, we found that the apatite from the rocks derived from continental crust (group B and cratonic adakites) is generally reduced, whereas apatite from the mafic rocks and high BaSr granitoids is highly oxidized. The highest degree of oxidation was found for the altered apatite from the group A granitoids and unaltered apatite in the orogenic adakites, possibly due to interactions with fluid derived from oceanic crust. Moreover, unaltered group A granitoids exhibit a transitional trend from an oxidized to a reduced state. Apatite geochemistry can be used to constrain the redox state of adakitic rocks and whether they have a continental or oceanic crustal origin. The high Sr contents of high BaSr granitoids are inherited from their mantle source.

Petrogenesis of the Early Paleozoic Dioritic–Granitic Magmatism in the Eastern North Qilian Orogen, NW China: Implications for Tethyan Tectonic Evolution

Abstract The North Qilian Orogen witnessed the opening, subduction, and closure of the Proto-Tethys Qilian Ocean and the post-subduction of multiple exhumation events from Late Neoproterozoic to Early Paleozoic. The Early Paleozoic dioritic–granitic magmatic suites, prominently exposed in the eastern North Qilian Orogen, offer valuable insights into the evolution of the Proto-Tethys Ocean. However, their petrogenesis, magma source, and tectonic evolution remain controversial. Here, we investigate the Leigongshan, Zhigou, and Dalongcun intrusions and present geochronological, geochemical, and isotopic data, aiming to refine the comprehension of their timing and petrogenesis, which will contribute to understanding the tectonic evolution of the Proto-Tethys Ocean. Zircon U-Pb dating reveals mean ages of 471–427 Ma for these intrusions, consistent with compiled formation ages of dioritic–granitic intrusions in the eastern North Qilian Orogen, indicating close temporal links with the tectonic evolution of the Proto-Tethys Ocean during the Early Paleozoic. The studied magmatic rocks could be categorized into two major types: granitoids and diorites. The granitoids are majorly I-type granitoids that are generated through partial melting of the mafic lower crust and fractional crystallization at the middle-upper crust, with the involvement of mantle-derived materials. The diorites underwent limited crustal contamination and fractionation of hornblende, plagioclase, and some accessory minerals. They were derived mainly from the mixture of fertile mantle and reworked crustal components, with minor contributions from subduction-related slab fluids and sediment melts. In addition, all the studied Early Paleozoic dioritic–granitic intrusions (ca. 471–427 Ma) formed within subduction-related arc settings. Combined with the tectonic evolution of the Early Paleozoic Qilian orogenic system, we interpret these Cambrian to Silurian dioritic–granitic intrusions as tectonic responses to the subduction (ca. 520–460 Ma) and closure (~440 Ma) of the Proto-Tethys Ocean, whereas the Devonian Huangyanghe intrusion witnessed the final stage of extensional collapse of the Qilian orogenic system at ca. 400–360 Ma.

Local controls on magma fertility in the Mio-Pliocene metallogenic belt of central Chile: Exploration implications

Understanding what produces and how to detect fertile magmas, capable of forming large, economic porphyry-type deposits, is not only a major scientific quest, but it is also relevant economically, as it could lead to better exploration models and higher success rates in mineral exploration. In this work, we use new and previous geochemical and geochronological data from the Mio-Pliocene metallogenic belt of Central Chile to assess the existing tools to detect fertile magmas, study their spatiotemporal variations at different scales and discuss the geological and exploration implications of our findings.Our results suggest that the geochemical ratios currently used to detect fertile magmas are effective, as there is a good correlation between fertile signatures and the presence of known porphyry Cu deposits and prospects, with few false positives, which might be explained by the exhumation and erosion of most of the metal-rich part of the hydrothermal systems.Regarding the genesis of fertile magmas, the spatiotemporal variations we document here are not satisfactorily explained by the processes commonly brought forward to produce this type of intrusions, such as crustal thickening and subduction of sediments, crustal fragments or oceanic fracture zones. These processes operate at regional- or continental-scales in entire arc segments, while magma fertility shows strong contrasts in several coeval intrusions separated only by a few kilometers or tens of kilometers, emplaced within the same arc segment, under the same tectonic regime, crustal thickness and subduction parameters. This implies that there is a strong local control on the production of fertile magmas.Our results support the notion that the most important variable affecting the generation of fertile magmas in subduction margins, is magma residence time at different crustal levels. This variable has a strong local control, as it will depend not only on the overall crustal thickness or the regional tectonic regime, but also on the local orientation of structural pathways controlling magma ascent through the crust, relative to the predominant stress tensor. If magma pathways are oriented at higher angles relative to the maximum stress, magma residence times will be longer, allowing the fractionation of abundant hornblende (giving the residual magma its characteristic fertile or “adakite-like” signature) and the accumulation of magmatic sulfides which can be remobilized by later magmatic pulses; becoming progressively more oxidized due to intra-crustal processes; and concentrating large amounts of incompatible volatiles and metals in the residual melts. This can explain the strong variability observed in magma fertility in nearby coeval intrusions, and also in cross-cutting intrusions separated by very short time periods, which might have followed different ascent pathways.Our results also suggest that the Teno-Maule segment, in the southernmost part of the Mio-Pliocene metallogenic belt, shows striking differences in magma fertility patterns compared with the rest of the belt. There, fertile magmas are scarce and, contrary to the northern part of the belt, there is no evidence of an enhanced magma fertility during the late Miocene – early Pliocene.

Petrogenesis of the Morobe Granodiorite and their shoshonitic mafic microgranular enclaves in Maramuni arc, Papua New Guinea

Abstract The Miocene tectonics of Papua New Guinea, where subduction, arc-continent collision, and changes in subduction direction are considered to have occurred, is very complex and various tectonic models have been proposed. The Maramuni arc, active in the Miocene, is composed of a chain of granitoid bodies. As the chain-like distribution indicates the generation of igneous activities in a wide range of the same tectonic settings, the study of the Maramuni arc magmatism is important for elucidating the geologic events of the time. We provide data on the petrological and geochemical characteristics of the Morobe Granodiorite that form part of the Maramuni arc. The Morobe Granodiorite consists of metaluminous I-type granitoids, belonging to the medium-K to high-K series. The whole-rock major element variations in the granitoids can be explained by the fractionation of hornblende and plagioclase. They are generally within the composition range of experimental partial melts of amphibolites, and the whole-rock trace element compositions have characteristics of slab failure magma rather than arc. This suggests that the granitoids were generated by partial melting of the torn slab after slab failure. The mafic microgranular enclaves (MMEs) in the granitoids are classified as shoshonite, and their trace element compositions suggest that they were formed by partial melting of phlogopite-bearing mantle. The occurrences of native gold and barite within the MME show that MMEs transport Au from the mantle metasomatized by slab-derived sediment melt and/or fluid to the crustal magma chamber.

The Petrology and Geochemistry of REE-Enriched, Alkaline Volcanic Rocks of Ambitle Island, Feni Island Group, Papua New Guinea

Ambitle in the Feni Island Group is located within the NW trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, Melanesian Arc, northeastern Papua New Guinea. The TLTF chain is renowned for its alkaline magmatism, geothermal activity, copper–gold mineralization, and world-class gold mining. Although its geochemical patterns indicate island arc signatures (i.e., high LILE and depleted HFSE), TLTF volcanism is not directly related to the older Melanesian Arc subduction system. However, it may have been influenced by source mantle metasomatism linked to the older subduction. The purpose of this study is to (1) present and interpret the petrographic, mineralogical, and geochemical data from Feni within the context of the tectonic evolution of the TLTF and (2) propose a geodynamic, petrogenetic model for the Feni volcanic rocks. The key methodologies used in this study are field mapping and sampling, petrographic analysis using the optical microscope, whole-rock geochemical analysis via XRF and ICP MS, and mineralogical analysis using an electron microprobe. The main rock types sampled in this study include feldspathoid-bearing basalt, trachybasalt, phonotephrite, trachyandesite, and trachydacite. Minerals identified include forsteritic olivine, diopside, augite, labradorite, andesine, anorthitic plagioclase, nepheline, and leucite in the primitive mafic suites, whereas the more evolved intermediate and felsic hypabyssal suites contain amphibole, albite, orthoclase, biotite, and either rare quartz or feldspathoids. Amphibole composition is primarily magnesiohastingsite with minor pargasite formed under polybaric conditions. Accessory minerals include apatite, titanite, and Ti-magnetite. We propose that limestone assimilation followed by fractional crystallization are plausible dominant processes in the geochemical evolution of the Ambitle volcanics. Clinopyroxene fractionation is dominant in the mafic volcanics whereas hornblende fractionation is a major petrologic process within the intermediate suites proven by the enrichment of LREE and depletions in MREE and HREE. Feni magmas are also highly enriched in REEs relative to neighboring arcs. This study is globally significant as alkaline magmas are important sources of Cu, Au, and REE as critical elements for green energy and modern technology.

Wet calc-alkaline magmatic fractionation in the middle−upper crustal sections of the continental arc: Insights from the Neoproterozoic Nanba intrusive complex, western Yangtze Block, South China

Elucidating the petrological and geochemical characteristics of continental arc crustal fragments can provide unique insights into the magmatic evolutional path and physicochemical conditions (e.g., P−T−H2O) of continental magmatic arcs. This contribution provides a comprehensive set of petrological, mineralogical, geochronological, and geochemical data for the Neoproterozoic Nanba intrusive complex of the western Yangtze Block, South China, which features well-exposed middle−upper crustal sections of the Neoproterozoic continental magmatic arc, to evaluate their magmatic source and fractionation process. The Neoproterozoic Nanba intrusive complex, composed of cumulate gabbros, hornblende gabbros, gabbro-diorites, diorites, granodiorites, K-feldspar granites, and monzogranites, was emplaced at ca. 796−790 Ma. The different lithologies of the Nanba intrusive complex display relatively homogeneous Sr-Nd-Hf isotopes, which indicates a cogenetic evolutional process. The predominantly depleted isotopic characteristics, together with high values of Ba, Rb/Y, and Ba/La, demonstrate that these rocks mainly originated from a subduction fluid−metasomatized mantle source. Petrological and geochemical characteristics, as well as hornblende thermobarometric data, collectively support that the ca. 796−790 Ma Nanba intrusive complex, following the calc-alkaline trend, was dominated by the wet cogenetic fractionation of hornblende accompanied by plagioclase and accessory minerals beneath the middle−upper crustal levels (2.4−4.7 kbar). The cumulate gabbros represent the residue of accumulated hornblende + plagioclase + Fe-Ti oxides from fractionated basaltic melts. The high-SiO2 K-feldspar granites and monzogranites display complementary trace elements, which represent residual silicic cumulates and extracted interstitial liquids in the shallower crystal mush reservoir, respectively. In combination with previous studies, we propose that the middle−upper crustal sections of the Neoproterozoic continental arc of the western Yangtze Block underwent hornblende-dominated fractionation that was followed by significant crystal-liquid separation, which led to the genesis of high-SiO2 melts at the shallower crustal level. The cogenetic evolution played a significant role in molding the petrological and geochemical diversity of Neoproterozoic igneous rocks and the differentiation process of continental arc crust in the western Yangtze Block.

The statherian anorogenic magmatism in the Paramirim Aulacogen, são Francisco-Congo paleoplate: New data, synthesis and regional correlations

Fragmentation of continental masses is usually associated with anorogenic magmatism, and in this context, major silicic magmatic events may be associated with Large Igneous Provinces (LIPs). The Paramirim Aulacogen evolves between 1.77 and 0.67 Ga, filling anorogenic siliciclastic and acidic volcanic rocks with subordinate carbonates. A rhyolite from the Algodão Formation, the oldest unit of the Aulacogen, was dated in this work using the U–Pb method (LA-ICPMS), finding a crystallization age of 1777±1 Ma, with a model age (TDM) of 3.10 Ga and εNd (t) equals −10.4. The anorogenic plutonic magmatism is represented by the Lagoa Real Intrusive Suite, with over 5000 km2. New geochemical and geochronological data are presented, and the integration with the literature allowed the interpretation of four plutonic magmatism cycles: (i) Cycle 1: represented by Lagoa do Barro granitoids (1762 ± 9 Ma) and the metavolcanic rocks of the Algodão Formation (1775 ± 77–1777±1 Ma); (ii) Cycle 2: with the São Timóteo (1741 ± 4–1746 ± 5 Ma) and Caetité (1743 ± 5 Ma) granitoid and syenitoid, in addition to charnockitic rocks (1742±7 Ma); (iii) Cycle 3: is related to phonotephrite, tephriphonolite and phonolite. Trachyte, trachyandesite and trachydacite of São Simão Formation (1731 ± 5 Ma) and tuff layer of Ouricuri do Ouro Formation (1722 ± 13 Ma), as well as rhyolites and dacites that ocurr in the basement of Paramirim Aulacogen. The Caldeirão granitoid and syenitoid (1724 ± 5 Ma) is associated with this cycle; and (iv) Cycle 4: with the Suçuarana intusive rocks (1686 ± 6 Ma) and pyroclastic rocks of trachyandesitic composition. The Sm–Nd model ages (TDM) obtained in this work and the literature varies between 3.1 and 2.2 Ga, with values of εNd (t) between −15.8 and −8.6. The formation of the Statherian magmas possibly occurred by melting the enriched mafic mantle, followed by underplating in the continental crust and assimilation of Archean crustal rocks, with fractionation of hornblende, biotite, apatite, and Fe–Ti oxides during magma evolution. The Lagoa Real Intrusive Suite, and their volcanic correlates, represent part of a Statherian Silicic Large Igneous Province (SLIP) developed in a crust of the Archean-Rhyacian-Orosirian age of the São Francisco-Congo Paleoplate. This magmatism marks the attempt at the breakup of the Columbia supercontinent, with correlates in other regions of Brazil and the world.

Growth of the upper crust in intra-oceanic island arcs by intrusion of basaltic magmas: the case of the Koloula Igneous Complex, Guadalcanal, Solomon Islands (SW Pacific)

The Pleistocene (2.2–1.5 Ma) Koloula Igneous Complex (KIC) on Guadalcanal in the Solomon island arc consists of a low-K calc-alkaline sequence of ultramafic to felsic plutonic rocks. We present whole-rock major and trace element and Sr–Nd-Pb isotope data, as well as mineral compositions that record the magmatic evolution of the complex. The intrusive sequence is grouped into two cycles, Cycle 1 and 2, comprising gabbroic or dioritic to granodioritic rocks. The major and trace element data of each cycle forms a single calc-alkaline fractional crystallisation trend. The distinct radiogenic isotope and incompatible element compositions of the Cycle 1 and 2 intrusions imply slightly different mantle sources. The KIC formed by shallow (0.1 GPa) fractional crystallisation of mantle-derived Al-rich basaltic parental magmas (6–8 wt.% MgO) that were formed by deeper-level (0.7 GPa) fractionation of olivine and pyroxene from Mg-rich (~ 11 wt.% MgO) primary magmas in the Solomon intra-oceanic island arc. Olivine, clinopyroxene, plagioclase, amphibole, biotite, apatite, and Fe–Ti oxides fractionated from the KIC’s high-Al basaltic parental magmas to form calc-alkaline magmas. Liquid line of descent trends calculated using mass balance calculations closely match major element trends observed in the KIC data. The KIC crystallised at shallow, upper crustal depths of ~ 2.0–3.0 km in ~ 20 km-thick island arc crust. This complex is typical of other Cenozoic calc-alkaline ultramafic to felsic plutons in Pacific intra-oceanic island arcs in terms of field relationships, petrology, mineral chemistry and whole-rock geochemistry. Hornblende fractionation played a significant role in the formation of the calc-alkaline felsic plutonic rocks in these Cenozoic arc plutons, causing an enrichment of SiO2 and light rare earth elements. These plutons represent the fossil magma systems of arc volcanoes; thus, the upper arc crust is probably generated by migration of magmatic centres.

Formation of the Qiyugou porphyry gold system in East Qinling, China: insights from timing and source characteristics of Late Mesozoic magmatism

The Qiyugou gold deposit, located in the East Qinling Orogen, is characterized by porphyry, breccia pipe-hosted and fracture-controlled hydrothermal mineralization. Four intrusive phases are identified in this deposit: pre-mineralization quartz porphyry, syn-mineralization granite porphyry associated with breccia pipe-hosted mineralization, syn-mineralization hornblende monzogranite associated with porphyry mineralization and post-mineralization monzogranite porphyry. These granitoids are metaluminous, alkalic to calc-alkalic, shoshonitic to high-K series, and belong to highly fractionated I-type granitoids. Most of the Qiyugou granitoids show relatively lower Sr/Y (<40) and (La/Yb) N (average = 12.0) than the typical adakite-like rocks. The granitoids are strongly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) and depleted in heavy rare earth elements (HREE) with slightly negative europium anomalies (except for hornblende monzogranite) and moderate depletion of high field strength elements (HFSE), mostly plotting in the range of the basement Taihua Group rocks. Zircon U–Pb ages of granite porphyry, hornblende monzogranite and monzogranite porphyry from the Qi189 pluton, and hornblende monzogranite beneath the J4 breccia pipe are 133.4 ± 0.8, 131.3 ± 0.9, 128.0 ± 0.7 and 129.9 ± 0.9 Ma, respectively. The ε Hf ( t ) values and T DM2 ages show a variation trend from −37.4 to −11.4 and from 3530 to 1912 Ma, consistent with Hf isotopic features of the Taihua Group. The ε Hf ( t ) values have a positive correlation with MgO and negative correlation with SiO 2 , indicating sustained involvement of mantle-derived materials. Notably, these granitoids have comparable δ 18 O zircon values ranging from 5.7 to 5.9‰. The covariations of geochemical and isotopic data indicate that these granitoids were produced by mixing of the partially melted Taihua Group basement with mantle-derived components. The Qiyugou granitoids were generated at different crustal levels by a lithospheric thinning process linked to the tectonic transition from compression to extension at 133–128 Ma. The hornblende monzogranite from the Qi189 pluton was probably more hydrous and resulted from a highly evolved magma after hornblende fractionation, which was favourable for porphyry mineralization. Our study provides insights into the formation of a porphyry gold system associated with a prolonged and active magmatic–hydrothermal system, upwelling of hot asthenosphere and intense mantle–crust interaction. Supplementary material : Tables giving compositions of the granitoids are available at https://doi.org/10.6084/m9.figshare.c.5835494

Magmatic origin and petrogenesis characterization of syenite rock from Pakkanadu alkaline complex, Southern Granulite Terrain, India: Implication on emplacement and petrogenetic history

The present study mainly focused on understanding the magmatic origin and petrogenesis characterization based on the Petrography, major, trace and Rare Earth Element (REE) signatures in the alkaline syenite from Pakkanadu alkaline carbonatite complex. The alkaline plutons from South Indian granulite terrain are intruded along with Archaean epidote-hornblende gneisses. The study area was carbonatite complexes of Tamil Nadu and is characterized by a group of rock associations Carbonatite-Syenite-Pyroxenite - Dunite. From Harker various patterns Pakkanadu alkaline complex syenite showed increasing trends of SiO2, Al2O3, Na2O + K2O opposite to decreasing order of CaO, Fe2O3, MgO, TiO2, P2O5 and MnO trend, suggest fractionation of clinopyroxene, hornblende, sphene, apatite and oxide minerals and feldspar that ruled the fractionation. The concentration of trace elements enriched in Large Ion lithophile elements (LILE) (Ba, Sr, and Rb) elements and High Field Strength Elements (HFSEs) indicated that the dyke intrusion by differentiation of magma from a mantle source. Rare earth element (REE) distribution of Light rare earth element (LREE) enriched and High rare earth element (HREE) depleted pattern show strongly fractionated pattern with moderate Eu anomalies. Plots of tectonic discrimination diagrams of Pakkanadu samples fall in the field of syn-COLG field to the VAG syn- COLG field. For the first time, this type of study was carried out in the study region in a detailed manner. The present study significantly exposed the petrography, petrogenesis and magmatic origin process in the Pakkanadu alkaline carbonatite complex.

Geochronology, geochemistry and Sr-Nd-Hf isotopes of the Heihuashan granite porphyry, NE China, and implications for Cu-Mo mineralization

Heihuashan is a newly discovered porphyry Cu-Mo prospect in Heilongjiang Province, northeastern China, with considerable Cu and Mo mineralization. Until now the ages of the Cu-Mo ores and the causative granite porphyry, the magma origin and characteristics, as well as the mineralization potential at depth are still unclear. In this contribution, whole rock geochemical and Sr-Nd isotopic compositions, zircon U-Pb ages and Hf isotopes, and molybdenite Re-Os dating, are presented to constrain the timing of the magmatism and mineralization, and the petrogenesis of the causative intrusion. The results also provide an opportunity for evaluating the potential of the deep Cu-Mo mineralization.Zircon U-Pb dating using LA-ICP-MS yields a crystallization age of 146.4 ± 1.8 Ma for the Heihuashan granite porphyry, which is consistent with the molybdenite Re-Os isochron age of 143.7 ± 3.1 Ma. These results indicate that the porphyry Cu-Mo mineralization at Heihuashan occurred at late Jurassic to early Cretaceous. The whole rockgeochemicaldatasuggest that the ore-related intrusion is mildly peraluminous and belongs to the high-Kcalc-alkalineseries, I-type granitoids. The granite porphyry samples are enriched in Rb, Ba, Th, U, and Sr, but depleted in Nb and Ta, showing typical arc magma affinities. They also have initial 87Sr/86Sr ratios of 0.70546–0.70550 and εNd(t) values between +0.4 and +0.7. The zircon εHf(t) values for the granitic intrusion range from +3.5 to +9.2, with two-stage Hf model ages of 977–614 Ma. The geochemical and Sr-Nd-Hf isotopic compositions suggest that the parental magma was dominantly derived from the slab-metasomatized mantle, which could have mixed with some lower crust components.The Fe2O3/FeO ratios of the Heihuashan granite porphyry are relatively high, ranging from 0.52 to 1.17, indicating a high magmatic oxidation state. This is also supported by the high zircon Ce/Ce*, Eu/Eu* and Ce/Nd ratios (33–2433, 0.22–0.70 and 2.4–44.0, respectively) and the high magmatic ΔFMQ values (+0.1 to +3.5 with an average of +2.1) calculated using the zircon trace elements. The intrusion has relatively high Sr/Y ratios (40–94) and displays negligible Eu anomalies (Eu/Eu* = 0.68–1.07); the zircons from the intrusion have high (Eu/Eu*)/Y (mostly > 0.0001) and (Ce/Nd)/Y ratios (mostly > 0.01), but low Dy/Yb ratios (generally < 0.4). These suggest that the fractionation of hornblende rather than plagioclase is dominant during the magma crystallization, which reveals a high magmatic water content. The above results indicate that the ore-related magma at Heihuashan is oxidized and hydrous, both of which are critical for porphyry Cu-Mo mineralization. The Heihuashan granite porphyry is geochemically similar to other Cu-related intrusions in NE China and also comparable to many porphyry Cu (-Mo) systems worldwide, implying a high Cu-Mo mineralization potential. Considering the surface and shallow levels at Heihuashan are characterized dominantly by sericite alteration, we suggest exploration and drilling focusing on the deeper levels beneath the currently mineralized domains, where major potassic alteration zones, likely associated with more extensive Cu-Mo mineralization, could be discovered.

Zircon U–Pb dating, geochemical, and Sr–Nd–Pb–Hf isotopic constraints on the age and origin of intermediate to felsic igneous rocks at South Altyn, Xinjiang, China

As a part of a giant trending fault system in the Asian continent and one where a strong zone of left strike-slip fault is present, the Altyn Orogenic belt (AOB) has become an important focus for research. Magmatic rocks are widely distributed across the AOB. However, many investigations have focused primarily on Paleozoic igneous rocks; discussion of Mesozoic related igneous activity is often ignored. Here we present the result of studies of representative diorite and granite rocks outcropping in the AOB, within the Xinjiang Uygur Autonomous Region, South Altyn, China. We present new zircon LA-ICP-MS U–Pb age, geochemical, and Sr–Nd–Pb–Hf isotopic data for these sample suites, identifying them as typical igneous rocks formed between 238 ± 1.5 and 238.8 ± 1.1 Ma. The rocks that we studied fall into the alkaline series, also enriched in light rare earth elements (LREE), some large ion lithophile elements (LILE; e.g., Rb, Ba, Sr, and K), Pb, Th and U, and depleted in heavy rare earth elements (HREE), Nb, Ta, Hf, and Ti. The granite and diorite have high initial 87Sr/86Sr ratios (0.7062–0.7114), negative eNd (t) values (− 8.8 to − 11.3), eHf (t) values (− 8.7 to − 18.7), and relatively constant Pb isotopic ratios ((206Pb/204Pb)i = 6.74–17.884, (207Pb/204Pb)i = 15.51–15.58, and (208Pb/204Pb)i = 35.36–38.04), respectively. This suggests that the magmas parental to these rocks were generated from the partial melting of the ancient crust. The parental magmas to these rocks experienced a degree of fractionation of plagioclase, K-feldspar, and hornblende, possibly during rapid magma ascent. Based on these studies, we propose a reasonable model for the origin of the investigated rocks from the Xinjiang Uygur Autonomous Region of South Altyn, which involves crustal thickening, lithospheric extension, and asthenosphere upwelling, that induced crustal melting.

Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration

The North Qiangtang continental block in central Tibet is a critical piece of the Pangea puzzle. This paper uses integrated geochronological and geochemical data for selected mafic dykes and dioritic enclaves in this block to evaluate its tectonic evolution in the Triassic. Zircons from two mafic dykes and the dioritic enclaves of a large arc granodiorite pluton in eastern North Qiangtang yield indistinguishable U-Pb ages from 248 ± 2 to 251 ± 3 Ma, contemporaneous with widespread arc basaltic andesites and crust-derived rhyolites in the region. The mafic dykes and coeval arc basaltic andesites have almost identical Sr-Nd isotopes (initial 87Sr/86Sr = 0.707 to 0.708, εNd = −4.4 to −3.6), and are all characterized by light REE enrichments and pronounced negative Nb-Ta anomalies. The dioritic enclaves and the hosts have indistinguishable zircon U-Pb ages, almost identical Sr-Nd isotopes (initial 87Sr/86Sr = 0.709 to 0.711, εNd = −7.4 to −5.9), and similar zircon εHf (−13.7 to −5.7), but contrasting chondrite-normalized REE patterns due to hornblende fractionation. The Sr-Nd isotope data indicate that the dioritic enclaves formed from the hybrid melts produced by mixing at depth between the arc basaltic andesites and the crust-derived rhyolites. We propose that the Early Triassic arc igneous suites are related to the northward subduction of the southern Paleo-Tethys beneath the North Qiangtang block from Early to Middle Triassic. The occurrence of several Late Triassic porphyry Cu deposits plus a VMS Ag-Pb-Zn deposit in the Yidun arc, which is the product of the southward subduction of the northern Paleo-Tethys beneath the North Qiangtang block in the Late Triassic, indicates that the arc magmas generated during the subduction of the Paleo-Tethys are fertile in ore metals. Therefore, exploration for Early–Middle Triassic porphyry Cu and VMS deposits in the southern part of the North Qiangtang block is warranted.

Generation and maturation of Mesoarchean continental crust in the Anshan Complex, North China Craton

The mechanisms underlying the generation of tonalite–trondhjemite–granodiorite suites and the transition to K-rich granitoids, which marks the most profound stage of Archean intracrustal differentiation, remain controversial. The Archean Anshan Complex (AAC) of the North China Craton (NCC) preserves diverse 3.8–2.5 Ga granitoids, providing an excellent opportunity to address these problems. We conducted a detailed field, petrological, geochemical, and zircon isotopic study on AAC units, demonstrating that: (1) the Li–Dong–Ying trondhjemite intrudes and encloses ca. 3.3 Ga gneiss–migmatite complex across the Anshan, and crystallized at ca. 3.14 Ga; and (2) the porphyritic and fine-grained Chentaigou monzogranites were emplaced contemporaneously with the trondhjemite, both at ca. 3.14 Ga.The Li–Dong–Ying trondhjemites show high SiO2, Na2O, low MgO, significant variations in Sr, Y, and Yb, REE fractionation (LaN/YbN = 9–140), and variable Eu anomalies (Eu/Eu* = 0.4–1.3). The magmatic zircons yield δ18O of +4.8 ± 0.2‰ to +6.9 ± 0.4‰ and initial εHf of −1.1 ± 0.7 to +4.4 ± 0.9, which translate to TDM ages of 3.6–3.3 Ga. The Chentaigou monzogranites have higher SiO2 and K2O, lower Na2O and CaO, similar δ18O of +5.1 ± 0.3‰ to +7.0 ± 0.3‰, and more negative Eu anomalies as compared with the trondhjemites. The Hf–O isotopes, geochemistry, particularly considerable variations in pressure-sensitive proxies of the trondhjemites, suggest that they were derived by melting of low-temperature altered basaltic rocks at 1.0–1.5 GPa and 900–950 °C, leaving a garnet amphibolite residue with variable amounts of plagioclase but no rutile; the most plausible source is ca. 3.3 Ga basaltic rocks. Polybaric melting of basaltic sources and fractionation of hornblende and plagioclase from trondhjemitic magmas account for the chemical variability of AAC granitoids, providing a mechanism for the coeval emplacement of diverse Archean granitoids. Secular compositional changes in AAC trondhjemites imply that the depth and interval of melting of basaltic sources increased from the Eoarchean to Mesoarchean. Changes in the nature of the source and melting depth characterized the early stage of Archean intracrustal differentiation in the NCC.

Petrochemical characterization of Neoproterozoic Colomine granitoids, SE Cameroon: Implications for gold mineralization

Biotite granites and K-feldspar granites are the dominant intrusive rock types of the Colomine gold district in the Neoproterozoic terrain of SE Cameroon. The silica contents of these granitoids range from 68.93–75.39 wt% and Au contents from 0.6 to 32 ppb. They are weakly peraluminous (ASI = 1.0–1.2) I-type granitoids with high-K calc-alkaline to shoshonitic character, emplaced within a syn-collisional arc setting. They have fractionated trace element patterns (LaN/YbN: ~ 11–331) with enrichment in LREE compared to HREE and weak to strong negative Ba, Nb, Sr, P, Ti, Lu, and Eu (Eu/Eu* = 0.12–0.52) anomalies. The geochemical characteristics suggest that the granitoids formed by partial melting of crustal source rocks with some input from greywackes. Differentiation of the hydrous parent magma was characterized by fractionation of muscovite, biotite, hornblende, plagioclase, and K-feldspar with minor zircon, apatite and Fe-Ti oxide. Mafic garnet-bearing inclusions represent an early crystallization phase that was incorporated in the later felsic melt. The granitoids became emplaced into Proterozoic schist belts. Along the contacts, they became brecciated, sheared, foliated and sericitized. These “contact granites” have higher gold contents (5–32 ppb) than the fresh granitoids (0.6–3 ppb). Remobilisation of gold from early magmatic sulfide minerals by post-magmatic hydrothermal fluids enriched the shear-related granitoids. Hydrothermal alteration results to enrichment in Cu, Ni, Cr, Mo, Ga, V, Tl, and Au and depletion in LREE (e.g., La, Ce, Pr). Micro-shear and fault zones in the vicinity of the granitoids constitute favourable sites for exploration.

Eocene tonalite–granodiorite from the Havza (Samsun) area, northern Turkey: adakite-like melts of lithospheric mantle and crust generated in a post-collisional setting

ABSTRACT Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Taşlık Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (± 1.4) and 40.5 (± 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95–1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO2 (67–68 wt%) and Al2O3 (15.5–16.0 wt%) content and Ba/La (17–23), Sr/Y (40.7–61.6), and LaN/YbN (14.4–23.7) ratios and low Y (8.2–9.9 ppm) and YbN (3.1–4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (LaN/LuN = 14.2–21.6) with insignificant negative Eu anomalies (EuN/Eu* = 0.86–1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have 87Sr/86Sr ratios of 0.704767 to 0.704927, 143Nd/144Nd ratios of 0.512767–0.512774, εNd values of (+2.52) – (+2.65), and δ18O values of 7.9–9.7‰. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.

Late Permian intermediate and felsic intrusions in the eastern Central Asian Orogenic Belt: Final-stage magmatic record of Paleo-Asian Oceanic subduction?

The Central Asian Orogenic Belt (CAOB), which formed by closure of the Paleo-Asian Ocean, is one of the largest Phanerozoic accretionary orogens on Earth. However, the timing of final closure of the Paleo-Asian Ocean remains debated. Here we present new zircon UPb dating, oxygen fugacity estimates, and whole-rock geochemical and SrNd isotopic data from intermediate and felsic intrusive rocks in the Chunhua area of NE China to constrain their petrogenesis and assess the tectonic implications for the evolution of the eastern CAOB. Zircon UPb dating indicates that the intrusive rocks formed in the late Permian (257–261 Ma). Based on their chemical compositions, two groups of rocks can be identified. Group Ι consists mainly of dioritic rocks (e.g., gabbroic diorites, diorites and tonalites) with relatively low SiO2 but high MgO contents and Mg# values. Petrological and geochemical characteristics show that Group Ι rocks were formed by mixing of mantle- and crust-derived magmas. Mafic microgranular enclaves within Group Ι rocks have high MgO contents (up to 9 wt%) and depleted isotopic compositions (εNd(t) = +4.3 to +7.2), and probably represent mafic end-member magmas derived from a metasomatized lithospheric mantle, which subsequently were injected into the felsic crustal magmas. In contrast, Group ΙI rocks are mostly granodiorites characterized by high SiO2 and low Mg# values, with high Sr/Y and La/Yb ratios. These rocks were probably generated by partial melting of mafic lower crust with little or no addition of mantle materials. Trace element modeling suggests that intracrustal differentiation was dominated by hornblende fractionation and caused the high Sr/Y and La/Yb signatures of Group ΙI rocks. Their low (87Sr/86Sr)i (0.70345–0.70485) and positive εNd(t) values (+3.8 to +4.6) suggest predominantly juvenile sources, typical of newly underplated mafic lower crust. The low oxidation states (FMQ–1.1 to FMQ–7.9) for both Group Ι and Group ΙI rocks, as indicated by Ce4+/Ce3+ ratios in zircon, imply reducing conditions for the lithospheric mantle and the overlying lower-crustal source rocks. The Chunhua intrusions show petrological and geochemical characteristics of arc-type igneous rocks, including abundant euhedral hydrous minerals, calc-alkaline affinities, and enrichment in large ion lithophile elements (e.g., Rb, Ba, and Pb) and depletion in high field strength elements (e.g., Nb, Ta, and Ti), indicating that they formed in a subduction-related setting. Given the occurrences of Early Triassic syn-collisional granites (235–252 Ma) in the area, the late Permian intrusions (257–261 Ma) likely represent the final stages of magmatism related to subduction of Paleo-Asian Ocean, signifying that final closure of the Paleo-Asian Ocean in the easternmost CAOB occurred during the end-Permian.

The genetic relationship between JTA–like magmas and typical adakites: An example from the Late Cretaceous Nuri complex, southern Tibet

Phanerozoic adakites typically represent modern analogues of Middle–Late Archean (3.5–2.5 Ga) tonalite–trondhjemite–granodiorite rocks (TTGs) due to their similarities in geochemical compositions. However, no Phanerozoic analogues have been reported for the Early Archean TTGs (>3.5 Ga), which are characterized by low Sr, Y and HREEs contents with low MgO and compatible elements contents, until recent years. The newfound Phanerozoic analogues have been defined as a new adakite subgroup named as “Jamaican–type adakites” (JTAs), and been regarded as the results of partial melting of subducted oceanic plateau crust. JTA–like magmas also occur in the Nuri complex, southern Tibet. We report whole–rock geochemical, SrNd isotopic data, zircon UPb ages and in-situ Hf isotopic data for adakitic quartz diorites, quartz monzonites and JTA–like quartz porphyries in the Nuri complex. Zircon UPb dating indicates that these rocks were emplaced in the Late Cretaceous at 93.5–92.1 Ma. Combined with the continuous variation trends of major and trace elements, the similarities in terms of whole–rock SrNd and zircon Hf isotopic compositions indicate that the three types of rocks are genetically related, while the quartz diorites should represent the relatively primitive melts. High MgO (5.00–5.41 wt%), Cr (138–159 ppm), Ni (65.9–96.8 ppm) contents and Mg# values (66–69), high εHf(t) values (10.5–11.9), εNd(t) values (3.7), and low (87Sr/86Sr)i values (0.7046–0.7050) indicate that the quartz diorites were derived from partial melting of subducted Neo–Tethyan oceanic slab, and subsequently underwent metasomatic reaction with the mantle wedge during their ascent. Quartz monzonites and JTA–like quartz porphyries were originated from quartz diorites by two stages of fractional crystallization. Low MgO and compatible elements contents of the quartz porphyries were caused by fractionation of hornblende, biotite, with minor magnetite and allanite, while low Sr contents and Sr/Y ratios were mainly controlled by plagioclase fractionation. This means that the JTA–like geochemical features of the quartz porphyries were generated by fractional crystallization from the adakites derived from partial melting of subducted slab, rather than directly derived from partial melting of subducted oceanic plateau. Therefore, various petrogenetic possibilities of the JTA–like magmas should be considered when using them as modern analogues.

Physicochemical Control of the Early Permian Xiangshan Fe-Ti Oxide Deposit in Eastern Tianshan (Xinjiang), NW China

The Xiangshan mafic-ultramafic complex is one of the major Early Permian mafic-ultramafic intrusions in eastern Tianshan (Xinjiang, NW China), and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian (280.1 and 279.2 Ma, respectively). Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce4+/Ce3+ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, H2O-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization.

Important role of hornblende fractionation in generating the adakitic magmas in Tongling, Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

ABSTRACTTongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of −3.00 to −4.52 and initial 87Sr/86Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.