Petrological Observations Research Articles

Petrogenesis and tectonic implications of the quartz diorites and mafic microgranular enclaves in the Asiha gold ore deposit in the East Kunlun orogenic belt: Evidence from zircon U–Pb dating, geochemistry, and Sr–Nd–Hf isotopes

Calc‐alkaline granitoid intrusions with abundant mafic microgranular enclaves (MMEs) are widespread in the East Kunlun orogenic belt (EKOB). Here, we present petrological observations, zircon U–Pb ages, whole‐rock geochemical, and Sr–Nd–Hf isotopic data for quartz diorites and the MMEs in the Asiha gold ore deposit in the EKOB, to constrain their petrogenesis and tectonic setting. The laser ablation inductively coupled plasma mass spectrometer 206Pb–238U ages of zircons indicate that the host quartz diorites and their MMEs from the Asiha gold deposit crystallized at 228.4 ± 1.7 Ma and 229.0 ± 1.7 Ma, respectively, which is similar to the gold mineralization age. Whole‐rock geochemical data indicate that the quartz diorites are I‐type granites. The MMEs have εHf(t) values of −7.6 to −0.2, −2.1 on average, whereas those of the quartz diorites range from −3.2 to −0.8, with a mean value of −2.1, which is identical within analytical uncertainty. Moreover, the host quartz diorites and their MMEs have relatively homogeneous Sr–Nd isotopic compositions, with high initial 87Sr/86Sr ratios and low εNd(t) values ranging from 0.70914 to 0.70963 and from −5.9 to −5.5, respectively. In summary, the MMEs and host quartz diorites have similar geochemical characteristics, indistinguishable Sr–Nd–Hf isotopic composition, and almost simultaneous crystallization age. Geochemical evidence shows that they formed due to the mixing of crust‐ and mantle‐derived magmas. Combined with our new results and published geological, geochronological, and geochemical data, this paper proposes that the EKOB had evolved into a post‐collisional environment at about 230–228 Ma, and the host quartz diorites and their MMEs from the Asiha complex formed in the transitional stage from the syn‐collisional to the post‐collisional setting.

Tracing carbonate dissolution in subducting sediments by zinc and magnesium isotopes

Carbonate dissolution in subduction zones transfers surface carbon to the mantle and plays an important role in regulating global carbon cycles through geologic time. Petrological observations and thermodynamic models predict substantial dissolution of subducting carbonates induced by slab-derived fluids at arc mantle depths, but this dissolution process has rarely been verified by existing geochemical tracers. In this paper we show that zinc and magnesium isotopes can serve as such tracers through a comparative study on carbonate-bearing and carbonate-free sediments from the South China Sea and the Philippine Sea that are going to be subducted. Leaching experiments are further performed to characterize the isotopic difference between dissolved carbonates and silicate components in sediments. Bulk carbonate-bearing sediments and carbonate-free sediments surprisingly have indistinguishable δ66Zn (0.26 ± 0.04‰ versus 0.25 ± 0.04‰), but the former display systematically lower δ26Mg (−0.85‰ to −0.17‰, n = 10) than the latter (−0.26‰ to +0.13‰, n = 13). Leaching experiments show that dissolved carbonates have much higher δ66Zn (0.82 ± 0.26‰) and lower δ26Mg (−2.70 ± 0.35‰) compared with the silicate residues (δ66Zn = 0.21 ± 0.06‰; δ26Mg = −0.06 ± 0.14‰). These results indicate that the presence of carbonate components has insignificant influence on Zn isotopic compositions of bulk sediments but prominent influence on their Mg isotopic compositions. In this respect, the final fate of carbonates in subducting sediments can be tracked by a combined Mg and Zn isotope analysis on mantle-derived magmas. For instance, bulk carbonate-bearing sediment addition (i.e., no carbonate dissolution) during subduction would produce low δ26Mg but normal δ66Zn values (i.e., Mg-Zn isotopic “decoupling”) in mantle-derived magmas. By contrast, if carbonate dissolution occurs during subduction, the high δ66Zn and low δ26Mg (i.e., Mg-Zn isotopic “coupling”) signals of dissolved carbonates will be transferred to the sources of mantle-derived magmas. We test this application for a Cenozoic subduction-related, arc-like volcanic exposure from Tengchong, Southwestern China and find significant Mg-Zn isotopic coupling, which demonstrates carbonate dissolution during Indian Oceanic slab subduction. Our results also shed constraints on the amount of dissolved carbonates in the sub-arc mantle. Available data reveal the absence of high δ66Zn and low δ26Mg in global arc lavas, for which we suggest to be the result of a relatively low proportion (<20%) of dissolved calcium carbonates in the sub-arc mantle.

Clumped isotope thermometry reveals diagenetic origin of the dolomite layer within late Ordovician black shale of the Guanyinqiao Bed (SW China)

The origin of widespread dolomite deposits in Paleozoic strata is a longstanding open question. Here we use a suite of geochemical methods to investigate a prominent Paleozoic dolomite occurrence in SW China. The upper Ordovician Wufeng Formation of the upper Yangtze Block (SW China) covers the time interval just before the Ordovician-Silurian Mass Extinction event, consisting of wide-spread black shale with an interlayer of bioclastic limestone, the Guanyinqiao Bed. In the western part of the Yangtze Block, in Shizhu county however, the Guanyinqiao Bed contains a particular organic-rich dolomite interlayer of which the formation mechanism is unclear. To distinguish between potential primary dolomite precipitation and late stage diagenetic/magmatic alteration, we studied this dolomite interlayer with petrological and geochemical techniques including microscopy, REE content and carbonate clumped isotope thermometry.The petrological observations show micritic dolomite with euhedral-subhedral shape and relatively low ordering (0.33 to 0.39). Conventional stable isotope analysis revealed negative carbon (δ13C of −3.17 ± 0.04‰ to −2.48 ± 0.03‰) and oxygen isotope compositions (δ18O of −11.28 ± 0.18‰ to −10.99 ± 0.20‰), and the organic carbon content ranged from 3.69% to 4.33%. The averaged clumped isotope temperature estimate revealed hot dolomite formation conditions of 87−10+11 °C, in a presumable fluid source with isotopic composition of δ18Ow = −1.8−1.5+1.5 ‰. The elevated formation temperature estimates, the slightly depleted δ18O of dolomite (~−11.14‰) with respect to contemporary deposited limestone (~−10‰) and the δ18Ow which is similar to the assumed late Ordovician seawater points at diagenetic alteration of primary limestone at a burial depth of approximately 3000 m. The analyzed dolomite samples with low contents of Fe, Zn and small Eu anomaly show further evidence for a diagenetic fluid source and exclude magmatic/hydrothermal alteration. We suggest that at this burial depth the temperature pressure conditions triggered significant Mg release from the surrounding clay to the diagenetic fluid, providing ideal conditions to transform the primary limestone into diagenetic dolomite.

Multiple shock events recorded in the Northwest Africa 2139 LL6 chondrite: Implications for collisional histories of the LL chondrite parent body

AbstractLL chondrites have experienced multiple shock events; however, the relations of each shock event and their timing have rarely been investigated. To demonstrate the relations between each shock texture and shock chronological ages, we conducted both petrological and chronological (40Ar/39Ar and I‐Xe ages) studies using aliquots subsampled from the same chip of the Northwest Africa (NWA) 2139 LL6 chondrite. Our 40Ar/39Ar studies and petrological observation reveal that NWA 2139 recorded at least three impact events before 4.17 ± 0.10 Ga, thus resulting in a complex brecciated texture, silicate darkening, and thick shock veins. An intense heating event occurred at 4.17 ± 0.10 Ga, which recrystallized the thick veins and healing cracks. Then, a weak shock event occurred at &lt;3.9 Ga. Combined with 40Ar/39Ar data of other LL chondritic materials, this study supports that the LL chondrite parent body was possibly broken up by 1.7 Ga, and that most of the breakup likely occurred within 3.8–4.2 Ga.

The syn‐rift tectono‐stratigraphic record of rifted margins (Part I): Insights from the Alpine Tethys

AbstractThe tectono‐stratigraphic evolution of rift systems is at present poorly understood, especially the one preceding the onset of oceanic seafloor spreading. Improving our understanding of the complex, polyphase tectonic evolution of the fossil Alpine Tethys, one of the best calibrated magma‐poor rift systems worldwide, offers great potential for the interpretation of the distal part of global rifted margins, which notoriously lack data. In this paper, we propose a tectono‐stratigraphic model for the former Alpine Tethys, whose remnants are exposed in the Alps of Western Europe. We first review the historical evolution of some concepts grounding present knowledge on the Alpine‐ and global magma‐poor rifted margins. Then we present a spatial and temporal template for the Alpine Tethys rift system using a ‘building block (BB)’ approach. This new approach is powerful in that it can integrate high‐resolution, structural, stratigraphic, thermochronological, and petrological observations from dismembered outcrops into a margin‐scale template compatible with the first‐order seismic architecture defined at present‐day, magma‐poor rifted margins. The detailed analysis of the syn‐rift sequence of the Alpine Tethys margins demonstrates that extension migrated and localised while evolving from the stretching to the necking phase. During hyperextension, rifting was asymmetric and controlled by in‐sequence detachment faulting. Then, the rift system evolved back into a more symmetric, embryonic spreading system. With this contribution, we aim to allow readers without knowledge of the Alps to access this unique ‘archive’ preserving some of the world's best‐exposed rift structures. Recognising these structures is critical to understand the origin of some new concepts used to explain present‐day, deep‐water rifted margins, and to interpret and predict the tectono‐stratigraphic evolution during advanced stages of rifting.

Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan

AbstractWe present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modelling give estimated peak P–T conditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks.

The impact and implications of aragonite-to-calcite transformation on speleothem trace element composition

Trace elements in cave carbonates (both calcite and aragonite) have been considered major proxies in paleo-hydroclimatic research. However, their imprints in authigenic minerals might be altered by neomorphic processes (e.g., aragonite-to-calcite transformation). Therefore, it is crucial to understand the impacts of neomorphism on the trace element compositions of secondary calcite and understand the mechanisms of trace elements redistribution during neomorphic processes. In this study, two aragonite stalagmites (SN3 and SN15) from Shennong Cave, part of which were transformed into calcite, were further inspected and analyzed in terms of petrological observations, trace element geochemistry, and Mg compositional mapping. The results show that (1) the aragonite-to-calcite transformation in SN3 and SN15 occurs by a dissolution-precipitation mechanism and is influenced by several factors, including internal fluid compositions, microstructure (such as porosity and permeability) of the parent material, and crystal defects. (2) The primary aragonite is enriched in Sr but depleted in Mg compared with secondary calcite because of the crystallographic disparity. The non-equilibrium absorption of Mg and slow diffusive element transport may contribute to the extremely high Mg/Ca ratios in secondary calcite near the mineral phase boundary. Incorporation competition among trace elements also plays an important role in affecting the distribution of Sr and Mg in neomorphic calcite. Considering the previous study on the alteration of isotopes (δ18O, δ13C) associated with the aragonite-to-calcite transformation, we propose that intervening fluids play an important role in altering the secondary calcite oxygen isotopic composition and Mg/Ca ratio, while the composition of primary aragonite dictates the secondary calcite carbon isotopic composition and Sr/Ca ratio, provided that the transformation of aragonite to calcite caused by a dissolution–reprecipitation process occurring in trapped pore water. These results highlight the indispensability of petrographic analysis of speleothem specimens before using trace elements and isotopes for paleoclimate research.

Formation of Rare Earth Deposits in Carbonatites

Carbonatites and related rocks are the premier source for light rare earth element (LREE) deposits. Here, we outline an ore formation model for LREE-mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. The LREEs can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and LREEs concentrate in the residual melt due to their incompatibility in early crystal-lising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution of the residual liquid into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These primary carbonates are rarely preserved owing to dissolution by later fluids, and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.

Origin of the Early to Middle Triassic polyhalite minerals in the Sichuan Basin, SW China: New evidence from calcium and sulphur isotopes and microfabrics

Polyhalite is a special potash fertilizer resource, which is widely distributed in the Triassic evaporite strata in the Sichuan Basin, but its genetic mechanism has always been controversial. This contribution presents new petrological, geochemistry, electron probe analyses, and Ca-S isotope systematics on Triassic polyhalite minerals for the first time from the Sichuan Basin aiming to better understand their origin. We present petrological observations of long columnar and granular polyhalites co-existing with halites (rock salt) and anhydrites that are not metasomatic in origin. Metasomatic polyhalites have also been studied, which have a fibrous and radial habit interwoven with anhydrite. The primary (sedimentary) polyhalite is solely formed from the evaporation of seawater, and the formation of metasomatic polyhalite involved impure meteoric water. The Ca and S isotopic values show that the sulphur and calcium in diamond-drillholes ZK001 and ZK601 are derived from seawater, whereas the sulphur and calcium in Well YS01 are derived from meteoric water. The sulphur and calcium in Well ZK02 are derived weathered terrigenous gypsum. This study demonstrates that the combined sulphur (δ34S) and calcium (δ44/40Ca) isotopes can be used to reveal the genesis of polyhalite.

Gold occurrences in copper-magnetite-apatite deposit at Seruwila, Sri Lanka

This study presents petrology and evidence for possible gold occurrences in Seruwila copper–iron oxide–apatite (IOA) deposit, hosted in an ultramafic intrusion which is located at the boundary between the Highland and Vijayan complexes, within the intermediate-granitic basement in north-eastern Sri Lanka. The study is complemented with petrological observations and XRD and SEM analysis, respectively, to investigate the petrology/subsurface geology of the deposit and identify possible gold occurrences in the deposit. The ore-bearing rocks are mainly composed of magnetite and apatite in various proportions, hosted in an ultramafic intrusion with cumulate features within the granitic-intermediate basement. The secondary veins contain magnetite, chalcopyrite, pyrrhotite, and pyrite together with apatite and scapolite, tremolite, diopside, and minor actinolite and calcite, serpentinite, anhydrite, or gypsum. The clinopyroxene euhedral crystals show cumulus textures including grain triple junctions and large dihedral angles (∼120°), showing magmatic origin. Texturally two types of amphiboles are identified as coarse-grained (0.5–1 mm), pale green euhedral amphibole that is free of inclusions, and fine-grained (<0.1 mm) and brownish, occurring as anhedral inclusions in clinopyroxene. The deposit contains varying amounts of sulfides in which pyrite is the potential gold carrier in magmatic–hydrothermal processes. By the results of XRD analysis, it is evident that the presence of Au (111), Au (200), Au (220) and Au (311), although with low count values (50–500), probably due to the low concentration of gold. Therefore, particularly in the samples with veins or veinlets, gold was inferred to be present in pyrite/chalcopyrite as invisible structurally-bonded gold and/or gold nanoparticles. Hence, the results of this study, although at non-ore grade, veinlets of gold-bearing pyrite/chalcopyrite may serve as a promising target for gold occurrence, being a potential site of gold-mineralization in the context of the East-Gondwana.

Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)

In Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partially eclogitised within hydrous shear zones, that have been interpreted as widening over time with fluid influx and strain. We here present a detailed petrological description of two different metre-scale shear zones from this area. Within a few tens of centimetres of each shear zone, the granulite protolith (initially plagioclase + garnet + two pyroxenes) is transformed into an albite + zoisite + garnet + clinopyroxene assemblage. The outer edge of the shear zones consists in a fine-grained heterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz. The core of the shear zones is an homogeneous eclogite assemblage of coarser omphacite + kyanite + garnet + zoisite + phengite ± quartz. As the shear zones widened over time, this lateral evolution from the edge to the core of the shear zones reflects the temporal evolution of the granulite from the beginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite facies assemblage. Field and petrological observations suggest that this transient assemblage is mechanically weaker than both the starting strong granulite and the final eclogite. We here investigate the impact of transient weakening during syn-tectonic metamorphism using a one-dimensional numerical model of a fluid-fluxed, reacting shear zone. We show that transient weakening is required to explain our field and petrological observations. Furthermore, although the widening of the shear zones was primarily controlled by fluid infiltration, we show that strain hardening during the end of the eclogitisation reactions sequence had a noticeable widening effect on the shear zones.

Tracking the summit activity of Mt. Etna volcano between July 2019 and January 2020 by integrating petrological and geophysical data

The evolution of volcanic activity observed at the New South East Crater (NSEC) and Voragine (VOR) between July 2019 and January 2020 has been deciphered by taking into account the changes of textures and chemical zoning of plagioclase and olivine crystals from the erupted lavas and tephra. The petrological observations have been integrated with analyses of the amplitude and source location of volcanic tremor and infrasound data. Characteristics of crystals erupted on July 2019 at the NSEC reflect protracted intrusions of magma into the mid-upper section of the plumbing system, approximately within 290–120 MPa, which acted as the main zone of magma accumulation and crystallization before the beginning of the eruptive activity. Textures and compositions of crystals erupted at VOR emphasize the beginning of volcanic activity driven by recharge/discharge phases that mostly affect the shallowest portion of the Mt. Etna plumbing system (<40 MPa). At the end of 2019, mineral compositions and zoning patterns changed again in accordance with eruption dynamics. The observed changes reflect the transition from an early phase, between November and December 2019, characterized by substantial equilibrium during magma storage and transport toward higher disequilibrium conditions and eruptive frequency, in January 2020. This has been associated to episodes of deep replenishment of mafic magmas displacing the resident one. Diffusion chronometry applied to zoned olivines shows that most of the episodes of magma intrusion correlate temporally with changes in the features of both volcanic tremor and infrasonic events in terms of amplitude and source location, providing evidence that such geophysical signals are directly related to the magma dynamics in the upper plumbing system.

The Architecture of the Southern Puna Magmatic System: Integrating Seismic and Petrologic Observations With Geochemical Modeling

AbstractImprovements in geochemical, petrologic, and geophysical methods and data have led to a paradigm‐shift in understanding magmatic system architecture away from isolated magmatic chambers toward transcrustal “mushy” magmatic systems. Geochemical and petrologic studies indicate that magma storage and differentiation occurs over a range of depths throughout the crust, while geophysical techniques delineate in situ spatial distributions of melt at similar scales. However, seismic properties are nonunique with respect to composition and melt percentage, and must be integrated with geochemical observations to better understand the architecture and chemical evolution of magmatic systems. The southern Puna Plateau represents a locality where extensive geochemical and seismic data can be combined to investigate the role of depth‐dependent processes in generating compositionally diverse volcanism. By combining geochemical modeling with seismic images of shear‐wave velocity and attenuation, we find that magma storage and differentiation occurs predominantly at two depth ranges: (a) near the base of the crust (&gt;40 km), where shear‐wave velocities are anomalously slow compared to what is expected (&lt;4.0 km/s) and geochemical signatures indicate fractional crystallization and crustal assimilation in the garnet stability field, and (b) in the mid‐crust (∼20 km) beneath the Cerro Galan Caldera, where shear‐wave velocities necessitate the presence of melt (∼2.7 km/s) and europium (Eu) anomalies indicate low‐pressure differentiation. These geochemical and seismic results provide evidence for multi‐level melt evolution and are consistent with petrological features of exposed arcs, whose transcrustal architecture is characterized by discrete and compositionally evolved intrusive bodies in the mid‐crust and a compositionally heterogeneous lower crust.

Paleomagnetism from multi-orogenic terranes is “not a simple game”: Pyrenees' Paleozoic warning

Summary Paleomagnetism is a versatile tool in the Earth sciences: it provides critical input to geological time scales and plate tectonic reconstructions. Despite its undeniable perks, paleomagnetism is not without complications. Remagnetizations overprinting the original magnetic signature of rocks are frequent, especially in orogens which tend to be the areas with better rock exposure. Unraveling the magnetic history of the rocks is a complicated task, especially in areas that underwent several orogenic pulses. In turn, constraining the timing of remagnetization represents an opportunity to solve post-magnetization structural and tectonic kinematics. Here, we evaluate the magnetization history of Silurian-Devonian carbonates from the Axial Zone of the Pyrenees. The Pyrenees are a multi-orogenic mountain belt where Silurian-Devonian rocks have seen the Variscan collision (late Paleozoic), the opening of the Atlantic/Bay of Biscay (early Cretaceous) and the Alpine orogeny (late Cretaceous to Miocene). Our results show widespread remagnetization(s) carried by magnetite and pyrrhotite in the Silurian-Devonian series of the Pyrenees. The majority of the samples show a post-folding but pre-alpine tilting magnetization. Considering the equatorial inclinations found in such samples, we suggest that they likely acquired their magnetization during the late Carboniferous and early Permian times. Two of the studied sites (located at the western Axial Zone) were subsequently remagnetized at the end of the Alpine orogeny. The paleomagnetic results constrained that the Variscan orogeny was responsible for the main folding event affecting Paleozoic rocks in the Axial Zone, whereas the Alpine orogeny produced the large-scale thrusting and antiformal stacking of these units. In addition, we observed a general clockwise rotational pattern which could be related with the formation of the Cantabrian Orocline and/or rotations associated with the Alpine orogeny. The Silurian-Devonian carbonates are thus useful to understand the tectonic evolution of the Pyrenean mountain range after a systematic combination of paleomagnetism with structural and petrological observations. In contrast, the secondary character of magnetization and complications associated with the Variscan tectonics indicate that a reassessment of Siluro-Devonian poles from the Variscan elsewhere in Europe might be appropriate.

The Major Controlling Factors and Different Oolitic Shoal Reservoir Characteristics of the Triassic Feixianguan Formation, Eastern Longgang Area, NE Sichuan Basin, SW China

AbstractBased on comprehensive analyses of occurrence, petrological observation, pore structure and geochemistry, the different reservoir characteristics and reservoir evolutionary pathways between different oolitic shoal reservoir types of the Feixianguan Formation on the west side of the Kaijiang‐Liangping Trough have been studied. There exist three stages of high‐energy slope break belts in the Feixianguan period, the corresponding three stages of oolitic shoals gradually migrating in the direction of the trough. Three types of oolitic shoal reservoirs, namely, residual‐oolitic dolomite, mold‐oolitic dolomite and sparry oolitic limestone, were formed during sedimentary‐diagenetic evolution, the pore types being intergranular dissolved pore, mold pore (or intragranular dissolved pore) and residual intergranular pore, respectively. The petrology, physical properties and pore structure of the different types of oolitic shoal reservoirs are quite different. Residual‐oolitic dolomite reservoirs have the best quality, while sparry oolitic limestone reservoirs have the poorest. Combined with analyses of trace elements, rare earth elements and carbon‐oxygen isotopes, it is suggested that the formation of residual‐oolitic dolomite reservoirs is jointly controlled by penesaline seawater seepage‐reflux dolomitization and hydrothermal dolomitization. Mold‐pore oolitic dolomite reservoirs are controlled by penesaline seawater seepage‐reflux dolomitization and meteoric water solution. The burial dissolution of organic acid not only further improves the reservoir qualities of previously formed oolitic dolomite reservoirs, but also preserves residual intergranular pores in the sparry oolitic limestone reservoirs.

Evolution of Archean Sanukitoids from the Otto Stock by Magma Mixing and Na–K Metasomatism: Evidence from Petrological Observations and Lithium Isotope Geochemistry

AbstractPetrogenetic models for the genesis of Archean sanukitoids are diverse in the literature but most point to the initiation of plate tectonics and the construction of the first proto-continents during the Late Archean. These rocks include diorites, monzonites, monzodiorites, quartz monzodiorites, trachyandesites, granodiorites and tonalites, and are defined as having SiO2 = 55–60 wt%, Na2O + K2O = 6 wt%, MgO &amp;gt; 6 wt%, Mg# &amp;gt; 60, Ni–Cr &amp;gt; 100 ppm, Sr and Ba = 600–1800 ppm, chondrite-normalized Ce and Yb values of 80–250 and 4–10, respectively, and no Eu anomalies. Petrogenetic models include the partial melting of mantle peridotite previously metasomatized by fluids of crustal or mantle origin, partial melting of subducted slabs and assimilation of peridotite, and partial melting of undepleted peridotite with subsequent mixing with tonalite–trondhjemite–granodiorite (TTG) crustal partial melts and addition of carbonatite, all of which may or may not include subsequent differentiation by fractional crystallization. Here, extraordinary textural relations unequivocally show that at the Late Archean Otto stock, Abitibi, Canada, sanukitoids resulted from the assimilation of clinopyroxenite by monzonitic magmas and coeval magmatic–hydrothermal Na–K metasomatism. The metasomatized monzonites (± quartz), melanogranites, granodiorites, and sanukitoidal melanoporphyries were subsequently cut by swarms of lamprophyre dykes hosting a different set of hydrothermal alteration assemblages, including propylitic alteration, biotitization, chloritization, and pyritization, along with associated enrichments in Au. These alterations are shown to be associated with the nearby Cadillac–Larder Lake Fault Zone and the world-class Kirkland Lake gold deposit. The Li isotopic compositions of these rocks correlate with Au concentrations and range from magmatic values of ∼ +4 ‰ up to +10·4 ‰ in Na–K metasomatized and Au-depleted rocks, and from magmatic values of ∼4 ‰ down to 0·6 ‰ in rocks hosting propylitic, chloritic, and biotitic alteration and Au enrichments of up to 2·9 ppm Au.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

On the volcanic architecture, petrology and geodynamic setting of the 3.48 Ga Barberton komatiite suite, South Africa

Abstract This study presents new field and petrological observations combined with geochemical data on a range of komatiitic to tholeiitic volcanic rocks from the ca. 3.48 Ga mid-lower Komati Formation type-section of the Barberton Greenstone Belt, South Africa. A range of mafic-ultramafic rocks is identified across a 1.44 km profile, leading to the proposition of a new preliminary volcanic architecture for the mid-lower Komati Formation type-section. Major, trace and rare earth element (REE) data in conjunction with Lu-Hf isotopic constraints indicate that the tholeiites, newly recognized high-magnesium basalts, basaltic komatiites and komatiites in the volcanic sequence have a primitive mantle signature with no geochemical affinity to Archaean or modern-day supra-subduction zone boninites. The whole rock initial εHf values of spinifex and massive komatiite flows in the lowermost part of the Komati type-section are negative, ranging between -1.9 and -3.1, whereas the second overlying spinifex and massive flow unit records positive initial εHf values between +0.5 and +4.7. A new geodynamic model involving crustal contamination of the mafic-ultramafic lavas is proposed for the Barberton mid-lower Komati Formation type-section, involving mantle plume-crust interaction. The new observations and data indicate that the komatiites erupted as a result of a mantle plume from a hot (&amp;gt;1 600oC) mid-Archaean mantle, in which the earliest volcanic flows were variably affected by crustal contamination during their ascent and eruption. The possibility of incorporation of lower crustal material and/or recycled crust residing in the mantle source region cannot be excluded. This indicates that modern-style plate tectonic processes, such as subduction, may not have been a requirement for the formation of the 3.48 Ga Barberton komatiite suite, with implications for the hydration state, geodynamic processes and secular thermal evolution of the Archaean mantle.

Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt &amp;lt;1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.