2020 Vol. 11, No. 1
The proto-Philippine Sea Plate (pPSP) has been proposed by several authors to account for the origin of the Mesozoic supra-subduction ophiolites along the Philippine archipelago. In this paper, a comprehensive review of the ophiolites in the eastern portion of the Philippines is undertaken. Available data on the geology, ages and geochemical signatures of the oceanic lithospheric fragments in Luzon (Isabela, Lagonoy in Camarines Norte, and Rapu-Rapu island), Central Philippines (Samar, Tacloban, Malitbog and Southeast Bohol), and eastern Mindanao (Dinagat and Pujada) are presented. Characteristics of the Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison. Nearly all of the crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages. The geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and suprasubduction signatures. Although paleomagnetic information is currently limited to the Samar Ophiolite, results indicate a near-equatorial Mesozoic supra-subduction zone origin. In general, correlation of the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are fragments of the Mesozoic pPSP.
New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age. Radiometric dating of its plutonic and volcanichypabyssal rocks yielded middle Eocene ages. On the other hand, the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge e island arc affiliated Coto block of the ophiolite complex, together with isotopic age datings of its dikes and mafic cumulate rocks, also yielded Eocene ages. This offers the possibility that the Zambales Ophiolite Complex could have: (1) evolved from a Mesozoic arc (Acoje block) that split to form a Cenozoic back-arc basin (Coto block), (2) through faulting, structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or (3) through the interplay of slab rollback, slab break-off and, at a later time, collision with a microcontinent fragment, caused the formation of an island arc-related ophiolite block (Acoje) that migrated trench-ward resulting into the generation of a back-arc basin (Coto block) with a limited subduction signature. This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation.
The New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution. The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91e93 mol.%), high Mg# of orthopyroxene (0.91 e0.93) and high Cr# of spinel (0.44e0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (DyN/ YbN ¼ 0.07e0.16) and fractionated LREE-MREE segments (LaN/SmN ¼ 2.1e8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%e18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr eHf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al2O3, and CaO-poor orthopyroxene (Al2O3 ¼ 0.88e1.53 wt.%, CaO ¼ 0.31e0.56 wt.%) and clinopyroxene with low Na2O (0.03e0.16 wt.%), Al2O3 (0.66e1.35 wt.%) and TiO2 (0.04e0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subductionrelated melts with a depleted trace element signature. Nd isotopic ratios range from unradiogenic to radiogenic (0.80εNdi þ13.32) and negatively correlate with Sr isotopes (0.7025787Sr/86Sr 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.
The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.
We present new, geological, metamorphic, geochemical and geochronological data on the East Anatolian eLesser Caucasus ophiolites. These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region. All these data allow the reconstruction of a large obducted ophiolite nappe, thrusted for >100 km and up to 250 km on the Anatolian-Armenian block. The ophiolite petrology shows three distinct magmatic series, highlighted by new isotopic and trace element data: (1) The main Early Jurassic Tholeiites (ophiolite s.s.) bear LILEenriched, subduction-modified, MORB chemical composition. Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle‒Late Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites, while minor volumes of pillow-lava flows infilled the rift grabens. Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone. (2) Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite. They have an OIB affinity. These lavas are featured by large pillow lavas interbedded a carbonate matrix. They show evidence for a large-scale OIB plume activity, which occurred prior to ophiolite obduction. (3) Early‒Late Cretaceous calc-alkaline lavas and dykes. These magmatic rocks are found on top of the obducted nappe, above the post-obduction erosion level. This series shows similar Sr-Nd isotopic features as the Alkaline series, though having a clear supra-subduction affinity. They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume. Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc, which highlight a dissymmetry in the obducted oceanic lithosphere from East to West. The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite: (1) P-T-t data from the northern Sevan-Akera suture zone (Armenia) highlight the presence and exhumation of eclogites (1.85 0.02 GPa and 590 5 C) and blueschists below the ophiolite, which are dated at ca. 94 Ma by Ar-Ar on phengite. (2) Neighbouring Amasia (Armenia) garnet amphibolites indicate metamorphic peak conditions of 0.65 0.05 GPa and 600 20 C with a U-Pb on rutile age of 90.2 5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8 3.0 Ma and 90.8 1.2 Ma, respectively. These data are consistent with palaeontological dating of sediment deposits directly under (Cenomanian, i.e. 93.9 Ma) or sealing (Coniacian‒Santonian, i.e., 89.8 Ma), the obduction. (3) At Hınıs (NE Turkey) PT- t conditions on amphibolites (0.66 0.06 GPa and 660 20 C, with a U-Pb titanite age of 80.0 3.2 Ma) agree with previous P-T-t data on granulites, and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity (ca. 70.6 Ma). Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3 3.7 Ma. We propose that the HT-MP metamorphism was coeval with the monzonites, about 10 Ma after the obduction, and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin. Numerical modelling accounts for the obduction of an “old” w80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling, which increased the oceanic lithosphere buoyancy. The long-distance transport of a currently thin section of ophiolites (<1 km) onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hınıs detachment at ca. 80 Ma, and southward gravitational propagation of the ophiolite nappe onto its foreland basin.
The Göksun (Kahramanmaras¸ ) ophiolite (GKO), cropping out in a tectonic window bounded by the Malatya metamorphic unit on both the north and south, is located in the EW-trending lower nappe zone of the southeast Anatolian orogenic belt (Turkey). It exhibits a complete oceanic lithospheric section and overlies the Middle Eocene Maden Group/Complex with a tectonic contact at its base. The ophiolitic rocks and the tectonically overlying Malatya metamorphic (continental) unit were intruded by I-type calc-alkaline Late Cretaceous granitoid (w81e84 Ma). The ultramafic to cumulates in the GKO are represented by wehrlite, plagioclase wehrlite, olivine gabbro and gabbro. The crystallization order for the cumulate rocks is as follows: olivine chromian spinel/clinopyroxene/plagioclase. The major and trace element geochemistry as well as the mineral chemistry of the ultramafic to mafic cumulate rocks suggest that the primary magma generating the GKO is compositionally similar to that observed in the modern island-arc tholeiitic sequences. The mineral chemistry of the ultramafic to mafic cumulates indicates that they were derived from a mantle source that was previously depleted by earlier partial melting events. The highly magnesian olivine (Fo77e83), clinopyroxene (Mg# of 82e90) and the highly Ca-plagioclase (An81e89) exhibit a close similarity to those, which formed in a supra-subduction zone (SSZ) setting. The field and the geochemical evidence suggest that the GKO formed as part of a much larger sheet of oceanic lithosphere, which accreted to the base of the Tauride active continental margin, including the _Ispendere, Kömürhan and the Guleman ophiolites. The latter were contemporaneous and genetically/tectonically related within the same SSZ setting during the closure of the Neotethyan oceanic basin (Berit Ocean) between the Taurides to the north and the Bitlis-Pütürge massif to the south during the Late Cretaceous.
The Anatolian peninsula is a key location to study the central portion of the Neotethys Ocean(s) and to understand how its western and eastern branches were connected. One of the lesser known branches of the Mesozoic ocean(s) is preserved in the northern ophiolite suture zone exposed in Turkey, namely, the Intra-Pontide suture zone. It is located between the Sakarya terrane and the Eurasian margin (i.e., Istanbul-Zonguldak terrane) and consists of several metamorphic and non-metamorphic units containing ophiolites produced in supra-subduction settings from the Late Triassic to the Early Cretaceous. Ophiolites preserved in the metamorphic units recorded pervasive deformations and peak metamorphic conditions ranging from blueschist to eclogite facies. In the nonmetamorphic units, the complete oceanic crust sequence is preserved in tectonic units or as olistoliths in sedimentary melanges. Geochemical, structural, metamorphic and geochronological investigations performed on ophiolite-bearing units allowed the formulation of a new geodynamic model of the entire “life” of the Intra- Pontide oceanic basin(s). The reconstruction starts with the opening of the Intra-Pontide oceanic basins during the Late Triassic between the Sakarya and Istanbul-Zonguldak continental microplates and ends with its closure caused by two different subductions events that occurred during the upper Early Jurassic and Middle Jurassic. The continental collision between the Sakarya continental microplate and the Eurasian margin developed from the upper Early Cretaceous to the Palaeocene. The presented reconstruction is an alternative model to explain the complex and articulate geodynamic evolution that characterizes the southern margin of Eurasia during the Mesozoic era.
The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments. The Alpine-Apennine (A-A) ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow- to ultraslow-spreading settings. Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale, reflecting a long-lived multi-stage melt migration, intrusion and melt-rock interaction history, occurred at different lithospheric depths during progressive uplift. The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments. In this work, we review field, microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle. Focus will be on three most important stages: (i) old (pre-Jurassic) pyroxenite emplacement, and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts, (ii) melt-peridotite interactions during Jurassic mantle exhumation, i.e. the open-system reactive porous flow at spinel facies depths causing bulk depletion (origin of reactive harzburgites and dunites), and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization. We infer that migrating melts largely originated as shallow, variably depleted, melt fractions, and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration. Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow- to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists, as a consequence of slow-spreading rate.
The Canavese Zone (CZ) in the Western Alps represents the remnant of the distal passive margin of the Adria microplate, which was stretched and thinned during the Jurassic opening of the Alpine Tethys. Through detailed geological mapping, stratigraphic and structural analyses, we document that the continental break-up of Pangea and tectonic dismemberment of the Adria distal margin, up to mantle rocks exhumation and oceanization, did not simply result from the syn-rift Jurassic extension but was strongly favored by older structural inheritances (the Proto-Canavese Shear Zone), which controlled earlier lithospheric weakness. Our findings allowed to redefine in detail (i) the tectono-stratigraphic setting of the Variscan metamorphic basement and the Late Carboniferous to Early Cretaceous CZ succession, (ii) the role played by inherited Late Carboniferous to Early Triassic structures and (iii) the significance of the CZ in the geodynamic evolution of the Alpine Tethys. The large amount of extensional displacement and crustal thinning occurred during different pulses of Late CarboniferouseEarly Triassic strike-slip tectonics is wellconsistent with the role played by long-lived regional-scale wrench faults (e.g., the East-Variscan Shear Zone), suggesting a re-discussion of models of mantle exhumation driven by low-angle detachment faults as unique efficient mechanism in stretching and thinning continental crust.
Covered by ultrabasic units for more than a third of its surface, the New Caledonia (South West Pacific) is one of the largest world producers of Ni-ore from lateritic deposits. Almost all outcrops of geological units and open mines contain serpentine and amphibole, also as asbestos varieties. In this geological context, in which weathering processes had a great contribution in the production and dispersion of mineral fibres into the environment, the development of a routinely analytical strategy, able to discriminate an asbestiform fibre from a non-harmful particle, is a pivotal requisite. However, the acquisition of all these parameters is necessary for determining the risk associated to fibres exposition. A multidisciplinary routinely approach, based on the use of complementary simply-to-use but reliable analytical methods is the only possible strategy. In addition, the instrumental apparatus must be easily transportable on the field, directly on the mining site. The employment of specialized tools such as Polarized Light Microscopy associated to Dispersion Staining method (PLM/DS) and portable Raman spectroscopy for identification of environmental asbestos, are proved extremely effective in the improvement of the performance and rapidity of data acquisition and interpretation. Both PLM/DS and handheld Raman devices confirmed to be discriminant in the detection and characterization of asbestos fibres for both serpentine and amphibole. Furthermore, these techniques proved extremely effective even in the presence of strongly fibrous and altered samples.
Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin. Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics. In this study, we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition. The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426 26 Ma to 2650 38 Ma whereas the Sandur stromatolites gave an age of 3508 29 Ma to 2926 36 Ma suggesting Paleo- to Neoarchean provenance. The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761 31 Ma to 1672 38 Ma suggesting Neoarchean to Mesoproterozoic provenance. The Rare Earth Element (REE) patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements (LREE) and enrichment in heavy rare earth elements (HREE) with pronounced positive Ce and negative Eu anomalies, typical of magmatic zircons. The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites. The 3508 29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton. The overall detrital zircon ages (3.5e2.7 Ga) obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin (2.7e1.6 Ga) collectively reflect on w800e900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.
Six new high precision UePb zircon ID-TIMS ages plus thirteen in situ high spatial resolution UePb zircon LA-MC-ICPMS ages are reported from Jurassic plutonic (metaluminous to weakly peraluminous biotite granites) and Jurassic to Cretaceous hypabyssal (dacites) rocks from Macao. Despite its relatively small area (w30 km2), the new ages tightly constrain the Macao granitic magmatism to two periods ranging from 164.5 0.6 Ma to 162.9 0.7 Ma and 156.6 0.2 Ma to 155.5 0.8 Ma, separated by ca. 6 Ma. Inherited zircons point to the existence of a basement with ages up to Paleo-Proterozoic and late Archean in the region. In addition, younger dacitic rocks were dated at 150.6 0.6 Ma and <120 Ma. U ePb zircon ages and whole-rock REE data of Macao granites indicate that the first pulse is also represented in Hong Kong and Southeast (SE) China, while magmatism with the chemical characteristics of the second pulse seems to not be represented outside Macao. The two granitic magmatic pulses have distinct mineralogical and geochemical features that support their discrete nature rather than a continuum of comagmatic activity and suggest that the Macao granitic suite was incrementally assembled during a period of ca. 9 Ma, a hypothesis also extendable to the neighboring Hong Kong region for a time lapse of ca. 24 Ma. In Macao, the transition from granitic magmatism (Middle to Upper Jurassic) to the younger dacite dykes (Upper Jurassic to Lower Cretaceous) most likely corresponds to a change in the regional tectonic setting, from an extensional regime related with foundering of the subducting paleo- Pacific plate during the Early Yanshanian period to the reestablishment of a normal subduction system in SE China during the Late Yanshanian period.
Accurately mapping plate boundary types and locations through time is essential for understanding the evolution of the plate-mantle system and the exchange of material between the solid Earth and surface environments. However, the complexity of the Earth system and the cryptic nature of the geological record make it difficult to discriminate tectonic environments through deep time. Here we present a new method for identifying tectonic paleo-environments on Earth through a data mining approach using global geochemical data. We first fingerprint a variety of present-day tectonic environments utilising up to 136 geochemical data attributes in any available combination. A total of 38301 geochemical analyses from basalts aged from 5e0 Ma together with a well-established plate reconstruction model are used to construct a suite of discriminatory models for the first order tectonic environments of subduction and mid-ocean ridge as distinct from intraplate hotspot oceanic environments, identifying 41, 35, and 39 key discriminatory geochemical attributes, respectively. After training and validation, our model is applied to a global geochemical database of 1547 basalt samples of unknown tectonic origin aged between 1000 e410 Ma, a relatively ill-constrained period of Earth’s evolution following the breakup of the Rodinia supercontinent, producing 56 unique global tectonic environment predictions throughout the Neoproterozoic and Early Paleozoic. Predictions are used to discriminate between three alternative published Rodinia configuration models, identifying the model demonstrating the closest spatio-temporal consistency with the basalt record, and emphasizing the importance of integrating geochemical data into plate reconstructions. Our approach offers an extensible framework for constructing full-plate, deeptime reconstructions capable of assimilating a broad range of geochemical and geological observations, enabling next generation Earth system models.
The >2000 km Indus-Yarlung Tsangpo suture zone (IYSZ) is composed of the Neo-tethys oceanic remnants, flysch units and related continental rocks, which has been regarded as the boundary between the Eurasian and Indian terranes. Among the ophiolitic complexes, the Purang ophiolite is the biggest massif in the IYSZ, and many studies have been conducted on this ophiolite. However, previous studies have mainly focused on harzburgite, clinopyroxenite and dunite. Field observations show that mafic dykes were emplaced within the Purang ophiolite. However, petrogenetic evolutions of those mafic dykes are poorly understood. In this study, we present new LA-ICP-MS zircon UePb dating results, whole-rock geochemistry and SreNdeHf isotope analyses for microgabbro, gabbro and dolerite dykes from the Purang ophiolite of the southwestern IYSZ, respectively. Three samples yielded zircon UePb ages of 144.2 2.1 Ma, 127.9 2.3 Ma and 126.5 0.42 Ma, suggesting two different phases of magmatic activities distinctly. Whole-rock geochemical results suggest that the gabbro samples show alkaline features marked by enrichments of light rare earth elements (LREE) and large-ion lithophile elements (LILE), as well as NbeTa elements, suggesting an oceanic island basalt-like (OIB-like) geochemical affinity. However, the dolerite and microgabbro samples demonstrate sub-alkaline characteristics with normal mid-oceanic ridge basalt-like (N-MORB-like) geochemical features. Three distinct mafic dykes show significant Rb element depletion. The geochemical data and SreNdeHf isotopic features suggest that the microgabbro and gabbro rocks were derived from a depleted mantle that had been metasomatized by partial melts of sediments and enriched slab-derived fluids. The dolerite was also originated from a depleted mantle marked by significantly depleted SreNdeHf compositions, which was not influenced by enriched slab-derived fluids and sediments contamination during subsequent evolution. The isotope and geochemical data and tectonic diagrams suggest a tectonic transition from a within-plate to a midoceanic ridge basalt-like (MORB-like) setting during the period from ca. 144 Ma to 127 Ma. Combined with regional background and this study, we propose that these mafic dykes were formed in an oceanic back-arc basin setting. Additionally, integrated with previous studies, we suggest that the geodynamic evolution of the southwestern and central parts of the Neo-Tethys oceanic basin is comparable in Early Cretaceous.
Glacio-isostatic adjustment (GIA) and tectonic activity are important factors in the formation of marine terraces. Late Holocenewave-cut benches in the eastern part of theWest Sea of Korea, also called the Yellow Sea, can be divided into two steps: 531 cm above sea level (ASL) for the upper bench (T2) and 464e481 cm ASL for the lowerbench (T1). Sediments onthebenches are classified into four units,andare interpreted tobe beach deposits according to gravel shape, texture, and seaward inclination. The overlying sediment indicates that T2 was formed at approximately 530 cm ASL before 2900 yr BP, and T1 at approximately 460e480 cm ASL before 1520 yr BP. Late Holocene (4000e2000 yr BP) relative sea level (RSL) curves based on GIA models are inconsistent with thewave-cut bench elevations. Comparing T1 and T2 benches to the RSL curves of the West Sea, the upper and the lower benches were uplifted by approximately 5e8mand 4e7m, respectively. Although the area is several hundred kilometers away from plate boundaries, the high frequency of earthquakes in theWest Sea may have induced the uplift of wave-cut benches during the last 2000 years. These indicate that the west coast of the Korean Peninsula (KP) should no longer be considered an area of subsidence, but be assigned to a regime of uplift during the late Holocene.
The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape, including a northern inselberg area and a southern elevated plateau, separated by the Doce River valley. This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Araçuaí orogen, in Ediacaran to Ordovician times. Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous, those rocks of the Araçuaí orogen became the basement of a portion of the South Atlantic passive margin and related southeastern Brazilian highlands. Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes, and to provide a hypothesis on the geodynamic context since break-up. To this end, we combine the apatite fission track (AFT) and apatite (UeTh)/He (AHe) methods as input for inverse thermal history modelling. All our AFT and AHe central ages are Late Cretaceous to early Paleogene. The AFT ages vary between 62 Ma and 90 Ma, with mean track lengths between 12.2 mm and 13.6 mm. AHe ages are found to be equivalent to AFT ages within uncertainty, albeit with the former exhibiting a lesser degree of confidence. We relate this Late CretaceousePaleocene basement cooling to uplift with accelerated denudation at this time. Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system. We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network.We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny, potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly, as a probable cause for the uplift.
The Um Rus tonalite-granodiorite intrusion (w6 km2) occurs at the eastern end of the Neoproterozoic, ENE-trending Wadi Mubarak shear belt in the Central Eastern Desert of Egypt. Gold-bearing quartz veins hosted by the Um Rus intrusion were mined intermittently, and initially by the ancient Egyptians and until the early 1900s. The relationship between the gold mineralization, host intrusion, and regional structures has always been unclear. We present new geochemical and geochronological data that help to define the tectonic environment and age of the Um Rus intrusion. In addition, field studies are integrated with EPMA and LA-ICP-MS data for gold-associated sulfides to better understand the formation and distribution of gold mineralization. The bulk-rock geochemical data of fresh host rocks indicate a calc-alkaline, metaluminous to mildly peraluminous, I-type granite signature. Their trace element composition reflects a tectonic setting intermediate between subduction-related and within-plate environments, presumably transitional between syn- and post-collisional stages. The crystallization age of the Um Rus intrusion was determined by in situ SHRIMP 206Pb/238U and 207Pb/235U measurements on accessory monazite grains. The resultant monazite UePb weighted mean age (643 9 Ma; MSWD ¼ 1.8) roughly overlaps existing geochronological data for similar granitic intrusions that are confined to major shear systems and are locally associated with gold mineralization in the Central Eastern Desert (e.g., Fawakhir and Hangaliya). This age is also consistent with magmatism recognized as concomitant to transpressional tectonics (D2: w650 Ma) during the evolution of the Wadi Mubark belt. Formation of the gold-bearing quartz veins in NNE-SSW and NeS striking fault segments was likely linked to the change from transpressional to transtensional tectonics and terrane exhumation (D3: 620e580 Ma). The development of NeS throughgoing fault arrays and dike swarms (w595 Ma) led to heterogeneous deformation and recrystallization of the mineralized quartz veins. Ore minerals in the auriferous quartz veins include ubiquitous pyrite and arsenopyrite, with less abundant pyrrhotite, chalcopyrite, sphalerite, and galena. Uncommon pentlandite, gersdorffite, and cobaltite inclusions hosted in quartz veins with meladiorite slivers are interpreted as pre-ore sulfide phases. The gold-sulfide paragenesis encompasses an early pyrite-arsenopyrite loellingite assemblage, a transitional pyrite-arsenopyrite assemblage, and a late pyrrhotite-chalcopyrite-sphalerite galena assemblage. Free-milling gold/electrum grains (10s mm-long) are scattered in extensively deformed vein quartz and in and adjacent to sulfide grains. Marcasite, malachite, and nodular goethite are authigenic alteration phases after pyrrhotite, chalcopyrite, and pyrite and arsenopyrite, respectively. A combined ore petrography, EPMA, and LA-ICP-MS study distinguishes morphological and compositional differences in the early and transitional pyrites (Py I, Py II) and arsenopyrite (Apy I, Apy II). Py I forms uncommon small euhedral inclusions in later Py II and Apy II. Py II forms large subhedral crystals with porous inner zones and massive outer zones, separated by narrow As-rich irregular mantles. The Fe and As contents in Py II are variable, and the LA-ICP-MS analysis shows erratic concentrations of Au (<1 to 177 ppm) and other trace elements (e.g., Ag, Te, and Sb) in the porous inner zones, most likely related to discrete sub-microscopic sulfide inclusions. The outer massive zones have a rather homogenous composition, with consistently lower abundances of base metals and Au (mean 1.28 ppm). The early arsenopyrite (Apy I) forms fine-grained euhedral crystals enriched in Au (mean 17.7 ppm) and many other trace elements (i.e., Ni, Co, Se, Ag, Sb, Te, Hg, and Bi). On the other hand, Apy II occurs as coarsegrained subhedral crystals with lower and less variable concentrations of Au (mean 4 ppm). Elevated concentrations of Au (max. 327 ppm) and other trace elements are measured in fragmented and aggregated pyrite and arsenopyrite grains, whereas the undeformed intact zones of the same grains are poor in all trace elements. The occurrence of gold/electrum as secondary inclusions in deformed pyrite and arsenopyrite crystals indicates that gold introduction was relatively late in the paragenesis. The LAICP- MS results are consistent with gold redistribution by the NeS though-going faults/dikes overprinted the earlier NNW-SSE quartz veins in the southeastern part of the intrusion, where the underground mining is concentrated. Formation of the Um Rus intrusion and gold-bearing quartz veins can be related to the evolution of the Wadi Mubarak shear belt, where the granitic intrusion formed during or just subsequent to D2 and provided dilatation spaces for gold-quartz vein deposition when deformed by D3 structures.
With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB), an integrated study was conducted on the geochronological and geochemical data for dioritic, granitic and diabase dykes from the AqishaneYamansu belt in the eastern Tianshan, NW China. Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous (w311 Ma and w315 Ma). The dioritic dykes show adakitic characteristics and have high Na2O and positive εHf(t) values (þ12 to þ17), which suggest an origin from partial melts of a subducted oceanic slab. The granitic dykes have high SiO2 and K2O contents and are characterized by enriched light rare earth elements (LREE) and slightly flat heavy rare earth elements (HREE), with negative Eu and Nb eTaeTi anomalies. These dykes are alkali-calcic and show geochemical features of highly fractionated Itype granites. Their positive εHf(t) values (þ16 to þ17) suggest that they were derived from a juvenile accreted oceanic crustal source. The coeval diabase dykes have low SiO2 and K2O contents but high TiO2, MgO and Mg# (54e59). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Ba/Th, slightly low Th/Ta ratios, and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids. Thus, these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the AqishaneYamansu belt was a continental arc formed by southdipping subduction of the Kangguer oceanic plate.
The North China Craton (NCC) hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization. The diverse models for the genesis of these deposits remain equivocal, and mostly focused on the craton margin examples, although synchronous deposits formed in the interior domains. Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC. In the Archean tectonic framework of the NCC, the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents. In the Paleoproterozoic framework, they are markedly aligned along two major collisional sutures e the Trans North China Orogen and the Jiao-Liao-Ji Belt. Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold, we explore the deep roots based on available geophysical data. We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle, and that the mineralization is located above regions of high heat flow representing mantle upwelling. The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly enriched it. The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning. Based on these results, and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization, we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction, underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC. The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs, leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.
Sandstone-type U mineral resources are among the important sources for nuclear energy. The U deposits in the Ordos Basin in China form part of the northern segment of the sandstone-hosted Central Asian Uranium Mega- Province. Two types of mineralizations are recognized in this basin: “phreatic permeable type” and “interlayer permeable type”, both exhibiting features equivalent to roll-front subtypes. The “interlayer permeable type” is widely accepted as the dominant mineralization type for sandstone-type uranium deposits within large-scale basins, also designated as the “interlayer oxidation zone type”, based on the horizontal color zoning model representing changing redox conditions. Here we synthesize data from several drill holes within the Ordos Basin, which suggest that major Mesozoic tectonic movements controlled the evolution of the sedimentary system in the basin. These tectonic movements contributed to the formation of three angular unconformities and four parallel unconformities as inferred from the stratigraphic relationships. In addition, other features such as vertical color zoning, paleo-channel controlled tabular or lentoid ore bodies (without roll-type) and a group interlayer horizontal zoning of altered minerals are also documented. Sequence stratigraphic analysis indicates that the Ordos Basin generally witnessed four cycles of water level variations during Mesozoic. During the variations, three high water level and three low water level events were recorded. Biological characteristics imply that the Ordos Basin went through multiple arid to humid climatic evolutions during Mesozoic. Combining the newly documented features with some novel concepts on the hydrodynamic mechanism for supergene ore-forming fluids, we propose a metallogenic model which invokes the importance of tectonic movements and water level fluctuations to explain the genesis of uranium deposits along the northern margin of the Ordos Basin.