high-K Calc-alkaline Compositions Research Articles

Petrogenesis of the Weideshan Pluton in Jiaodong and Its Implications for Gold Polymetallic Mineralization: Constraints from Zircon U-Pb-Hf Isotopes, Petrogeochemistry, and Whole-Rock Sr-Nd Isotopes

The Early Cretaceous Weideshan granites are associated with large-scale Au and polymetallic Cu-Mo-Pb-Zn mineralization. To investigate the petrogenesis of the Weideshan granite and constrain its tectonic setting during the Early Cretaceous, we conducted a zircon U-Pb-Hf isotope and whole-rock geochemical and Sr-Nd isotopic study of the granite. In situ zircon U-Pb dating of three granite samples yielded Early Cretaceous ages of 112.83 ± 0.80, 112.64 ± 0.91, and 111.82 ± 0.78 Ma. The samples had high-K calc-alkaline compositions and were enriched in the light rare earth and large-ion lithophile elements (e.g., K, Rb, Ba, Th, and U) and depleted in high-field-strength elements (e.g., Nb, Ti, and P). The samples had small negative Eu anomalies and initial 87Sr/86Sr and εNd(t) values of 0.70853–0.71029 and –18 to –14, respectively. The zircon εHf(t) values varied between −16 and −12, with corresponding two-stage model ages (tDM2) of 2180–2000 Ma. These characteristics indicated that the Weideshan pluton was formed in a back-arc extensional environment caused by subduction of the Paleo-Pacific Plate toward the Asian continent during the early Cretaceous. The magma was generated by crust–mantle interaction during lithospheric thinning. The diagenetic age of the Weideshan granites was the same as the formation age of gold and polymetallic ores in the Jiaodong area. The extensive fluid circulation induced by the magma emplacement may be the main source of ore-forming materials for the gold and polymetallic Cu-Mo-Pb-Zn deposits.

Physicochemical evolution and mechanism of a skarn system: Insights from the world-class Mazraeh Cu deposit, NW Iran

The Mazraeh Cu skarn deposit, NW Iran, is situated at the lithological contact between an Oligo−Miocene granodiorite and Cretaceous limestones. The causative granodiorite has I-type medium- to high-K calc-alkaline composition that formed in a subduction arc setting. Metasomatism generated exoskarn and endoskarn characterized by a temporal and spatial mineral-chemistry zonation with massive garnet and garnet-pyroxene proximal and epidote-amphibole skarn at the distal zones of the exoskarn. Our data reveal a multi-stage evolution of the hydrothermal system. Accordingly, the prograde stage was formed by barren, oxidized, high-temperature and hypersaline magmatic fluids, which suggests that the parental magma was emplaced at low lithostatic pressure conditions (∼1.5 km depth). By contrast, during the retrograde stage the regime changed from lithostatic to hydrostatic conditions, resulting in a significant decrease in pressure and oxygen fugacity, boiling, cooling, and mixing, respectively. Dilution by meteoric fluids led to additional cooling and lower salinity of the ore fluids. Sulfur isotope compositions (δ34S = −2.8‰−1.3‰) of sulfides indicate a unique magmatic source of the sulfur. The δ18Ofluid and δDfluid values of the prograde stage fluid also indicate a magmatic origin, while their gradual depletions from the prograde to retrograde stage imply increased mixing with meteoric water. We propose that the fluid mixing scenario continued over the entire life cycle of the mineral system in an open hydrostatic regime as reflected in the oxygen isotope values. It is suggested that boiling and fluid mixing played a significant role in the formation of this giant Cu skarn deposit.

Lithospheric dripping triggered by slab break-off: A possible mechanism for Late Carboniferous magmatism in the eastern Central Asian Orogenic Belt

In this paper, we present zircon UPb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon UPb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive εNd(t) (+1.88 to +2.00) and εHf(t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted NdHf isotopic compositions (zircon εHf(t) = +12.5 to +18.9, whole rock εNd(t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive εHf(t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off.

Continental crust delamination in a retreating subduction zone: A case study in the southern Alxa (Inner Mongolia, China), Central Asian Orogenic Belt

Carboniferous-Permian igneous rocks in the southern Alxa (Inner Mongolia, China) of the southern Central Asian Orogenic Belt exhibit an oceanward migration trend, which provides an ideal area to reconstruct the process of crust-mantle material exchange within a retreating subduction zone. This study presents new geochronological and geochemical data for the Shouji plutons from the southern Alxa to address crust-mantle material exchange process. All these diorite, granodiorite, and K-feldspar granite samples from the Shouji batholith exhibit Early Permian zircon U-Pb ages (ca. 282−270 Ma). These diorite and granodiorite are characterized by high Sr contents as well as low Y, and Yb concentrations, coupled with distinctively high Sr/Y values. They exhibit fertile Nd-Hf isotopic compositions with negative whole-rock εNd(t) values, and zircon εHf(t) values, similar to those of continental crust-related adakites, which might originate from partial melting of lower crust, and followed by melt-mantle interactions. These K-feldspar granite samples display peraluminous and high-K calc-alkaline compositions, and negative zircon εHf(t) and whole-rock εNd(t) values, and originated from partial melting of Paleoproterozoic igneous rocks, with additions of mantle-derived melts in their sources, in low-temperature, low-pressure conditions. Previously published zircon U-Pb age data show that numerous mafic igneous rocks were mostly generated during the Early Permian, marking the onset of mafic volcanism in the southern Alxa. Moreover, compiled detrital zircon U-Pb age data for Late Carboniferous−Permian sedimentary succession imply a regional topographic uplift during the Early Permian. Both of these two lines of evidence suggest an Early Permian lower crustal delamination in the southern Alxa. These Shouji adakitic plutons were most likely to be generated during the lower crustal delamination. Furthermore, Sr-Nd isotope data of Carboniferous-Permian igneous rocks show that the continental crust was recycled back into the mantle during Early Permian within the southern Alxa by means of subduction erosion and lower crustal delamination.

Multiple Sources of Indosinian Granites and Constraints on the Tectonic Evolution of the Paleo-Tethys Ocean in East Kunlun Orogen

Numerous Indosinian granitoids occur in the East Kunlun Orogen (EKO). The Indosinian was a key transitional period associated with the evolution of the Paleo-Tethys Ocean. Here, we study the relationship between the petrogenesis of the granitoids and the regional tectonic setting based on a comprehensive analysis of the petrology, geochronology, and geochemistry of typical granitoids in the eastern part of the EKO. The Indosinian granitoid compositions are dominated by quartz diorites, granodiorites, monzogranites, porphyritic monzogranites, and syenogranites. Early Indosinian granitoids are large, granitic batholiths, while the middle and late Indosinian granitoids are smaller in size. From the early Indosinian to late Indosinian, the granitoids show a transition from a medium-K calc-alkaline to high-K calc-alkaline composition. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs), especially for the Helegangxilikete and the Kekeealong plutons. The late Indosinian granitoids have relatively low Y and Yb contents, high Sr contents, and high La/Yb and Sr/Y ratios, which suggests adakitic affinity. The zircon saturation temperatures of the early Indosinian syenogranite and the Keri syenogranite are above 800 °C. The zircon saturation temperatures of other Indosinian granites (average 749 °C) are lower than those of the biotite and amphibole partial melting experiment. In the early Indosinian (255–240 Ma), numerous granitoids were the products of the partial melting of the juvenile lower crust by mafic magma underplating. This underplating is geodynamically related to the continuous subduction of a branch of Paleo-Tethys Ocean, with slab break-off, rapid upwelling, and mantle decompression. In the middle Indosinian (240–230 Ma), the compression that accompanied the continent–continent collision was not conducive to fluid activity, and hence, the formation of magma could be attributed to dehydration partial melting of muscovite, biotite, or amphibole. In the late Indosinian (230–200 Ma), the delamination of thickened crust would provide heat and channels for fluid migration, leading to a flare-up of the magmas. The composition and petrogenesis of the Indosinian granitoids in the eastern EKO are the result of processes associated with the subduction, collisional, and post-collisional stages, during the evolution of the Paleo-Tethys Ocean.

Late Cretaceous Transition From Calc-Alkaline to Alkaline Magmatism in the Eastern Anatolian Plateau: Implications for Microblock Collision Timing

Abstract In convergent belts, a collision between two blocks can reshape upper mantle geometry and processes responsible for a change in mechanisms of magma generation with distinct geochemical compositions. Therefore, identifying the turning point of a magma compositional shift can provide key constraints on collision timing, which is decisive in building a framework of regional tectonic evolution. The Eastern Anatolian Plateau is composed of a mosaic of rifted blocks assembled through successive collisional events, culminating with the terminal collision of Arabia with Eurasia and the closure of the southern branch of the Neotethys in the Cenozoic. The timing of the microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block, the southern Eastern Anatolian Plateau, is a matter of debate due to limited constraints on the timing and petrogenesis of the (post-)collision-related magmatism during the Late Cretaceous. This study identifies three compositionally distinct intrusive suites aged from ~87 to ~69 Ma in the Eastern Taurides Block, the southern Eastern Anatolian Plateau. Group 1 intrusive rocks were emplaced in the southern Eastern Taurides Block at ~87–77 Ma and are characterized by high-K calc-alkaline compositions with predominantly depleted Hf and Nd isotope compositions [εHf(t) = 0.9 to +16.5 and εNd(t) = −2.3 to +6.9]. In contrast, the younger (~77–69 Ma) Group 2A comprises nepheline (Ne)-normative alkaline compositions, and Group 2B consists of shoshonitic compositions; both groups are mainly distributed in the central and northern Eastern Taurides Block. Groups 2A and 2B overlap in age between ~77 and 69 Ma and show relatively enriched Hf–Nd isotope compositions [Group 2A: εHf(t) = −1.0 to +4.7 and εNd(t) = −2.3 to +1.2; Group 2B: εHf(t) = −4.0 to +4.0 and εNd(t) = −6.5 to −1.3]. Group 1 intrusive rocks relate to the sub-arc asthenosphere-derived melts that differentiated toward the granite minimum by fractional crystallization. Parental melts of Group 2A intrusive rocks are interpreted to be derived from metasomatized subcontinental lithospheric mantle and differentiated toward the phonolite minimum. Crustal assimilation during magma ascent pushed the initially silica-undersaturated magma (Group 2A) into silica-(over)saturated compositions (Group 2B) through fractional crystallization that ultimately evolved toward the granite minimum. We propose that the Group 1 magmatism was related to northward subduction and closure of the Berit oceanic lithosphere, whereas the Group 2 magmatism results from collision-induced lithospheric delamination ± slab rollback. Together with the P–T–t evolution of the high-pressure metamorphic rocks from the Bitlis Massif and the spatiotemporal and geochemical variations of the Late Cretaceous magmatism in the Eastern Taurides Block, this study suggests that microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block (Eurasia) most likely occurred at ~84–77 Ma. This study provides an example to constrain collision timing through a perspective of magmatic transition from calc-alkaline to alkaline series.

Early Eocene high-Sr/Y magmas from the Urumieh-Dokhtar paleo-arc, Iran: Implications for the origin of high-flux events in magmatic arcs

High-flux magmatic events, or flare-ups, are seen in many continental and transitional arcs but the underlying cause is not fully understood. Throughout Iran, voluminous calc-alkaline rocks emplaced between ~53 Ma and ~37 Ma are manifestations of a high-flux event. This high-flux event has been attributed to extension in the upper plate of one or more Neo-Tethyan subduction zones. Here, we present new findings in the Urumieh-Dokhtar paleo-arc, Iran where emplacement of Early Eocene granitic intrusions (ca. 55–52 Ma) immediately preceded the high-flux event. Geochemically, the studied rocks display high-K calc-alkaline compositions, continental crust-like trace element patterns, variable depletion of Y and heavy rare-earth elements, and SrNd isotopes that are close to or slightly more enriched than the Bulk Silicate Earth. The SrNd isotopic compositions indicate that the intrusions derived from upper plate materials. Relatively high Sr/Y and La/Yb of the studied rocks indicate that the residue of melt extraction, the crystalline assemblage that was left behind during magma differentiation, or both, to have existed in the deep arc crust. Following a ~ 20 Myr magmatic lull, the Early Eocene crustal thickening was most likely related to underthrusting of rear-arc crust towards the main arc, a process that not only promoted rapid root growth but also triggered the Eocene high-flux event. In the Iranian case example in particular, magmas formed during the high-flux event do not display elevated Sr/Y, implying that dense, locally thickened root might have foundered from the base of the arc lithosphere to the convecting mantle soon after their formation.

Geochronology, geochemistry and tectonic implications of Variscan granitic and dioritic rocks from the Odenwald-Spessart basement, Germany

New zircon U-Pb-Hf-O isotope, whole rock geochemical and Sr-Nd-Pb isotope geochemical data of Variscan felsic to intermediate rocks from the Odenwald-Spessart basement, Mid-German Crystalline Zone, Germany are presented. Peraluminous, high-K calc-alkaline S-type granite in the eastern Odenwald basement (Group 1 rocks) formed by partial melting of the lower crust at c. 425 Ma. Their high-K calc-alkaline composition indicates the presence of sedimentary and/or metasomatized lithospheric mantle material during formation of the melts. The protolithic melts of metaluminous, medium- to high-K calc-alkaline I-type granodiorite and diorite of the Spessart and western Odenwald basement as well as the Neustadt (Odenwald) outlier (Group 2 rocks) formed by partial melting of the upper mantle at c. 340 Ma. The melt source was enriched through previously subducted Mesoproterozoic sedimentary material. Eastern Odenwald basement leucocratic gneiss have zircon with ages ranging from 2803 Ma to 336 Ma and main age peaks between 620 and 570 Ma and 360 and 330 Ma. The presence of numerous >1800 Ma old zircon and a prominent age gap between 1800 and 1000 Ma imply a Gondwanan zircon source and indicate the presence of Cadomian material in the Odenwald-Spessart basement. Group 2 diorite is exposed over a distance of at least 60 km from the easternmost Spessart to the westernmost Odenwald and is expected to underlie the eastern Odenwald basement. We suggest that Group 2 diorite formation was related to the presence of a mantle plume, which was also responsible for the widespread Carboniferous magmatism and the associated high-temperature metamorphism in the Odenwald-Spessart basement and other areas of the Variscan orogen.

Evidence of late Palaeozoic and Middle Triassic magmatism in the Sakar-Strandzha Zone, SE Bulgaria: Regional geodynamic implications

ABSTRACT Late Palaeozoic granitoids, meta-granitoids and meta-volcanic rocks predominate in the metamorphic basement of the northern and western parts of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria, together with the subsidiary Triassic meta-granitoids and meta-volcanic rocks. Generally, igneous minerals and textures are preserved, except in the meta-granitoids and meta-volcanic rocks that experienced a low- to high-grade metamorphic overprint. The volcanic rocks have a peraluminous and high-K calc-alkaline composition, and the granitoids range between I- to S-type compositions, typical of volcanic arcs and syn-collisional settings. LILE and LREE-enrichment and Nb-Ta anomalies characterize the intrusive and extrusive rock suites. U-Pb zircon geochronology has yielded crystallization ages between 245 and 237 Ma for the majority of the studied igneous rocks, and between 297 and 295 Ma for a small group of igneous rocks. Early Permian and Middle Triassic igneous suites of the northern and western SASTZ have similar compositions and a similar tectonic setting when compared to Late Carboniferous-Early Permian intrusive and extrusive suites of the adjacent Sakar unit of the SASTZ, confirming a common regional late Palaeozoic-early Mesozoic tectono-magmatic event. As the Late Carboniferous-Permian to Middle Triassic magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Paleotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be linked with the Serbo-Macedonian-Rhodope zones to the southwest, where coeval meta-granitoids document the same geodynamic context. By contrast, the Triassic igneous suite of the SASTZ is unrelated to the Serbo-Macedonian-Rhodope zones, where Triassic meta-ophiolite and meta-granitoids record Neotethys rifting.

The Maravilha Orthogneiss: High-K calc-alkaline Cryogenian evidence of the pre- to early- collisional magmatism in the southern PEAL domain, SE Borborema Province

The study area is in the southern East Pernambuco–Alagoas Domain, Borborema Province, Southern sub-province, northeastern Brazil. The Maravilha Orthogneiss comprises an elongated intrusion of ~200 km2, with an ESE-WNW major axis, ranging in composition from tonalite to sienogranite, emplaced into metasedimentary rocks of the Inhapi Sequence, in the southern part of the Pernambuco-Alagoas Domain. Diorite sheets, amphibolite, leucocratic dikes and quatz-feldspar pegmatite veins are common. The Maravilha Orthogneiss show SiO2 contents ranging from 55.9 to 65.55 wt%, and Mg# (MgO/FeO + MgO) of 6.28–9.12. It has magnesian, metaluminous, and medium to high-K calc-alkaline composition. Chondrite-normalized spidergrams are characterized by troughs at Ba, Rb, Sr, Th, Nb and Ta. Chondrite-normalized REE patterns are moderately fractionated, with (Ce/Yb)N ratios ranging from 2.28 to 17.29 and display discrete negative Eu anomalies. This rock shows volcanic-arc geochemical signature. U–Pb zircon SHRIMP data yielded an age of 646 ± 5 Ma, interpreted as the crystallization age of the protolith. The Geochemical and isotopic data suggest that the source of the Maravilha orthogneiss protolith involved mixing of crustal and mantle melts. The resulted magma evolved by amphibole, plagioclase and apatite fractionation. A sample of migmatitic orthogneisses of the Inhapi Sequence basement show U–Pb zircon crystallization age of 983 ± 7.2 Ma. The lack of zircon grains with Tonian age in the studied orthogneiss indicates no contribution of Tonian crust in their source. The Maravilha Orthogneiss is a record of a pre-to early-collisional calc-alkaline magmatism with arc-related geochemical signature emplaced during the beginning of the Brasiliano Orogeny compressive stage.

Geology and geochronology of the Archean plutonic rocks in the northeast Democratic Republic of Congo

Here we report the first regional-scale study of Archean plutonic rocks from 50,000 km2 of the northeast Democratic Republic of Congo (DRC). We include 50 new U-Pb zircon Sensitive High-Resolution Ion MicroProbe (SHRIMP) ages supported by petrographic and whole rock geochemical data from a further ~400 samples, and 39 additional U-Pb zircon ages reported by Kabete et al. (submitted) and Bird (2016).Felsic-intermediate magmatism across the northeast DRC occurred from ~ 3200 to 2530 Ma. Plutonism before ~ 2670 Ma is restricted to the West Nile Gneiss in eastern and northern-most DRC, whereas plutonism across the remainder of the northeast DRC initiated between 2670 and 2640 Ma. Two Neoproterozoic granite plutons ~ 1000–950 Ma are also present within the West Nile Gneiss. Magmatism between ~ 2670 and 2530 Ma marks the principal period of crustal growth in the northeast DRC. Felsic-intermediate plutonism between ~ 2670 and 2600 Ma included two suites of magnesian, calc-alkaline rocks, (1) a sodic-calcic suite with high Sr/Y ratios (>40) comparable with tonalite-trondhjemite-granodiorite (TTG) suites in other Archean terranes, and (2) a calc-alkaline to high-K suite of granitoids with low Sr/Y ratios (<40) and lower Na and Al contents, comparable with “low-Ca” granites in the Yilgarn Craton. In contrast, most granitoids emplaced between ~ 2600 and 2530 Ma have ferroan, high-K calc-alkaline compositions, and are enriched in high field strength (HFSE) and rare Earth elements (REE). Although the HFSE-elevated granitoids are comparable with Closepet-type, and “mafic granites” in other Neoarchean terranes, their volume within the northeast DRC appears unprecedented.Before ~ 2600 Ma partial melting of hydrous metabasalts at depths > 40 km is inferred to have produced the high Sr/Y magnesian granitoids, whereas the low Sr/Y magnesian granitoids were derived from melting-assimilation-storage-hybridization (MASH) processes in the overlying low-mid crust during the same period. Trans-lithospheric extension after ~ 2600 Ma, enabled asthenospheric mantle to interact with the base of the crust, resulting in high-temperature melting that produced the HFSE-elevated granitoids. Inherited Meso- and Paleoarchean zircons are almost completely restricted to the area within 30–40 km of the West Nile Gneiss, implying most of the northeast DRC is underlain by juvenile Neoarchean crust.

From Ordovician nascent to early Permian mature arc in the southern Altaids: Insights from the Kalatage inlier in the Eastern Tianshan, NW China

AbstractThe Kalatage inlier in the Dananhu-Haerlik arc is one of the most important arcs in the Eastern Tianshan, southern Altaids (or Central Asian orogenic belt). Based on outcrop maps and core logs, we report 16 new U-Pb dates in order to reconstruct the stratigraphic framework of the Dananhu-Haerlik arc. The new U-Pb ages reveal that the volcanic and intrusive rocks formed in the interval from the Ordovician to early Permian (445–299 Ma), with the oldest diorite dike at 445 ± 3 Ma and the youngest rhyolite at 299 ± 2 Ma. These results constrain the ages of the oldest basaltic and volcaniclastic rocks of the Ordovician Huangchaopo Group, which were intruded by granite-granodiorite-diorite plutons in the Late Ordovician to middle Silurian (445–426 Ma). The second oldest components are intermediate volcanic and volcaniclastic rocks of the early Silurian Hongliuxia Formation (S1h), which lies unconformably on the Huangchaopo Group and is unconformably overlain by Early Devonian volcanic rocks (416 Ma). From the mid- to late Silurian (S2-3), all the rocks were exhumed, eroded, and overlain by polymictic pyroclastic deposits. Following subaerial to shallow subaqueous burial at 416–300 Ma by intermediate to felsic volcanic and volcaniclastics rocks, the succession was intruded by diorites, granodiorites, and granites (390–314 Ma). The arc volcanic and intrusive rocks are characterized by potassium enrichment, when they evolved from mafic to felsic and from tholeiitic via transitional and calc-alkaline to final high-K calc-alkaline compositions with relatively low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, and positive εNd(t) values, +4.1 to +9.2. These new data suggest that the Dananhu-Haerlik arc is a long-lived arc that consequently requires a new evolutionary model. It began as a nascent (immature) intra-oceanic arc in the Ordovician to early Silurian, and it evolved into a mature island arc in the middle Silurian to early Permian. The results suggest that the construction of a juvenile-to-mature arc, in combination with its lateral attachment to an incoming arc or continent, was an important crustal growth mechanism in the southern Altaids.

U–Pb zircon geochronology and petrology of the early Miocene Göloba and Şaroluk plutons in the Biga Peninsula, NW Turkey: Implications for post-collisional magmatism and geodynamic evolution

The Biga Peninsula (NW Turkey) is an area where magmatic and tectonic events are observed together, and post-collisional plutons are extensively seen. Two plutons in this peninsula were studied; the Göloba Pluton is composed of quartz monzonite, granite and granodiorite, and the Şaroluk Pluton is composed of quartz monzonite, monzonite and granodiorite. U–Pb zircon dating yielded 20.87 ± 0.31 Ma for the Göloba pluton, 19.69 ± 0.25 Ma for a quartzmonzonite porphyry vein cutting this pluton, and 22.18 ± 0.40 to 21.51 ± 0.37 Ma for the Şaroluk pluton, revealing an Early Miocene emplacement age for these pluton magmatism. These plutons are geochemically similar to each other with high-K calc-alkaline compositions. Their A/CNK values range between 0.84 and 1.48, suggesting I-type and metaluminous to slightly peraluminous characteristics. The rock samples show linear-like correlations in major oxide and trace element variation diagrams, which indicate main mineral phase’ fractionation. The Göloba and Şaroluk plutons also exhibit enrichment in large ion lithophile elements (K, Sr, Rb and Ba), Ce and Th, with decreases in high field strength elements (Ti and Y), Ta and Nb. Chondrite normalised REE diagrams of the plutonic rocks show concave upward patterns (LaN/LuN = 11–34), and slight negative Eu anomalies (EuN/Eu* = 0.51–1.03). In the Göloba pluton samples, 87Sr/86Sr(i), 143Nd/144Nd(i) and ƐNd(i) values vary from 0.706792 to 0.707250, from 0.512435 to 0.512489, and from −2.36 to −3.41, respectively. In the Şaroluk pluton samples, 87Sr/86Sr(i) and 143Nd/144Nd(i) isotopic values range within 0.706978–0.707669 and 0.512456–0.512520 respectively, and ƐNd(i) values are between −1.75 and −2.99. Fractional crystallisation, rather than assimilation and mixing, seems to have been important in the evolution of the studied plutons. All geochemical data suggest that the parental magma(s) of the studied plutons were hybrid magma that originated from enriched lithospheric mantle and lower/middle crust end-members. According to the regional geology and petrological data, Neogene magmatism in the Biga Peninsula has post-collisional feature, and was closely related to slab break-off.

Syn-exhumation melting of the subducted continental crust: Geochemical evidence from early Paleozoic granitoids in North Qaidam, northern Tibet

Syn-exhumation partial melting of the deeply subducted continental crust commonly occurs in collisional orogenic settings. The resulting felsic melts not only form granitoids, but can also modify the composition of the mantle wedge above continental subduction zones. This study investigates crustal anatexis and its role in granitoid formation and crust–mantle interaction during continental collision using new whole-rock geochemical, Sr–Nd–Hf isotope, and zircon U–Pb geochronological and Lu–Hf isotope data for early Paleozoic granitoids of the North Qaidam area of northern Tibet, a collisional orogen that contains ultrahigh-pressure (UHP) metamorphic rocks. Zircon U–Pb dating of the granitoids in this area yielded ages between 416 ± 6 Ma and 393 ± 2 Ma, coeval with the exhumation of the deeply subducted continental crust. This indicates that these granitoids formed as a result of syn-exhumation magmatism during the continental collision. Relict zircons in the granitoids yield concordant Neoproterozoic and early Paleozoic U–Pb ages that are consistent with the protolith and metamorphic ages of UHP metaigneous rocks in the orogen, respectively. The granitoids have calc-alkaline to high-K calc-alkaline compositions, with total alkali contents of 4.07–8.09 wt%. They are also metaluminous to weakly peraluminous with A/CNK ratios of 0.86–1.14. They are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), yielding arc-like trace element patterns. They also have high (87Sr/86Sr)i ratios (0.7058–0.7195) and negative εNd(t) values (−7.1 to −1.4) and have whole-rock and zircon εHf(t) values of −2.8 to +2.7 and − 8.6 to +4.7, respectively. These data indicate that the granitoids are compositionally similar to the UHP metaigneous rocks in the collisional orogen, suggesting a genetic link. This in turn indicates that the granitoids in the study area formed from magmas generated by the partial melting during the exhumation of deeply subducted continental crust. The resulting granitic melts not only generated the syn-exhumation granitoids in this area, but also caused metasomatism associated with crust–mantle interaction in continental subduction channels.

Carboniferous to Early Triassic magmatism and accretion in Alxa (NW China): implications for accretionary orogenesis of the southern Altaids

Alxa occupies a crucial position between the Tianshan–Beishan orogen to the west and the Solonker suture to the east and is important in our understanding of the accretionary orogenesis of the southern Altaids. To unravel the tectonic history of the Alxa region, we undertook an integrated study of the field geology, geochemistry and geochronology of magmatic rocks and an accretionary complex. Six granites and one rhyolite from the Zhusileng–Hangwula arc show a peraluminous, high-K calc-alkaline composition and one gabbro is tholeiitic. They show patterns rich in incompatible elements and negative Nb–Ta anomalies on primitive mantle-normalized trace element spider diagrams. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating reveals 348–329 and 267–250 Ma magmatic events. These data indicate that the Zhusileng–Hangwula arc was a Japan-type island arc from the late Paleozoic to Early Triassic. The Engger Us mélange comprises pillow basalts with a normal-type mid-ocean ridge basalt composition, tuffs, cherts, siliceous mudstones, limestones and turbidites, which are tectonically juxtaposed by a block-in-matrix structure. This mélange is interpreted as a Carboniferous–Permian accretionary complex. These new data, combined with previous studies, confirm that the Palaeo-Asian Ocean subducted bidirectionally in the Alxa region in the Carboniferous–Early Triassic. A consistent divergent subduction system existed from Alxa to Solonker before the terminal closure of the Palaeo-Asian Ocean in the Mid- to Late Triassic.Supplementary material: Major and trace element data and zircon U-Pb age results of all the samples are available at: https://doi.org/10.6084/m9.figshare.c.4962230

Gold in the lithosphere of the western South China Block, SW China: Insights from quartz porphyries from the giant Zhenyuan gold deposit

Numerous Au-rich deposits in the western South China Block, SW China, provide insights into the gold enrichment process at a lithospheric scale. Zhenyuan is a giant gold deposit in the western South China Block, but its genesis remains controversial with competing hypotheses of magmatic–hydrothermal and metamorphic–hydrothermal development. Here, we present new petrographic, major and trace element, and zircon U–Pb age data for quartz porphyry from the Zhenyuan deposit to evaluate the specific gold enrichment process. Zircon SHRIMP and LA–ICP–MS results demonstrate that the Zhenyuan quartz porphyries crystallized at ca. 255–247 Ma. This age is broadly consistent with the Re–Os age of gold-bearing pyrite measured in previous studies. The porphyries are characterized by high-K calc-alkaline compositions and exhibit significant enrichment in LILEs and LREEs, and strong depletion in HFSEs. They share the same source (Au-rich lower crust) with Cenozoic Au-bearing intrusions. The initial melts had a weak oxidation state (ΔFMQ = –0.15–0.55), which favored Au accumulation and migration. However, the gold was precipitated with the crystallization of Ti–Fe oxides in the middle crust, resulting in the emplacement of Au-poor magma in the upper crust. The gold orebodies in the Zhenyuan deposit formed due to Cenozoic hydrothermal processes, as evidenced by our observations of ore-bearing pyrite and Ar–Ar ages of phlogopite measured in previous studies. Combined with regional mineralization data, we propose lithosphere-scale processes for gold accumulation, migration, and precipitation from the Neoproterozoic to Cenozoic. Specifically, we predict an Au-rich middle crust beneath the western South China Block.

Geochronological and geochemical constraints on the genesis of Cu-Au skarn deposits of the Santa María de la Paz district (Sierra del Fraile, Mexico)

Cu-Au skarn deposits of the mining district of Santa María de La Paz are located in the Sierra del Fraile, in the northern part of San Luis Potosí, Mexico. Samples of four different intrusions and their related skarn deposits were taken underground, in order to obtain whole-rock and mineral geochemical data. U-Pb LA-ICP-MS zircon data indicate crystallization ages of ca. 37.6–35.0 Ma for skarn-related granitic and granodioritic stocks, which show dominant metaluminous to slightly peraluminous, high-K calc-alkaline compositions. These intrusions were emplaced in a post-Laramide continental arc setting under shallow conditions, as suggested by fast cooling inferred from K-Ar biotite ages of ca. 35–33 Ma. Skarn deposits were contemporaneous with emplacement and crystallization of stocks, as revealed by Re-Os molybdenite geochronological data of exoskarn rocks. Skarn rocks show a general depletion of LILE and REE and a slightly enrichment in HFSE in comparison with intrusive rocks. Pyroxene and garnet show dominant hedenbergite and andradite compositions, respectively. The association of hedenbergite, pyrite, chalcopyrite, native bismuth and pyrrhotite during skarn mineralization suggest reducing conditions, typical for Au but not for Cu skarns. Highest metal contents are concentrated in exoskarn rocks, including anomalies of Cu, Fe, Ni, Sn, Sb, Zn, W and Bi. Metal contents, ore minerals and environmental conditions of skarn rocks, together with whole-rock geochemical data of stocks and pyroxene-garnet compositions, reveal some singularities for the Sierra del Fraile deposits, as they exhibit characteristics of both Cu and Au skarn deposits but might be also compatible with Fe, W, and Zn skarns. Hence, skarn and associated vein deposits of the Sierra del Fraile might have a high potential not only for Cu-Au, but also for several base metals, as reported for comparable post-Laramide skarn and hydrothermal deposits of central and northern Mexico.

Mid–Late Cretaceous igneous activity in South China: the Qianjia example, Hainan Island

ABSTRACTBoth Pacific and Neo-Tethys plates had major influences on the Cretaceous magmatisms in southeastern China. The subduction of the Neo-Tethys plate is, however, not well studied. This paper reports zircon U–Pb ages, Lu–Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes for the Qianjia intrusive rocks in Hainan Island, southeast China. LA-inductively coupled plasma mass spectrometry zircon U–Pb dating of granites and dark enclave monzonite in the area yield magmatic crystallization ages of ca. 100 Ma, which are consistent with other Late Cretaceous granites, e.g. Baocheng, Tunchang, and Yaliang. Both rocks show high-K calc-alkaline compositions and metaluminous to weakly peraluminous signatures belonging to I-type rocks. They are enriched in the alkalis, Rb, Th, U, K, and light rare earth elements, depleted in Nb, Ta, Ti, and P, and characterized by high Al2O3 contents (14–15 wt%) and high Mg# values (50–53). Among them, some of granodiorites have geochemical affinities of adakitic rocks. Zircon εHf(t) values range from −5.97 to −1.18, with fairly constant whole-rock Sr–Nd isotopes (ISr = 0.7084–0.7086; εNd(t) = −4.97 to −4.29) similar with those of the Cretaceous mafic dikes (136–81 Ma) in Hainan Island, which are the result of partial melting of subduction-related sub-continental lithospheric mantle. Combined with Sr–Nd isotopes and negative Hf isotope, Qianjia intrusive rocks were likely derived from hybrid melts of underplated continental crust-derived with mantle-derived, then experienced varied degrees of fractional crystallization. According to the latest geophysical, sedimentological, and geochemical data, previous authors identified a Cretaceous E–W-trend subduction zone in the northern margin of the South China Sea. Combined with the southern margin magmatisms (110–80 Ma) and magmatisms of ~120 Ma distributed east–west ward from the Philippines to the Vietnam, We preferred that the subduction of the E–W-trend Neo-Tethys plate was the main geodynamic mechanism which induced the Cretaceous large-scale magmatisms in the southern margin of South China Block.

Paleoproterozoic andesitic volcanism in the southern Amazonian craton, the Sobreiro Formation: New insights from lithofacies analysis of the volcaniclastic sequences

The Sobreiro Formation (SF) records one excellent and well-preserved example of subaerial Precambrian (ca. 1.88Ga) volcanism on earth. It is located in the São Felix do Xingu region (SFX), in the eastern part of Pará State, southern Amazonian craton, northern of Brazil. The high-K calc-alkaline composition of the Sobreiro rocks indicates that this formation likely generated in an ocean-continent convergent margin. This paper documents the architecture of a series of basaltic-andesite to andesite and minor dacite lava flows and associated volcaniclastic rocks. These rocks are divided into primary and secondary lithotypes, depending if they resulted from a direct volcanic activity (pyroclastic) or reworked processes and massive to stratified, depending on the different transport and emplacement mechanisms. Primary lithofacies is formed of pyroclastic flow and surge deposits, volcanic breccias and welded ignimbrites; secondary volcaniclastic lithofacies consist of reworked debris. Mass-flows, hyperconcentrated flows and stream floods are interpreted as fluvial/alluvial deposits representing periods of stream and river reworking and re-establishment after an eruptive phase or an edifice failure event. These different lithofacies record the volcanic history of the Sobreiro Formation. A complex volcanic environment is thought to have existed with emission of large lava flows and explosive eruptions. The modern volcanological approach used here can serve as a model for the evolution of Precambrian volcano-sedimentary basins. Our approach sheds new light on the different processes operating on volcanic edifices and to constrain the depositional environment and thus geodynamic setting of Precambrian continental volcanic belts.

Petrogenesis of granitoids in the eastern section of the Central Qilian Block: Evidence from geochemistry and zircon U-Pb geochronology

The Caledonian-age Qilian Orogenic Belt at the northern margin of the Greater Tibetan Plateau comprises abundant granitoids that record the histories of the orogenesis. We report here our study of these granitoids from two localities. The Qingchengshan (QCS) pluton, which is situated in the eastern section of the Central Qilian Block, is dated at ~430–420 Ma. It has high-K calc-alkaline composition with high SiO2 (> 70 wt%), enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and varying degrees of negative Sr and Eu anomalies. The granitoids in the Tongwei (TW) area, 150 km east of the QCS, are complex, the majority of which are dated at ~440 Ma, but there also exist younger, ~230 Ma intrusions genetically associated with the Qinling Orogeny. The Paleozoic TW intrusions also have high SiO2, fractionated REE (rare earth element) patterns, but a negligible Eu anomaly. The whole rock Sr-Nd-Hf isotopic compositions suggest that all these Paleozoic granitoids are consistent with melting-induced mixing of a two-component source, which is best interpreted as the combination of last fragments of subducted/subducting ocean crust with terrigenous sediments. The mantle isotopic signature of these granitoids (87Sr/86Sri: 0.7038 to 0.7100, eNd(t): −4.8 to −1.3, eHf(t): −0.7 to +4.0) reflects significant (~70 %) contribution of the ocean crust derived in no distant past from the mantle at ocean ridges with an inherited mantle isotopic signature. Partial melting of such ocean crust plus terrigenous sediments in response to the ocean closing and continental collision (between the Qilian and Alashan Blocks) under amphibolite facies conditions is responsible for the magmatism. Varying extents of fractional crystallization (±plagioclase, ±amphibole, ±garnet, ±zircon) of the parental magmas produced the observed QCS and TW granitoids. We note that sample HTC12–01 in the TW area shows an A-type or highly fractionated granite signature characterized by elevated abundances and a flat pattern of REEs, weak Nb-Ta anomaly, conspicuous negative Sr and Eu anomalies (Sr/Sr* = 0.09, Eu/Eu* = 0.22), and thus the high 87Sr/86Sr ratio (0.7851), and moderate eNd(t) (−4.9) and eHf(t) (−2.0), pointing to the significant mantle contribution. Compared with the Paleozoic granitoids, the ~230 Ma granitoids in the TW area represented by sample JPC12–02 have higher initial 87Sr/86Sr (0.7073) and lower eNd(t) (−6.2) and eHf(t) (−4.5) values, offering an ideal opportunity for future studies on tectonic effects of juxtaposition of younger orogenesis on an older orogen.