Volcano-sedimentary Sequence Research Articles

Mid-Ordovician and Late Devonian magmatism in the Togtokhinshil Complex: new insight into the formation and accretionary evolution of the Lake Zone (western Mongolia)

New field, petrographic, geochemical and geochronological data from the Togtokhinshil Complex (western margin of the Lake Zone, Central Asian Orogenic Belt) reveal the presence of two separate Early Palaeozoic magmatic pulses: Mid-Ordovician and Late Devonian. The Mid-Ordovician (c. 460 Ma) magmatism produced various types of mafic plutonic rocks (gabbro–diorite suite) emplaced into the volcanosedimentary sequence of the Baatar Fm. Gabbros to gabbrodiorites have metaluminous and mostly low to normal-K calc-alkaline or possibly tholeiitic character. The majorand trace-element as well as Nd isotopic signatures suggest its magmatic-arc geotectonic setting and imply the derivation either exclusively from the mantle or in part from the juvenile mafic crust. The Late Devonian (c. 376 Ma) granite suite intruded the rocks of both the gabbro–diorite suite and the Baatar Fm. Granites are subaluminous and have a high-K calc-alkaline to shoshonitic character. Their magmatic arc-like geochemical signature is interpreted as inherited from the recycled arc-related crustal source. The granite suite seems a product of extensive mantle heat-induced crustal anatexis. The Depleted Mantle Nd model ages (c. 0.75 Ga) indicate derivation of the magmatic rocks of the both suites from relatively juvenile, and geochemically immature, sources. These results provide a clear evidence of the Mid-Ordovician arc-related magmatism in the western part of the Lake Zone. They thus prolong the assumed period of magmatic-arc activity and constrain onset of accretionary processes in this part of the Central Asian Orogenic Belt. The gabbro–diorite suite represents either part of a long-lived magmatic-arc or, more likely, a member of multiple island-arc system along the western margin of the Lake Zone. Newly described Late Devonian magmatism in the Togtokhinshil Complex provides further evidence of tectono-thermal event that was widespread in the Central Asian Orogenic Belt. The geodynamic cause for this event remains uncertain, but effects of lithospheric thinning and/or asthenospheric mantle upwelling are considered the most likely.

Digital Image Processing of Multispectral ASTER Imagery for Delineation of Alteration and Related Clay Minerals in Sakoli Belt: Maharashtra – A Case Study

Abstract: The extraction of spectral signature through remote sensing data in geological studies is very limited in Indian terrain. The present research is an attempt to delineate alteration and associated minerals using high resolution multispectral advanced space borne thermal emission and reflection (ASTER) data in Sakoli volcano-sedimentary sequence. The alteration zones are generally associated with certain types of mineral and mineral aggregates and are localized in the ground that can be mapped remotely using spectral mapping techniques. The band ratio and selective PCT were applied on calibrated ASTER image to extract spectral signature of index mineral. The results showed that high abundance of these minerals within pixels is corresponding to the alteration zones. The field observation, petrography and XRD analysis of the alteration zones confirmed the presence of dominant mineral phases that were identified remotely.

Near-surface ambient noise tomography in the Baogutu copper deposit area

The Baogutu copper deposit region is located in the northwestern margin of the Junggar basin, controlled by the NE–SW trend Darbut fault. This region developed mid-late Hercynian (330–320 Ma) intermediate-acid small porphyry bodies. Hosted in the lower carboniferous volcano-sedimentary sequences, the Baogutu porphyry copper deposit has experienced two epochs of fluid metallogenesis after the adakitic magma diagenesis. In the copper deposit area, the orebody distributes in the strata of the Baogutu and Xibeikulasi group, and the ground surface is covered by tuffaceous siltstone. The clear structure of the copper deposit has not been revealed yet. In this paper, we utilize the technique of ambient noise tomography (ANT) to image the phase velocity anomaly maps of different frequencies in the deposit area. Then, we invert the shear (S-) wave velocity structures by using a damped least squares inversion algorithm. The pattern of the tomographic images indicates that the copper deposit mainly distributes in the range of 0.3–0.5 km depth. The major orebody is identified as an obviously high-velocity anomaly. In addition, a small high-velocity anomaly is observed at a depth greater than 0.4 km in the northeast of the study area, and it might associate with another orebody of the Baogutu copper deposit, which has not previously been reported.

The use of decision tree induction and artificial neural networks for recognizing the geochemical distribution patterns of LREE in the Choghart deposit, Central Iran

Some evidences of rare earth elements (REE) concentrations are found in iron oxide-apatite (IOA) deposits which are located in Central Iranian microcontinent. There are many unsolved problems about the origin and metallogenesis of IOA deposits in this district. Although it is considered that felsic magmatism and mineralization were simultaneous in the district, interaction of multi-stage hydrothermal-magmatic processes within the Early Cambrian volcano-sedimentary sequence probably caused some epigenetic mineralizations. Secondary geological processes (e.g., multi-stage mineralization, alteration, and weathering) have affected on variations of major elements and possible redistribution of REE in IOA deposits. Hence, the geochemical behaviors and distribution patterns of REE are expected to be complicated in different zones of these deposits. The aim of this paper is recognizing LREE distribution patterns based on whole-rock chemical compositions and automatic discovery of their geochemical rules. For this purpose, the pattern recognition techniques including decision tree and neural network were applied on a high-dimensional geochemical dataset from Choghart IOA deposit. Because some data features were irrelevant or redundant in recognizing the distribution patterns of each LREE, a greedy attribute subset selection technique was employed to select the best subset of predictors used in classification tasks. The decision trees (CART algorithm) were pruned optimally to more accurately categorize independent test data than unpruned ones. The most effective classification rules were extracted from the pruned tree to describe the meaningful relationships between the predictors and different concentrations of LREE. A feed-forward artificial neural network was also applied to reliably predict the influence of various rock compositions on the spatial distribution patterns of LREE with a better performance than the decision tree induction. The findings of this study could be effectively used to visualize the LREE distribution patterns as geochemical maps.

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognised that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

Skarn formation and trace elements in garnet and associated minerals from Zhibula copper deposit, Gangdese Belt, southern Tibet

Trace element concentrations in garnet and associated minerals from the mid-Miocene Zhibula Cu skarn, Gangdese Belt, Tibet reflect a diversity of local environments, evolving fluid parameters and partitioning with coexisting minerals. Exoskarn occurs as massive but narrow intervals within a Lower Jurassic volcano-sedimentary sequence containing limestone, the main skarn protolith. Endoskarn is present at the contact with mid-Miocene granodiorite dikes. Prograde skarn associations are garnet-dominant but also include diopside-dominant pyroxene in variable amounts. Garnet compositions in exoskarn change from andradite (And)- to grossular (Gr)-dominant from the massive intervals to bands/lenses within marble/tuff, but not in endoskarn. In both cases however, associations at the protolith contact include anorthite and wollastonite, both indicative of skarnoid or distal (relative to fluid source) skarn formation. Exoskarns also contain vesuvianite. Retrograde clinozoisite, actinolite and chlorite replace pre-existing skarn minerals. Garnet displays brecciation and replacement by Al-richer garnet. Depending on partitioning among coexisting minerals, chondrite-normalised REY (REE+Y) fractionation trends for garnet depict endo- to exoskarn diversity, the dominance of And- vs. Gr-rich garnet (in turn related to proximal-to-distal relationship to fluid source), as well as prograde-to-retrograde evolution in the same sample. A strong variation in Eu-anomaly, from positive to negative, in And-dominant garnet can be correlated with variation in salinity of ore-forming fluids, concordant with published fluid inclusion data. Trends depicted by And- and Gr-dominant garnets are consistent with published data from skarns elsewhere, in which the dominant substitution mechanism for REY is YAG-type. Zhibula garnets are enriched in a range of trace elements less commonly reported, including W, Sn, and As, but also Mo (as high as 730ppm), an element seldom analysed for in silicates. Molybdenum, W, and Sn display excellent co-correlation and shared zonation patterns on LA–ICP–MS maps of garnet, indicating substitution in the crystal lattice. As well as assisting in interpreting skarn evolution in time and space, and providing constraints on ore genesis, the trace element data for garnet explain the range of colours observed. The discovery of garnets carrying significant concentrations of W, Sn and Mo is a valuable finding that deserves evaluation in post-collisional skarns elsewhere, and is potentially of critical significance in prospecting. Together with a conspicuous trace ore mineral signature, garnet compositions at Zhibula support a genetic connection and sharing of ore-forming fluids between the skarn and the Qulong porphyry Cu-Mo deposit, 2km to the north. Within the Gangdese belt, or in analogous settings elsewhere, the presence of deep-seated porphyry mineralization beneath exposed skarns could be tested for by studying garnet chemistry. As more data become available, such trace element signatures could be viable tools for distinguishing barren from mineralized skarn systems.

Rocking around a volcanic island shelf: Pliocene Rhodolith beds from Malbusca, Santa Maria Island (Azores, NE Atlantic)

Rhodoliths are a common producer of carbonates on modern and ancient shelves worldwide, and there is growing evidence that they thrive on volcanic insular shelves. However, little is still known on how rhodoliths cope with the demands of this particularly dynamic environment. In this study, the focus is placed on fossil rhodoliths from a Pliocene sequence at Santa Maria Island, Azores, in order to gain further insight into the life cycle (and death) of rhodoliths living within a mid-ocean active volcanic setting. These rhodoliths occur as a massive accumulation within a larger submarine volcano-sedimentary sequence that was studied from the macro- to the micro-scale in order to reconstruct the paleoenvironmental conditions under which the rhodolith accumulation was deposited and buried. All fossil rhodoliths from this setting are multi-specific and demonstrate robust growth forms with a lumpy morphology. Moreover, taphonomical analyses show the succession of several destructive events during rhodolith growth, suggesting life under a highly dynamic system prior to stabilization and burial. The rhodoliths therefore tell a story of an eventful life, with multiple transport and growth stages, owing to the environment in which they lived. Transport and deposition to their final resting place was storm-associated, as supported by the general sedimentary sequence. In particular, the sequence features an amalgamation of tempestites deposited under increasing water depths, sediment aggradation, and before burial by volcanic activity. This transgressive trend is also attested by the overall characteristics of the volcano-sedimentary succession, which exhibits the transition to subaerial environment in excess of 100 m above the rhodolith bed.

The genesis of Archean supracrustal rocks in the western Shandong Province of North China Craton: Constraints on regional crustal evolution

Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province (WSP) is located at the eastern part of the North China Craton (NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.

Ore deposit types and tectonic evolution of the Iberian Pyrite Belt: From transtensional basins and magmatism to transpression and inversion tectonics

A new interpretation of the structural evolution of the Iberian Pyrite Belt (IPB) and volcanogenic massive sulfide mineralization (VMS) is presented in this work, based on a review of the ore deposit types, the analysis of the hosting volcanic sequences and the tectonic evolution. The VMS deposits of the IPB are hosted by volcanic and siliciclastic rocks. Four main volcano-sedimentary sequences (VSC), from VSC0 to VSC3, have been assumed, the main deposits being located in the VSC0 and at the top of the VSC2.We have defined three main sectors oriented approximately E-W and hosting the VMS deposits. In the Northern sector, which is mostly located in Spain, graben basins and local pull-aparts are the main structures. In this sector, two belts can be distinguished, the deposits being located at the top of the VSC2 felsic volcanism (Rio Tinto-type IPB deposits). In the Central sector, both in Spain and Portugal, half-graben basins are the most common structures, and the deposits are mostly located in the VSC0 andesitic volcanic-sedimentary sequence (Tharsis-type IPB deposits). In the Southern sector, which is only located in Portugal, a graben basin with a pull-apart is again the main structure, and the deposits are located in black slates and at the top of a felsic volcanism, Strunian in age (VSC0). The deposits located in graben basin with a pull-apart are essentially felsic volcanic-hosted with some siliciclastic material, mostly black shales. By contrast, those located in half-graben basins are mainly hosted by black-shales with minor amounts of andesitic rocks.The tectonic evolution shows that as a result of a counterclockwise rotation of the stress axes, the formation of the IPB and the associated ore deposits took place during several episodes, from transtension (with the development of both graben with pull-aparts and half-graben basins), through left lateral E-W shearing, to transpression. At the beginning of the transtensional process, several extensional, roughly E-W trending faults that developed graben and half-graben basins were generated and the first volcanic andesite-rhyolite rocks (VSC0) formed. The Tharsis-type deposits, mainly hosted by black slates with some volcanic rocks, were formed in the Central sector while the Neves Corvo-type deposit, hosted by black slates and felsic volcanism formed in the Southern one. After a period characterized by barren mafic volcanism (VSC1), a sinistral shear affected the previous fractures due to the stress axis rotation and felsic crustal volcanism started (VSC2). Rhyolites and dacites were particularly abundant in two graben basins, which developed rollovers in pull-apart zones, forming the Rio Tinto-type deposits in the Northern sector. The thermal increase associated with VSC0 and VSC2 gave rise to the development of crustal-scale hydrothermal convective cells, which generated both types of deposits.After a barren VSC3 felsic volcanism, subsequently, during the Variscan transpressional phase, the E-W extensional faults were reactivated as reverse faults, affecting the volcanic sequence (VSC0 to VSC3) as well as the interbedded sedimentary rocks (mostly black shales). As has been recognized at the Rio Tinto deposit, buttressing must have played a significant role in the geometry of inverted structures, and the VMS ores were intensely recrystallized.It should be emphasized that this new regional geological model for the IPB is an approach to provide a better insight into VMS deposits and could be a key-point for further studies, providing a new tool to improve knowledge of the VMS mineralizations and exploration guidelines elsewhere in the IPB.

Geochemical Signatures and Inclusions in Apatite as Markers of a Hidden Ultrahigh-Pressure Event (Betic Cordillera, Spain)

AbstractDiamond inclusions have been found in apatite from a specific type of orthogneiss of a volcanosedimentary sequence from the deepest tectonic complex (Nevado-Filabride) of the Alpine Betic-Rif belt (Spain). This unusual finding led us to make a detailed petrographical and geochemical study, with the aim of characterizing ultrahigh-pressure (UHP) apatite and comparing it with the younger and widespread magmatic and/or metamorphic generations. The UHP apatite is abundant in biotite-rich orthogneisses, whereas it is lacking in the most common biotite-poor orthogneisses, which instead contain abundant magmatic apatite. In addition to diamond, UHP apatite contains abundant biphased inclusions consisting of chlorides and hydrocarbons. The late alteration of fluid inclusions caused oligomerization of hydrocarbons and graphitization of diamond, leading to a characteristic irregular outer brownish zone. The UHP apatite shows bell-shaped rare earth element (REE) profiles, with weak Eu anomalies and Sr and Th...

A preliminary Geological and Generalized Stratigraphy of Western Margin of Northern Afar Depression, Dallol Area, Northern Ethiopia

The western margin of northern Afar Depression constitutes various rock units of Neoproterozoic basement complex, Paleozoic to Mesozoic sedimentary successions and the Oligocene to present volcano-sedimentary sequences. As part of a reconnaissance survey for mineral resources in selected districts of Afar region, a geological and structural map of Dallol area has been produced with the help of the Landsat Thematic Mapper Plus imagery at a scale of 1:100,000 and a simplified stratigraphy is constructed for the western rift margin, Dallol area.Keywords: Geology, Stratigraphy, Western Rift Margin, Dallol, Northern Afar Depression, Ethiopia.

Geology, ore facies and sulfur isotopes geochemistry of the Nudeh Besshi-type volcanogenic massive sulfide deposit, southwest Sabzevar basin, Iran

The southwest Sabzevar basin is placed in the southwestern part of a crustal domain known as the Sabzevar zone, at the north of Central Iranian microcontinent. This basin hosts abundant mineral deposits; particularly of the Mn exhalative and Cu-Zn volcanogenic massive sulfide (VMS) types. The evolution of this basin is governed by the Neo-tethys oceanic crust subduction beneath the Central Iranian microcontinent and by the resulting continental arc (Sanandaj-Sirjan) and back-arc (Sabzevar-Naien). This evolution followed two major sequences: (I) Lower Late Cretaceous Volcano-Sedimentary Sequence (LLCVSS), which is indicated by fine-grained siliciclastic sediments, gray basic coarse-grained different pyroclastic rocks and bimodal volcanism. During this stage, tuff-hosted stratiform, exhalative Mn deposits (Nudeh, Benesbourd, Ferizy and Goft), oxide Cu deposits (Garab and Ferizy) and Cu-Zn VMS (Nudeh, Chun and Lala) deposits formed. (II) Upper Late Cretaceous Sedimentary Dominated Sequence (ULCSS), including pelagic limestone, marly tuff, silty limestone and marl with minor andesitic tuff rocks. The economically most important Mn (Zakeri and Cheshmeh-sefid) deposits of Sabzevar zone occur within the marly tuff of this sequence. The Nudeh Cu–Zn volcanogenic massive sulfide (VMS) deposit is situated in the LLCVSS. The host-rock of deposits consists of alkali olivine basalt flow and tuffaceous silty sandstone. Mineralization occurs as stratiform blanket-like and tabular orebodies. Based on ore body structure, mineralogy, and ore fabric, we recognize three different ore facies in the Nudeh deposit: (1) a stringer zone, consisting of a discordant mineralization of sulfides forming a stockwork of sulfide-bearing quartz veins cutting the footwall volcano-sedimentary rocks; (2) a massive ore, consisting of massive replacement pyrite, chalcopyrite, sphalerite and Friedrichite with magnetite; (3) bedded ore, with laminated to disseminated pyrite, and chalcopyrite. Chloritization, silicification, sericitization and epidotization are the main wall-rock alterations; alteration intensity increases towards the stringer zone. The δ34S composition of the sulfides ranges from −1.5‰ to +3.69‰ with a general increase of δ34S ratios of massive ore facies to stockwork zone. The heavier values indicate that some of the sulfur was derived from seawater sulfate that was ultimately thermochemically reduced in deep hydrothermal reaction zones. Sulfur isotopes, along with sedimentological, textural, petrological, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a Besshi-type VMS ore deposit.

A Neoarchean paleomagnetic pole from the Kisii Series of western Kenya: Implications for crustal mobility

The Kisii Series lavas of western Kenya were the target of a paleomagnetic study. The Kisii Series is a volcano-sedimentary sequence dated to 2531±3Ma (U–Pb) that rests unconformably over the Mesoarchean–Neoarchean Nyanzian and Kavirondian Series. The paleomagnetic study reported in this paper expands on an earlier study by Brock et al. (1972) using modern demagnetization and analysis techniques. In spite of the advanced methods, the results of both our new study and the previous study are statistically indistinguishable. We therefore combine the results of both investigations to arrive at a grand mean pole at 7°S, 166°E (A95=8°). Due to the relatively minor tilt-corrections, a fold test is inconclusive; however, the dual-polarity magnetization passes a reversal test. We argue for a primary remanence on the basis of a comparison to other paleomagnetic data in Kenya and Tanzania and geological information. We examine the relationships between age-equivalent paleomagnetic poles from the Tanzanian, Zimbabwe and Kaapvaal cratons. Based on this limited dataset, we cannot conclude whether or not the observed apparent polar wander was due to true polar wander or modern-style plate tectonics, such as would be implied by the (variable) apparent polar wander path segments. We favor the latter explanation based on the disparate lengths of the Zimbabwe versus Tanzanian apparent polar wander paths during the 2700–2500Ma interval. The Protopangea model is not consistent with paleomagnetic data from these three cratons.

Age and origin of the gabbros in the Jilotlán pluton, Jalisco: primitive magmatic rocks in the southern part of the Guerrero terrane

An outstanding feature of the Mexican Cordilleran magmatic arc between Sonora and Chiapas is the presence of numerous plutonic and batholitic bodies of Cretaceous to Miocene ages. In this contribution, we investigated the petrogenesis and age of the gabbros from Jilotlan, Jalisco, in order to better understand their genetic relation with the intruding and surrounding Late Cretaceous calc-alkaline mantle-type granitoids. Gabbros from Manzanillo (Colima), Tepalcaltepec, and Aquila (Michoacan) were also studied. The objective of this work is to contribute new findings to the tectonic setting and magma generation processes of the Cretaceous magmatic arc in the coastal Zihuatanejo terrane, which is part of the Guerrero terrane. All gabbros are hornblende rich, which indicates hydrated con- ditions. Clinopyroxene crystals and hornblende pseudomorphs have plagioclase inclusions, suggesting exsolution by a rapid uplift. Geochemically, most gabbros are calc-alkaline. The relatively flat trace element patterns display incompatible element (LILE and LREE) en- richment relative to the less incompatible elements (HFSE and HREE), which show minor enrichment or are even depleted with respect to N-MORB. Positive Pb and Sr anomalies as well as LILE enrichment in comparison to HFSE and REE suggest a magmatic arc origin. The Sr and Nd isotopic compositions indicate that the Jilotlan, Tepalcaltepec, and Manzanillo gabbros have a common source, comparable to N-MORB. Depleted mantle model ages (TDM) are similar for the gabbros and a quartz monzodiorite (averaging ~500 Ma) reflecting participation of a crustal component in their genesis. Isotopic data indicate that the gabbros could have been generated from a combination of mantle and crustal components. The existence of a Cretaceous, Barremian to Albian (~130–100 Ma), extensional arc proposed by other studies suggests that the gabbros were generated in a thinned crust setting. Hornblende 40Ar/39Ar ages and U-Pb zircon ages of Jilotlan gabbros are undistinguishable at ~114 Ma, which indicates that they crystallized ~40 Ma earlier than the surrounding granitoids dated at ~70 Ma. The identical ages for both isotopic systems in the gabbros give evidence for their shallow emplacement and/or rapid uplift. The ~118 Ma (K-Ar hornblende) age of an andesite from the Tecalitlan Formation host rock suggests that the gabbros may represent the plutonic counterpart of this volcanosedimentary sequence. The crystalline rocks of the study area are magmatically very primitive (initial epsilon Nd (eNdi) up to ~+7, TDM max. ~0.7 Ga), without evidence of evolved crustal components in comparison to the Puerto Vallarta batholith, located NW of the study area, which contains Precambrian crustal components (eNdi as low as ~-7, TDM up to ~1.55 Ga). This evidences different tectonic settings for these two areas. This study supports the hypothesis that a long-lived subduction zone (at least since the Early Cretaceous) generated poorly evolved arc rocks in a continental arc setting in the Guerrero terrane.

Myliobatid and pycnodont fish from the Late Cretaceous of Central India and their paleobiogeographic implications

Bulk sampling from Upper Cretaceous (Maastrichtian) Deccan volcano-sedimentary sequences at Kisalpuri (District Dindori, Madhya Pradesh) and Pisdura–Dongargaon (Chandrapur District, Maharashtra) in Peninsular India has yielded the dental remains of myliobatid and pycnodont fish. This fish fauna comprises Igdabatis indicus, Pycnodontoidea indet. and Pycnodontidae indet., and resembles assemblages known from Upper Cretaceous deposits in Africa and Europe. While paleobiogeographically speaking, the presence of Igdabatis suggests a series of shallow marine dispersals that may have occurred between Africa and India, possibly along the margins of the Kohistan–Ladakh island arc during the latest Cretaceous; the record of pycnodont fish favours instead a Gondwanan dispersal event.

The formation of the Suyunhe large porphyry Mo deposit in the West Junggar terrain, NW China: Zircon U–Pb age, geochemistry and Sr–Nd–Hf isotopic results

Newly-discovered Suyunhe Mo deposit (0.57Mt Mo metal), located in the Barluk Mountains of the West Junggar terrain, Xinjiang, NW China, is the largest porphyry Mo deposit discovered in Xinjiang so far. The Suyunhe Mo deposit is associated with the Suyunhe intrusive complex which was emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Group. The Suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry, granodiorite porphyry, monzonitic granite porphyry, and tonalite porphyry stocks which are cut by the felsic dikes. Dominant Mo mineralization is hosted in the Barluk Group volcano-sedimentary rocks but is minor in the granite, granite porphyry, granodiorite porphyry, and monzonitic granite porphyry of the Suyunhe complex. We report in situ zircon U–Pb ages, whole rock geochemistry, Sr–Nd isotopic and zircon Lu–Hf isotopic data for the ore-bearing intrusive rocks from the Suyunhe deposit.Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the deep granite emplaced at 298.4±1.9Ma–295.3±3Ma and the shallow granodiorite porphyry intruded during 294.7±2.1Ma–293.7±2.3Ma. The Suyunhe granite intrusion, including fine- and medium-grained granite, belongs to the high-K calc-alkaline series. Most of them have low Zr+Nb+Y+Ce (<350ppm) and 10,000Ga/Al (<2.7) values, showing a geochemical affinity to I-type granite. These granites are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE) and Eu, Ba, Sr, Nb, P and Ti, indicating that they have experienced advanced fractional crystallization. They have low initial 87Sr/86Sr ratios (0.70318–0.70380), high whole-rock εNd(t) values (+4.4 to +6.0), high zircon εHf(t) values (+10.8 to +14.6), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (<0.35). These features demonstrate that the ore-bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization. The Suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite, indicating that they share a common magma source and process with the granite. The Suyunhe granodiorite porphyries belong to slightly high-K calc-alkaline series and have LREE enrichment and HREE, Nb and Ti depletion. They have low (87Sr/86Sr)i ratios (0.70421–0.70444), high εNd(t) values (+4.9 to +6.2), high zircon εHf(t) values (+9.6 to +15.2), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (<0.39) with one reception of 0.48. These features indicate that the granodiorite porphyries have a relatively deeper magma source region of the lower crust and have not experienced as much fractional crystallization. The Suyunhe barren tonalite porphyries show enrichment of LREE and depletion of HREE, Nb and Ti depletion, and have chemical composition (e.g., Mg#=0.41–0.48) different from the Suyunhe granodiorite porphyries, though isotopically they are indistinguishable. This suggests that the parental magma of the barren tonalite porphyries originated from melting of a lithospheric mantle. Therefore, the Suyunhe ore-bearing magmas derived from the juvenile lower crust which has a high potential to bear porphyry Mo deposit and result in the formation of the Suyunhe large porphyry Mo deposit.

Reconstructing multiple arc-basin systems in the Altai–Junggar area (NW China): Implications for the architecture and evolution of the western Central Asian Orogenic Belt

The Altai–Junggar area in northwestern China is a critical region to gain insights on the tectonic framework and geological evolution of the western Central Asian Orogenic Belt (CAOB). In this study, we report results from integrated geological, geochemical and geophysical investigations on the Wulungu Depression of the Junggar Basin to determine the basement nature of the basin and understand its amalgamation history with the Chinese Altai, within the broad tectonic evolution of the Altai–Junggar area. Based on borehole and seismic data, the Wulungu Depression is subdivided into two NW-trending tectonic units (Suosuoquan Sag and Hongyan High) by southward-vergent thrust faults. The Suosuoquan Sag consists of the Middle–Late Devonian basaltic andesite, andesite, dacite, tuff, tuffaceous sandstone and tuffite, and the overlying Early Carboniferous volcano-sedimentary sequence with lava flows and shallow marine sediments from a proximal juvenile provenance (zircon εHf(t)=6.0–14.9), compared to the Late Carboniferous andesite and rhyolite in the Hongyan High. Zircon SIMS U–Pb ages for dacites and andesites indicate that these volcanics in the Suosuoquan Sag and Hongyan High erupted at 376.3Ma and 313.4Ma, respectively. The Middle–Late Devonian basaltic andesites from well LC1 are calc-alkaline and exhibit primitive magma-like MgO contents (7.9–8.6%) and Mg# values (66–68), with low initial 87Sr/86Sr (0.703269–0.704808) and positive εNd(t) values (6.6–7.6), and relatively high Zr abundance (98.2–116.0ppm) and Zr/Y ratios (5.1–5.4), enrichment in LREEs and LILEs (e.g., Th and U) and depletion in Nb, Ta and Ti, suggesting that they were probably derived from a metasomatized depleted mantle in a retro-arc extensional setting. The well LC1 andesitic tuffs, well L8 dacites, well WL1 dacitic tuffs and well L5 andesites belong to calc-alkaline and metaluminous to peraluminous (A/CNK=0.8–1.7) series, and display low Mg# values (35–46) and variably positive εNd(t) (4.5–8.5) and εHf(t) (10.2–16.8) values, as well as young isotopic model ages. These Devonian–Carboniferous intermediate–felsic volcanics are interpreted as the products of partial melting of a juvenile lower crust with some contributions from mantle components in an evolved island arc setting from immature to mature island arc. The basin filling pattern and the distribution of arc volcanics and their zircon Hf model ages with the eruptive time suggest that the Wulungu Depression represents an island arc-basin system with the development of a Carboniferous retro-arc basin. In combination with previous work, we propose that the northern Junggar area comprises three arc-basin belts from south to north: the Darbut–Luliang–Karamaili, Wulungu–Yemaquan, and Saur–Fuhai–Dulate. Such tectonic subdivisions are consistent with the regional gravity and magnetic anomaly data. The recognition of the Wulungu arc-basin system demonstrates that the Junggar Basin is likely underlain by juvenile continental crust rather than ancient Precambrian basement, and also implies that the CAOB was built by amalgamation of multiple linear arcs and accretionary complexes.

A geochemical modeling to predict the different concentrations of REE and their hidden patterns using several supervised learning methods: Choghart iron deposit, bafq, Iran

The Bafq mining district hosts some Kiruna-type iron oxide-apatite (IOA) deposits which are commonly formed as a result of the multistage interaction of hydrothermal-magmatic processes within the Early Cambrian volcano-sedimentary sequence. Rare earth elements (REEs) are potentially concentrated under different physicochemical conditions in IOA deposits. Choghart orebody is one of the main magnetite-apatite deposits in the region. A wide range of primary and secondary geological events have affected the Choghart deposit, causing the behavior of REEs to vary in different zones. This study proposes a suitable exploration technique using various classification methods to identify the different concentrations of REE and their hidden patterns. To provide the required data, a systematic lithogeochemical sampling was performed in the north and NE of the Choghart orebody. The REE contents of samples were transformed into discrete values as distinct classes based on the results of clustering analysis. All possible combinations of features, being the geographical location and the major oxide composition of samples, were selected as subsets of predictors in every classification method. For each REE, 455 prediction models were constructed using these predictors. The performances of the classification methods were evaluated by error criteria with regard to all cases. Having the least amount of errors, the decision tree method was selected as the most suitable method. Based on decision tree results, the best subsets of predictors were chosen for each element. The existence of a significant relationship between the distribution patterns of each REE and the related predictors was assessed by its prediction errors. The assessment illustrated that some REE are reasonably predictable, and others are too irregular to be modeled. The extracted classification rules describe the geochemical relationships among the most important factors influencing the different concentrations of REE in the Choghart orebody. These results can be extended to other similar deposits to predetermine some REE-enriched zones based on major elements analysis. Merely by employing such techniques in REE exploration projects, a great savings in time and cost will be affected.

Geochemistry and zircon geochronology of the Neoarchean volcano-sedimentary sequence along the northern margin of the Nilgiri Block, southern India

The Nilgiri Block is one of the major Archean crustal blocks that define the tectonic framework of southern India. Here we report geologic, petrologic, geochemical, and zircon U–Pb, –REE, and –Lu–Hf data of a highly metamorphosed and disrupted sequence of amphibolite, meta-gabbro, websterite, volcanic tuff, meta-sediment, and banded iron formation (BIF) from the northern fringe of the Nilgiri Block. Geochemically, the amphibolite shows altered ocean floor basalt signature, whereas the meta-gabbro and the websterite samples form part of a volcanic arc. The metamorphosed volcanic tuff shows subalkaline rhyolitic signature. U–Pb isotope analysis of zircon grains from the volcanic tuff and meta-gabbro shows 207Pb/206Pb ages of 2490±12Ma and 2448±16Ma, respectively. Zircons from the meta-sediments show an age range of 2563±33Ma to 2447±34Ma. The dominantly positive εHf (t) values of the zircons in the analyzed rock suite suggest that the magmas from which the zircons crystallized evolved from a Neoarchean depleted mantle source. The Hf model ages (TDM) of volcanic tuff, meta-sediment and meta-gabbro samples are ranging between 2908–2706Ma, 2849–2682Ma, and 2743–2607Ma, respectively. The ca. 2500Ma ages for the arc-related magmatic rock suite identified along the northern periphery of Nilgiri Block suggest prominent Neoarchean arc magmatism and early Paleoproterozoic convergent margin processes contributing to the early Precambrian crustal growth in Peninsular India.

The ∼860 Ma mafic dikes and granitoids from the northern margin of the Yangtze Block, China: A record of oceanic subduction in the early Neoproterozoic

There are voluminous Neoproterozoic arc-related volcano-sedimentary sequences and small intrusions on the northern margin of the Yangtze Block, South China. The understanding the origin of the Sanligang granitoid intrusion and the spatially associated mafic dikes in the region is crucial for unraveling the tectonic evolution and continental crust growth processes in the Yangtze Block. Zircon U–Pb dating suggests that the mafic dikes (ca. 871Ma) and granitoids (ca. 860Ma) are contemporaneous. The mafic dikes have low SiO2 (45.37–46.55wt.%), K2O (0.32–0.82wt.%) and Na2O (2.01–2.85wt.%), and are characterized by enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), suggesting that their mantle source was modified by subducted materials. The Sanligang granitoids have intermediate to high SiO2 (60.35–71.38wt.%), intermediate K2O (1.38–3.67wt.%) and Na2O (3.97–5.33wt.%), and high MgO (1.03–3.16wt.%). They show LREE-enriched REE patterns (La/YbN=7.2–12.3) with no or minor negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dikes and granitoids share similar zircon ɛHf(t) values (+10.5 to +12.9, +7.9 to +11.7, respectively), whole-rock initial 87Sr/86Sr ratios (0.7051–0.7057, 0.7033–0.7041, respectively) and ɛNd(t) values (+4.0 to +7.1, +3.4 to +4.9, respectively), suggesting that the granitoids were generated by partial melting of juvenile basaltic crust. High Mg# values (49–58) in the granitoids may have resulted from assimilation of residual mafic minerals in their source region. Based on its arc-related geochemical affinity and contemporaneous arc-related magmatism, the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting during crustal growth and reworking. The early Neoproterozoic assemblage from the Sangligang–Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.