The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were esti-mated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and -calcic granites were derived by 20%-40% partial melting of intra-crustal feldspathic rocks, whereas the -sodic granites are extreme fractionates ( 90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, -water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcani-clastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.
Whole-rock geochemical and Sr Nd isotopic data, and zircon U–Pb–Hf isotopic data are reported for mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, located in the westernmost segment of the North Qaidam Orogenic Belt (NQOB), NW China. Ni Cu sulfide ores of economic interest are found within these intrusions, with the Niubiziliang III and IV intrusions hosting most of the Ni–(Cu) mineralization. The Niubiziiang mafic–ultramafic intrusions intrude biotite–plagioclase gneiss of the Paleoproterozoic Dakendaban Group. Zircons separated from the gabbro, plagioclase-bearing clinopyroxenite, and olivine websterite yield U Pb ages of 389 ± 2, 373 ± 2, and 380 ± 2 Ma, respectively, indicating a genetic linkage with Middle–Late Devonian regional magmatism (390–370 Ma). Gabbro zircons have εHf(t) values of 4.2–10.9 with older T ages (0.86–0.59 Ga). The Niubiziliang intrusions contain many xenoliths of Paleoproterozoic biotite–plagioclase gneiss, and the intrusions have similar trace element patterns with strong enrichment in LILEs (e.g., K, Rb, and Th) and moderate depletion in HFSEs (e.g., Nb, Ta, P, and Ti). The major and trace element characteristics, and Sr–Nd–Hf isotopic compositions indicate the parental magmas originated from a metasomatized, asthenospheric mantle source that had previously been modified by subduction-related fluids. The magmas experienced significant crustal contamination both in the magma chamber and during ascent, triggering sulfur over-saturation that resulted in the deposition and enrichment of sulfides. Considering the tectonic evolution of this region, we infer that the Niubiziliang mafic–ultramafic intrusions formed in a post-collisional extensional setting related to delamination of the subcontinental lithosphere.
A detailed investigation of potential provenance is still lacking in the southwestern Tarim Basin, which restricts our complete understanding of Cenozoic source-to-sink relations between the basin interior and the Pamir salient - western Kunlun Mountain Range. Debate also exists concerning the potential sources of the Paleogene and Cretaceous igneous detritus present in the Cenozoic sedimentary sequences. Here, we present U-Pb (LA-ICP-MS) ages of detrital zircons from the continuous Eocene-Pliocene sediment series in the well-exposed Aertashi section to investigate changes in sediment provenance through time. The U-Pb detrital zircon ages range widely from 45 to 3204 Ma and can be divided into seven main groups: 45–65 Ma (sub-peak at 49 Ma), 67–103 Ma (sub-peak at 95 Ma), 196–251 Ma (sub-peak at 208 Ma), 252–416 Ma (sub-peak at 296 Ma), 417–540 Ma (sub-peak at 446 Ma), 550–1429 Ma (sub-peaks at 614 Ma, 828 Ma and 942 Ma) and 1345–3204 Ma (sub-peaks at 1773 Ma and 2480 Ma). These zircons were mainly derived from the western Kunlun Mountain Range and northern Pamir salient to the west and south. The evolution of the provenance and source-to-sink relationship patterns in the southwestern Tarim Basin can be divided into three stages: (1) The Middle Eocene to Lower Oligocene sediments display a wide variety of detrital zircon ages, suggesting that the source area was extensive. (2) A major change in provenance occurred during the Late Oligocene to Early Miocene and was characterized by an abrupt increase in the proportion of Triassic and Lower Paleozoic igneous components, implying a significant adjustment in topography induced by the initial uplift and exhumation of the western Kunlun Mountain Range and northern Pamir salient. (3) In the Late Miocene, the source-to-sink system transformed again, and contributions of Triassic to Lower Paleozoic material weakened substantially due to the sufficient indentation of the Pamir salient. Our integrated analyses of zircon geochronology indicate that the main source terranes of the Paleogene and Cretaceous igneous detritus are the central and southern Pamir salient, respectively, which are speculated to have been continuously connected to the study area during Eocene-Pliocene times, although such detritus is scarce in certain formations and has not yet been detected.
The tectonic framework of the North China Craton (NCC) during late Archean to early Paleoproterozoic (circa 2.5 Ga) is still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. Circa 2.5 Ga magmatic and metamorphic activities are widely spread throughout the NCC, which can be used as an efficient target to better understand the tectonic evolution at this period. In this study, based on a detailed field, structural, geochemical, geochronological and Sm–Nd isotopic study, we focus our work on the Haozhuang granitoids in the Zanhuang Massif located at the eastern margin of the Central Orogenic Belt of the NCC. The granitoids mainly include undeformed pegmatite and granodiorite. One pegmatite and two granodiorite samples yield zircon Pb/ Pb ages of 2513 ± 29 Ma, 2511 ± 36 Ma and 2528 ± 18 Ma, respectively. The granodiorites show metaluminous and shoshonitic to high-K calc-alkaline series characteristics with A-type granite affinity. The circa 2.5 Ga granodiorites have highly negative ε (t) values (− 29.22 ~ − 33.12) and T model ages between 2671 Ma and 3151 Ma. This work shows clearly, from whole-rock major and trace elements and Sm–Nd isotopic studies, that the Haozhuang granodiorites were derived from partial melting of old and thickened TTG crust rather than mantle sources, and formed in a subduction-related tectonic setting. With geochemical comparison studies to other similar-aged granitic rocks in the Zanhuang Massif, we suggest that these granitic rocks possibly have a certain correlation during the magma evolution. Coupled with our previous geochemical and isotopic studies on circa 2.5 Ga mafic dike swarms, we propose that the similar-aged granitic rocks and mafic dike swarms were produced by an east-dipping subduction polarity reversal event following an arc–continent collision between the Fuping/Wutai island arc and Eastern Block of the NCC above a west-dipping slab. The east-dipping subduction resulted in partial melting of the enriched lithospheric mantle, firstly forming the circa 2.5 Ga mafic dikes inducing the widespread circa 2.5 Ga metamorphism in the Central Orogenic Belt and the Eastern Block, and then the parental magma of these mafic dike swarms heated the old and thickened TTG crust causing the formation of similar-aged granitic rocks at a later stage.
The Shitoukengde Ni-Cu deposit, located in the Eastern Kunlun Orogen, comprises three mafic-ultramafic complexes, with the No. I complex hosting six Ni-Cu orebodies found recently. The deposit is hosted in the small ultramafic bodies intruding Proterozoic metamorphic rocks. Complexes at Shitoukengde contain all kinds of mafic-ultramafic rocks, and olivine websterite and pyroxene peridotite are the most important Ni-Cu-hosted rocks. Zircon U-Pb dating suggests that the Shitoukengde Ni-Cu deposit formed in late Silurian （426-422 Ma）, and their zircons have ~Hf（t） values of-9.4 to 5.9 with the older TDMm ages （0.80-1.42 Ga）. Mafic-ultramafic rocks from the No. I complex show the similar rare earth and trace element patterns, which are enriched in light rare earth elements and large ion iithophile elements （e.g., K, Rb, Th） and depleted in heavy rare earth elements and high field strength elements （e.g., Ta, Nb, Zr, Ti）. Sulfides from the deposit have the slightly higher ~34S values of 1.9-4.3%o than the mantle （0 ~ 2%o）. The major and trace element characteristics, and Sr-Nd-Pb and Hf, S isotopes indicate that their parental magmas originated from a metasomatised, asthenospheric mantle source which had previously been modified by subduction-related fluids, and experienced significant crustal contamination both in the magma chamber and during ascent triggering S oversaturation by addition of S and Si, that resulted in the deposition and enrichment of sulfides. Combined with the tectonic evolution, we suggest that the Shitoukengde Ni-Cu deposit formed in the post-collisional, extensional regime related to the subducted oceanic slab break-off after the Wanbaogou oceanic basalt plateau collaged northward to the Qaidam Block in late Silurian.
The Central Asian Orogenic Belt (CAOB) or Altaids contains several large porphyry Cu(-Au-Mo) and Au(-Cu) deposits. These deposits are genetically associated with equigranular and porphyritic tonalite, granodiorite and diorite. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that these hydrous, mineralized calc-alkaline intrusions were emplaced from the Cambrian to the Triassic. Trace element geochemistry and Sr–Nd–Hf–O isotopic compositions show two different signatures of the mineralizing intrusions. Mineralizing intrusions at Bozshakol, Aktogai, Nurkazghan, and Tuwu-Yandong have depleted Sr–Nd–Hf–O isotopic compositions ( = 0.7033 to 0.7047, εNd(t) = +3.6 to +6.6, εHf(t) = +9.6 to +16.4, δ O = 6.8 to 4.7‰) and variable high Nb/Ta (11.6 to 16.4) ratios, implying a dominantly juvenile lower crustal source with mantle involvement at Bozshakol and Aktogai and sedimentary rocks contamination at Nurkazghan. Mineralizing intrusions at Erdenet, Koksai and Yubileinoe have slightly depleted Sr–Nd–Hf isotopic compositions ( = 0.70403 to 0.7043, εNd(t) = +2.9 to +3.8, εHf(t) = +6.6 to +11.6) and low δ O (3.1 to 2.5‰), slightly enriched Sr–Nd–Hf–O isotopic compositions ( = 0.7044 to 0.7049, εNd(t) = +0.04 to +1.9, εHf(t) = 3.2 to 12.0, δ O = 5.2 to 6.4‰) and enriched Sr–Nd isotopic compositions ( = 0.7041 to 0.7081, εNd(t) = −2.6 to −2.9), respectively, as well as intermediate to low Nb/Ta ratios (13.4–8.1), implying a juvenile lower crustal source with wall rocks contamination at Erdenet and ancient crust contamination at Koksai and Yubileinoe. Therefore, Juvenile sources played a significant role in the generation of fertile magmas for porphyry-Cu and -Au style mineralization in the CAOB. Mineralizing intrusions include the adakitic, transitional, and normal arc rocks. Mineralizing intrusions in some porphyry Cu(-Mo) deposits (e.g., Aktogai, Erdenet) have adakitic affinities based on their high Sr/Y (30–193) and La/Yb (17–49) ratios coupled with low Y (1.5–10.3 ppm) and Yb (0.2–1.0 ppm) concentrations. Variable high Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a garnet-bearing amphibolite. These adakitic rocks were formed by partial melting of thickened lower crust at depths of >40 km. Mineralizing intrusions at most porphyry Cu(-Au) and Au deposits, including Bozshakol, Nurkazghan, Koksai, Tuwu-Yandong, and Yubileinoe, have variable intermediate Sr/Y (13–84) and La/Yb (7–23) ratios, intermediate to low Y (4.7–16.2 ppm) and Yb (0.5–1.3 ppm) concentrations, showing a transitional signature between adakitic and normal arc rocks. Variable moderate Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole. These transitional rocks were formed by the MASH (melting, assimilation, storage, and homogenization) processes at depths of 40 km) lower crust melting for adakitic magmas; (2) MASH and AFC for transitional magmas; and (3) thin (30–35 km) lower crust melting followed by AFC for normal arc magmas. A majority of transitional and normal arc magmas suggest that thinner crust was important to the formation of porphyry Cu(-Au) and Au deposits that dominate in the CAOB. Our geochronology and geochemistry results indicate that most of the mineralizing intrusions from the porphyry deposits in the CAOB formed in island arc settings from the Cambrian to Triassic. The intrusions at Erdenet, Koksai and Yubileinoe are exceptions, forming in continental arcs in the Permian, Silurian and Devonian, respectively. Paleozoic and minor Mesozoic porphyry deposits in the CAOB formed during six periods. The greatest metal endowment is associated with the Devonian porphyry deposits (374 to 372 Ma, e.g. Oyu Tolgoi). The largest number of porphyry deposits were emplaced in the Carboniferous (334 to 317 Ma, e.g., Aktogai, Kounrad, Tuwu-Yandong, Almalyk). The CAOB is a complex collage of Paleozoic and Mesozoic tectonic elements and has considerable potential for further discoveries of large porphyry Cu(-Au-Mo) and Au deposits.
Gravity and height changes, which reflect magma accumulation in subsurface chambers, are evaluated using analytical and numerical models in order to investigate their relationships and temporal evolutions. The analysis focuses mainly on the exploration of the time-dependent response of gravity and height changes to the pressurization of ellipsoidal magmatic chambers in viscoelastic media. Firstly, the validation of the numerical Finite Element results is performed by comparison with analytical solutions, which are devised for a simple spherical source embedded in a homogeneous viscoelastic half-space medium. Then, the effect of several model parameters on time-dependent height and gravity changes is investigated thanks to the flexibility of the numerical method in handling complex configurations. Both homogeneous and viscoelastic shell models reveal significantly different amplitudes in the ratio between gravity and height changes depending on geometry factors and medium rheology. The results show that these factors also influence the relaxation characteristic times of the investigated geophysical changes. Overall, these temporal patterns are compatible with time-dependent height and gravity changes observed on Etna volcano during the 1994–1997 inflation period. By modeling the viscoelastic response of a pressurized prolate magmatic source, a general agreement between computed and observed geophysical variations is achieved.
The Tarim large igneous province (TLIP) in northwestern China, covering an area of ca. 250,000 km , includes large volumes of basalts, basic dyke swarms, mafic-ultramafic intrusion and minor picrite. Here we report systematic Hf isotope data from basalt, diabase, olivine pyroxenite and syenitic porphyry from the TLIP and address the source components and magma evolution. The subdivision of the Tarim basalts shows that the Group 1 and Group 2 basalts are clearly differentiated based on different Nb/Y values, with two subgroups (Group 1a, Group 1b) identified based on distinct P O vs. Mg# trends. The Hf/ Hf isotopic composition of the basalts ranges from 0.282584 to 0.282837. The Hf( ) of the basalts belonging to the Group 1a, Group 1b and Group 2 are −0.4–4.4, 0.5–2.1, and 1.9–3.1, respectively, and those of the intrusive suite of olivine pyroxenite, diabase and syenitic porphyry show a range of 6.0–6.5, 4.5–5.7, and 6.5–8.3, respectively. The TLIP basalts generally show a good positive correlation between Hf/ Hf and Nd/ Nd, and fall in the field of the oceanic island basalts (OIBs) with low Hf/ Hf and Nd/ Nd, comparable to the basaltic lavas of the Pitcairn hotspot. These features, together with the enriched signature of the Tarim basalts might reflect the incorporation of partial melting of lithospheric mantle source in the early stages of the plume activity. The TLIP basalts show low Nd( ) and moderate Hf( ) OIB-like source. The Hf/ Hf- Nd/ Nd values of the Group 1a and Group 1b basalts of the TLIP basalts are comparable to those from the Karoo high-Ti basalts, and those of the Group 2 basalts are comparable to the features of the Karoo low-Ti basalts and diabases. The Group 1a and Group 1b basalts fall in the same Hf–Nd array, whereas the Group 2 basalts fall in a different array with much higher Nd( ). The olivine pyroxenite, diabase and syenitic porphyry fall in the higher Nd( ) and Hf( ) field, with the olivine pyroxenite and diabase having features close to OIB-like source. Our new Hf isotopic results suggest distinct sources for the Tarim basalts (285–290 Ma) and the intrusive rocks (274–284 Ma). Furthermore, the Hf( ) vs. Nd( ) plots show that the basalts and intrusive rocks might correspond to two different periods of magmatic activity in the Tarim Basin during the Early Permian, being comparable to the temporal evolution of different rock units in the TLIP. The Hf( ) and Nd( ) combined with other evidences address that the basalts could be explained by being derived from the asthenospheric (or plume) mantle and having interaction with lithospheric mantle source by mainly lower degree of partial melting in the early stage before the eruption, and should be much less proportion of crustal contamination during the period of 285–290 Ma, and the intrusive rocks might be derived from the primary magma and/or OIB-like mantle sources and underwent a magma process mainly by fractional crystallization and/or cumulation during the period of 274–284 Ma.
The thermal evolution of the Phlegraean magmatic system (southern Italy) is studied by analyzing the influence of the thermal property variations on the solution of the heat conduction equation. The aim of this paper is to verify if appropriate choices of thermal parameters can reproduce, at least to greater depths, the high temperatures measured in the geothermal wells, drilled inside the caldera, under the assumption of heat loss from a magma chamber by conduction. Since the main purpose is to verify the plausibility of such an assumption, rather simple models of the magmatic system are adopted and only major volcanic events (i.e., the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions) are considered. The results of the simulated two-dimensional model scenarios show that by assuming an extended source region, whose emplacement time is longer than 40 ka, heat conduction mechanisms can provide temperatures as high as those measured at depths deeper than about 2000 m. On the other hand, the 1D simulations show that appropriate choices for the thermal conductivity depth profiles can reproduce the observed temperatures at depths deeper than about 1000 m. These findings question the apparent consensus that convection is the only dominant form of heat transfer at Phlegraean Fields and might motivate new research for reconstructing the thermal evolution of the Phlegraean magmatic system.
The Prominent Hill deposit is a world-class iron oxide copper–gold (IOCG) deposit in South Australia, characterized by a high Cu/S ratio of the dominant Cu-(Fe) sulfides hosted by hematite breccias. It contains a total resource of 278 Mt of ore at 0.98% Cu and 0.75 g/t Au. Prominent Hill is one of several IOCG deposits and numerous prospects in the Olympic IOCG province that are temporally associated with the 1603–1575 Ma Gawler Range Volcanics, a large igneous province including co-magmatic granitoid intrusions of the Hiltaba Suite. Globally, IOCG deposits share many similar features in terms of their geological environment and mineral association. However, it is not yet clear whether sulfur and copper originate from the same source rocks and which hydrothermal redox processes created the characteristic iron oxide enrichment. Highly variable sulfur isotope compositions of sulfides and sulfates in IOCG deposits have previously been interpreted in terms of diverse sulfur sources that may include contributions from magmatic, sedimentary, seawater or evaporitic sulfur. In order to test these alternatives, we performed a detailed sulfur isotope study of Cu-(Fe) sulfides from Prominent Hill and IOCG prospects nearby. The Prominent Hill deposit shows a wide range in δ S between − 33.5‰ and 29.9‰ for Cu-(Fe) sulfides, and a narrower range of 4.3‰ to 15.8‰ for barite. Iron sulfides (pyrite, pyrrhotite) show a narrow range in sulfur isotope composition, whereas Cu-bearing sulfides show a much wider range, and more negative δ S values on average. We propose a two-stage sulfide mineralization model for the IOCG system in the Prominent Hill area, in which all hydrothermal sulfur is ultimately derived from a magmatic source that had a composition of 4.4 ± 2‰. The diversity in sulfur isotope composition can be produced by different fluid evolution pathways along reducing or oxidizing trajectories. A reduced sulfur evolution pathway is responsible for stage I mineralization, when intrusion-derived magmatic-hydrothermal fluids produced early pyrite and minor chalcopyrite at Prominent Hill, and iron ± copper sulfides in regional magnetite skarns and in some pervasively altered volcanic rocks of the Gawler Range Volcanics. Shallow-venting magmatic-hydrothermal fluids and subaerial volcanic gases that became completely oxidized by reaction with atmospheric oxygen produced sulfate and sulfuric acid with a sulfur isotope composition equal to their magmatic source. This highly oxidized ore fluid probably consisted dominantly of water from the hydrosphere, but contained magmatic solute components, notably sulfate, acidity and Cu. Sulfate reduction produced hydrothermal Cu sulfides with a wide range in sulfur isotope compositions from very negative to moderately positive values. Partial reaction of the Cu-rich stage II fluid with earlier stage I sulfides resulted in mixing of sulfur derived from sulfate reduction and from sulfides deposited during stage I. Modeling of the sulfur isotope fractionation processes in response to reducing and oxidizing pathways demonstrates that the entire spectrum of sulfur isotope data from stage I and stage II mineralization can be explained with a single, ultimately magmatic sulfur source. Such a magmatic sulfur source is also adequate to explain the complete spectrum of sulfur isotope data of other IOCG prospects and deposits in the Olympic province, including Olympic Dam. The results of our study challenge the conventional model that suggests the requirement of multiple and compositionally diverse sulfur sources in hematite-breccia hosted IOCG style mineralization.