Suite Of Rocks Research Articles

Organic geochemical heterogeneity of cretaceous mudrocks and reassessment of oil sources in the Tano Basin, Ghana

The Tano Basin in Ghana, situated within the South Atlantic margin, is a significant area for petroleum exploration. Previous studies have recognized the presence of source rocks rich in organic matter from the Cretaceous period in this basin. However, the specific origins of this organic matter, as well as the thermal maturity and depositional environment of the Cretaceous source strata, are not well characterized in existing literature. Here, geochemical data are reported on the bulk geochemical composition of 45 Maastrichtian to Albian-aged rock cuttings via pyrolysis and total organic carbon (TOC) analysis. Out of these samples, 29 were selected for further biomarker and stable carbon isotope (δ13C) analysis. The biomarker and isotopic signatures of the 29 rock extracts and 6 crude oils from the basin's productive Jubilee and TEN oil fields were compared to establish oil-oil and oil-source correlations. The Cenomanian and Turonian source rocks showed the greatest oil generation potential based on current bulk geochemical analysis. These rocks are dominated by oil-prone organic matter and are at a more favorable maturity stage compared to the Maastrichtian, Santonian, and Coniacian (M-S-C) rock suites. In contrast, the Albian rock samples, characterized by mixed Type II/III and Type III kerogen, and their high thermal maturity, indicate a high potential for the generation of gaseous hydrocarbons. The biomarker and isotope data from the Cenomanian, Turonian, and Albian (T-C-A) source rocks suggest that they were deposited in a Transitional Marine-Lacustrine environment characterized by reducing redox conditions. This environment effectively preserved organic matter from diverse sources such as bacteria, algae, and higher plants. Furthermore, biomarker ratios indicated that the T-C-A source rocks are in the early-peak mature window. On the other hand, the M-S-C source rocks appear to have been deposited under suboxic conditions in a Coastal Marine environment. Lower aquatic organisms dominate these rocks, which have an immature to marginally mature level. The analysis of lower molecular weight hydrocarbons (≤C20), characterized by an unresolved complex mixture, did not reveal any obvious correlation between the oils and the Cretaceous source rocks. However, the analysis of higher molecular weight hydrocarbons (≥C20) and isotopic ratios indicated that the commercial discoveries in the Jubilee and TEN fields share a genetic similarity and are primarily sourced from the T-C-A source rocks. Furthermore, the molecular and isotopic characteristics suggest that the M-S-C source rocks do not appear to have contributed to the oils found in the Jubilee and TEN fields. These new geochemical findings shed light on the paleoenvironmental conditions and source rock characteristics of the region, significantly contributing to our understanding of the petroleum evolution and potential of the Tano Basin.

Isotopic Geochronological Constraints on the Formation and Evolution of the Moon

One major task of studying the formation and evolution of the Moon is to construct a timeline of the important events with precise isotopic ages. Here, we review recent major isotopic geochronological progress in the past decade and the unsolved problems in isotopic geochronology. The Moon probably formed between 4.52 and 4.42 Ga. Recent high-precision whole-rock and mineral Sm-Nd isotopic dating results suggested that ferroan anorthosite and highlands magnesian suite rocks formed contemporarily around 4.37 to 4.33 Ga. Although the major mare basaltic volcanism took place from 3.85 to 2.93 Ga, new geochronological data from lunar meteorites and Chang’e-5 basalts suggested that lunar basaltic volcanism took place as old as up to 4.37 Ga and at least as young as 2.0 Ga, respectively. Impact events older than 3.9 Ga have also been revealed based on U-bearing minerals Pb/Pb ages and Ar-Ar ages and can provide important clues to understand the late heavy bombardment hypothesis. However, the reliable isotopic ages for the important events on the Moon are still far from conclusive, due to lack of pristine samples that directly crystallized from Lunar Magma Ocean and samples from impact melt sheets in large impact basins (e.g., the South Pole-Aitken basin). In the future, collection and return of pristine samples of ferroan anorthosite and highlands magnesian suite rocks from the farside, cryptomare basalts and late-stage basalts, quartz monzogabbros, granites/felsites, and rocks from impact melt sheets in large impact basins are required for better understanding the formation and evolution of the Moon.

Genesis of the Large-Scale Kamado Magnesite Deposit on the Tibetan Plateau

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical “Kraubath-type” magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata.

A Residual Middle Triassic Oceanic Island-Seamount in the Maoershan Area, Northwest Tibet: Implications for the Evolution of the Paleo-Tethys Ocean

The assemblage of oceanic islands and seamounts, arising from the widespread presence of mature oceans, plays a crucial role in reconstructing the evolutionary history of the paleoocean. Oceanic islands or seamounts within the Longmuco-Shuanghu metamorphic complex, a remnant of the Paleo-Tethys Ocean in the central Tibetan Plateau, have seldom been reported due to their remoteness. This study has identified an oceanic island-seamount in the Maoershan area, situated to the west of the Longmuco-Shuanghu metamorphic complex, composed of basalt, diabase, limestone, and siliceous rocks. Based on field observations, petrology, zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopes analyses, we have identified a suite of mafic rocks with OIB affinity. The youngest zircon U-Pb age cluster was concentrated at ~243–241 Ma. The geochemical characteristics of the siliceous rocks indicate a mixture of terrigenous material, suggesting that they formed in a continental margin. In combination with regional geological data, we conclude that the Longmuco-Shuanghu Paleo-Tethys Ocean remained open during the Middle Triassic. Furthermore, a fraction of the oceanic island-seamounts underwent scraping and transformed into a metamorphic complex, while other segments experienced deep subduction, resulting in the formation of high-pressure metamorphic rocks. Collectively, these processes gave rise to the distinctive high-pressure metamorphic complex within the central Qiangtang terrane.

Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa

The Paleoproterozoic Schiel Alkaline Complex (SAC) in the Limpopo Belt is a composite pluton that coevally formed with the layered intrusion of the Bushveld large igneous province. The SAC is mainly composed of alkaline felsic rocks, pyroxenite, gabbro, syenodiorite/−gabbro, and minor carbonatite and gabbroic dikes. Here we present a new geochemical, geochronological, and isotopic study of the felsic and (ultra)mafic rock suites of the western SAC. The whole rock SmNd isotope study of the felsic rocks revealed εNd2059 Ma values between −6.5 and − 3.8, while those of the syenodiorite/−gabbros and gabbros vary from −7.9 to −8.8, and − 4.3 to +1.1, respectively. The pyroxenites have εNd2059 Ma values of +7.4 to +2.7. The LuHf isotope study displays εHf2059 Ma values of −15.9 to −9.0 for the syenites, alkali granites and syenodiorite/−gabbros, and significantly higher εHf2059 Ma of −3.1 to +3.4 for the gabbros. LA-SF-ICPMS UPb zircon dating yields 2079 ± 10 Ma and 2088 ± 30 Ma for the syenodiorite/−gabbros, and 2086 ± 51 Ma and 2107 ± 30 Ma for the gabbros. In conjunction with published data from the eastern SAC, we propose a close petrogenetic link between the mafic and felsic rocks of the SAC. The positive εNd and εHf values of the pyroxenite and gabbro favor a depleted mantle source, while variable εNd and εHf values, and trace element ratios of the gabbros are ascribed to crustal contamination during magma ascent. The felsic rocks, however, mainly formed by fractional crystallization from the gabbros as revealed by major and trace element correlation trends, isotopic data and thermodynamic modelling.

Petrology of ijolite xenoliths entrained in a nephelinite dyke from the Kamthai area, Late Cretaceous polychronous Sarnu-Dandali alkaline complex, North-West India: Evidence for recurrent magmatic pulses and magma mixing

The Late Cretaceous (89–65 Ma) Sarnu-Dandali alkaline complex is intrusive into the Neoproterozoic (∼750 Ma) Malani Igneous Suite of rocks in North-Western India. This complex represents a polychronous setup owing to the available age groups of varied rock types involving nephelinites, alkali pyroxenites, syenites, phonolites, ijolites, carbonatites and lamprophyres. In this study, we report xenoliths of ijolite up to 2 mm in size, comprising minerals such as aegirine, nepheline, sodalite, apatite, sphene, fluorite, Ti-rich andradite, and perovskite entrained in the host nephelinite from this complex. Pyroxene thermobarometry for both host and xenoliths reveals that the xenoliths crystallized around 876–1114°C; 4.6–18 kbar (Kdcpx−melt ∼0.164), whereas the host magma crystallized around 800–1165°C and 4.5–17.9 kbar (Kdcpx−melt ∼0.44). The redox conditions of the magma are also estimated from Fe-Nb oxy barometry of perovskite and the log oxygen fugacity varies from -2.5 to -3.7 (ΔNNO), showing a resemblance with that of the world-wide kimberlites derived from the cratonic mantle. Furthermore, the composition as well as the barometric study of clinopyroxenes from both the host and the xenoliths, suggest multiple shallower magma chambers composed of ijolite and nephelinite magmas which were activated by different magmatic pulses, thereby forming a complex magmatic plumbing system.

The Paleocene high-silica granites and their associated diorite enclaves in central Lhasa subterrane, Xizang, China: Insights to crust-mantle mixing and crustal reworking

The history of crustal growth in the western Central Lhasa (CL) subterranean during India-Eurasian collision remains unclear. The crucial problem is lack of suitable and ideal magmatism recorded information of the early stages of continental collision. Here, we report the newly discovered high-silica granites (K-feldspar granite with aplite granite) hosting diorite enclaves from the Geji area in western CL subterrane. This suite of magmatic rocks is of great significance to reveal the crustal growth of CL during the India-Eurasia collision. Zircon U-Pb dating results show that the K-feldspar granite, aplite granite, and diorite enclaves were formed at 57.2–57.9 Ma, 58.7 Ma and 58.2 Ma, respectively. Petrology, mineralogy, and mineral chemistry of the plagioclase from the diorite enclave indicates that it was generated by magma mixing between mafic and felsic magmas. The mafic melt was sourced from partial melting of mantle wedge and metasomatized by subduction fluid and sediment melt. This has been supported by the positive zircon εHf(t) values (+0.6 to + 4.9), enrichment of large ion lithophile elements (Rb, K, Th, and U) and depletion of high field strength elements (Nb and Ta), and the high Th/Ce ratios of diorite enclaves. The K-feldspar granite also has positive zircon εHf(t) values (+1.3 to +6.2), arc magma geochemical features, and a lower crust Nb/Ta ratio (8.3). These indicate that the K-feldspar granite was originated from the juvenile lower crust. In conclusion, we think that the crust-mantle magma mixing has a substantial impact on the crustal growth of the western CL subterrane during the subduction slab roll-back of the Paleocene Neo-Tethys Ocean when paired with other research and the context of regional tectonic development.

Garnet versus amphibole: Implications for magmatic differentiation and slab melting

Abstract The garnet signature in the rare earth element (REE) abundances in adakites has been considered a key genetic indicator of these controversial rocks, whose proposed origins include direct melting of subducted oceanic crust (“slab melts”). We show that the garnet signature may be quantified using the shape coefficients of chondrite-normalized REE patterns. We applied this method to a global data set of Cenozoic and Quaternary volcanic samples described as “adakites.” The results indicate that many, but not all, suites of rocks labeled as adakites have undergone fractional crystallization of garnet, starting from parental melts attributable to partial melts of garnet-bearing sources. The extreme garnet signatures seen in many examples require hybrid sources, consisting of subducted sediment as well as igneous oceanic crust; however, extensive deep-crustal differentiation obscures the major and trace-element characteristics of these sources, casting doubt on their identification as primitive slab melts.

Geochemistry, tectonic setting, and petrogenesis of the Coldbrook Group and associated plutons, southern New Brunswick, Canada: a unique Late Ediacaran magmatic episode in Avalonia

The Coldbrook Group is a unique suite of late Ediacaran volcanic and epiclastic rocks deposited in an extensional setting in the Avalonian Caledonia terrane of southern New Brunswick. It is informally divided into a lower part composed of mainly andesitic to dacitic tuffs and epiclastic sedimentary rocks intruded by bimodal plutons, and an upper part composed mainly of mafic flows and tuffs interlayered with but mainly overlain by rhyolitic flows and tuffs and minor epiclastic sedimentary rocks. Recent U–Pb (zircon) dating of four formations in the Coldbrook Group and a subvolcanic granitic dome led to the proposal that the lower and most of the upper Coldbrook Group and associated plutons were formed within <760 kyr at about 551.5 Ma whereas the uppermost rhyolite (Fundy Trail Parkway rhyolite) and underlying basalt of the Hosford Brook Formation formed in a younger event at about 549.5 Ma. Overlapping dates and chemical data also suggest that the subvolcanic granitic domes are the plutonic counterparts of the ca. 551.5 Ma felsic volcanic rocks whereas the other granitic plutons represent extracted melts that evolved separately. Whole-rock geochemical and isotopic data combined with previous field work and map information and integrated with the geochronological data provide new insights into the genesis of these magmas and their change from intermediate to bimodal magmatism. The data indicate that the volcanic and plutonic rocks are mainly tholeiitic and were derived from varying proportions of juvenile melts and recycled older arc lithosphere. The mafic rocks have chemical similarities to flood basalts which inherited some calc-alkalic signatures from older arc-like rocks. Intermediate magmas that formed much of the lower Coldbrook Group had larger lithospheric inputs and thus have mainly calc-alkalic signatures. The Vernon Mountain, Blackall Lake and Silver Hill area rhyolites represent evolved melts derived from the lower Coldbrook Group magmas, and their chemical differences are interpreted to result from melt extraction and solid-liquid dripping magmatic processes. Enhanced extension and thinning led to formation of less lithosphere-contaminated melts that formed the ca. 549.5 Ma bimodal units of the upper Coldbrook Group.

Calcium isotopes track volatile components in the mantle sources of alkaline rocks and associated carbonatites

The volatile components CO2 and H2O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event.The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle.

Sedimentary record of Variscan unroofing of the Bohemian Massif

The Bohemian Massif in the eastern Variscan Orogen is the host of globally significant ultrahigh-pressure metamorphic rock suites that experienced enigmatic unroofing processes. In the eastern Bohemian Massif, the Moravosilesian Culm Basin (The Czech Republic) with ≤7.5 km thick Lower Carboniferous siliciclastic turbidities has recorded crustal unroofing associated with the late Paleozoic amalgamation of Gondwana and Laurussia. This study presents data from sandstone petrography, heavy mineral assemblages, and detrital zircon U-Pb geochronology from Moravosilesian Culm sediments. Sandstone modal compositions imply continental/recycled orogen affinities. High-grade metamorphic lithic fragments and high garnet contents in upper Culm sediments indicate the exhumation of the middle to lower crustal materials, consistent with those of the Bohemian Massif. Detrital zircon U-Pb data constrain maximum depositional ages of sediments in the Drahany Upland to 335–326 Ma (late Viséan–middle Serpukhovian). Detrital, igneous and metamorphic zircon U-Pb data compiled from surrounding regions support that source regions for lower Culm sediments (Moldanubian Unit and Sudetes) remained the same. Whereas, upper Myslejovice sediments might have derived from exhumation and erosion of the root of the Moldanubian Unit and/or Moravosilesian Nappes. Rapid exhumation of the Moldanubian Unit within the Bohemian Massif during the deposition of upper Culm sediments is inferred.

Differences in Geochemical Signatures and Petrogenesis between the Van Canh and Ben Giang-Que Son Granitic Rocks in the Southern Kontum Massif, Vietnam

Permian Ben Giang-Que Son and Triassic Van Canh granitic rocks are widely distributed across the southern Kontum Massif, the basement of which consists mainly of metasedimentary rocks. The Ben Giang-Que Son granitic rocks are classified as I- to S-type and ilmenite-series granitic rocks, while the Van Canh granitic rocks are classified as I-type and magnetite-series granitic rocks. Both granitic rock suites exhibit more or less adakitic properties, suggesting that the subduction of the high-temperature Song Ma Ocean crust, part of the Paleo-Tethys Ocean, beneath the Indochina Block produced adakitic magma. It is hypothesized that the differences between the two granitic rock suites were caused by differences in the quantities of incorporated continental crustal materials and carbon or graphite in clastic sedimentary rocks when their adakitic magma intruded into the continental crust. Based on their high initial Sr isotope ratios, the Ben Giang-Que Son granitic rocks evidently incorporated a higher quantity of continental crustal materials compared to the Van Canh granitic rocks, resulting in the former showing the signatures of ilmenite-series and I- to S-type granitic rocks. Consequently, the Ben Giang-Que Son granitic rocks have relatively high A/CNK ratios and high total Al contents in their biotite, whereas the Van Canh granitic rocks have low A/CNK ratios and low total Al contents in their biotite. The intrusion of the Ben Giang-Que Son granitic rocks caused high-temperature metamorphism, which decomposed some of the carbon or graphite in the surrounding continental crustal materials, such as clastic sedimentary rocks. Meanwhile, the Van Canh granitic rocks, which intruded later than the Ben Giang-Que Son granitic rocks, incorporated smaller quantities of carbon or graphite in continental crustal materials, resulting in them retaining the chemical characteristics of adakitic, magnetite-series, and I-type granitic rocks, different from the Ben Giang-Que Son granitic rocks.

Geochemistry, mineral chemistry and Re–Os isotopes of refractory peridotites of the North Balkhash ophiolite zone in the West Central Asian Orogenic Belt (Central Kazakhstan): multi-stage melt evolution of a late Precambrian forearc mantle

In this study we report on the petrography, major and trace element and mineral chemistry, platinum-group elements and Re–Os isotope systematics of depleted ultramafic rock suites from the Itmurundy Block in the North Balkhash ophiolite zone in Kazakhstan. Represented mainly by variably serpentinized harzburgites and dunites, our samples are characterized by low whole-rock Al 2 O 3 (0.33–0.86 wt%), CaO (0.51–0.86 wt%) and Na 2 O (0.07–0.25 wt%) concentrations, and high-Mg olivine (Fo = 91–92) and orthopyroxene (Mg# = 92–93) contents, together with moderately high spinel Cr-numbers (Cr# = 63–68). They are depleted in incompatible elements ( Σ REE, Nb, Sc) and enriched in compatible elements, such as Cr (up to 2817 ppm) and Ni (up to 2327 ppm), representing highly refractory mantle residues derived from a forearc mantle wedge. They underwent 19–23% hydrous partial melting to produce boninitic melts. 187 Os/ 188 Os values vary from 0.1202 to 0.12599, and 187 Re/ 188 Os ratios from 0.230 to 0.316. The Re–Os model ages (T MA ) and maximum Re depletion model age (T RD ) were calculated based on the obtained data. Re–Os isotope systematics suggests that the analysed peridotites formed in two stages: a first stage around 1.5 Ga and a later stage around 668–589 Ma. Supplementary material: A supplementary figure is available at https://doi.org/10.6084/m9.figshare.c.6845683 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Geochemistry, Sr-Nd isotopes and zircon U-Pb dating of magmatic rocks from the Talesh range, western Alborz: New insights into Late Cretaceous evolution of the southern Eurasian margin

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross-cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon UPb ages, bulk-rock major and trace element geochemistry, and SrNd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc-related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon UPb age of Group 1 rocks is 95.6 ± 1.8 Ma (2σ), whereas for Group 2 rocks, cross-cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87Sr/86Sr(i) varying from 0.7038 to 0.7070 and εNd(t) values ranging from +0.5 to +5.0. Group 2 rocks have 87Sr/86Sr(i) = 0.7040–0.7065 and εNd(t) from −3.0 to +3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of ∼6–10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by ∼1–5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from ∼6–13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through ∼1–3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north-dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back-arc setting.

Evaluation of portable Raman spectroscopic analysis for source-rock thermal maturity assessments on bulk crushed rock

This study presents a simplified method and empirical relationships for determining organic matter thermal maturity using a portable Raman system equipped with a 785 nm laser, for analysis of crushed, whole-rock samples. Suites of rocks represented by shale and coal samples with various mineralogical composition, thermal maturity, and total organic carbon (TOC) were used to test the method and build correlations between Raman band separation (RBS) values and traditional thermal maturity indicators, organic matter reflectance (Ro), and programmed temperature pyrolysis (Tmax) values. A set of disparate shale samples, where both vitrinite and solid bitumen reflectance values were reported, have Ro values that range from 0.40 to 4.62%. Above 3.35% Ro, the corresponding RBS values plateau at ∼290 cm−1, thus correlations were evaluated with a linear regression (R2 = 0.96) between 0.40 and 3.35% Ro. Shale samples with Ro < 2% and Tmax < 551 were also used to correlate Tmax and RBS, yielding a linear correlation with an R2 of 0.94. For the coal data set, Ro values range from 1.21 to 4.08% and correlated RBS values plateau at ∼250 cm−1 above Ro = 3.0%, suggesting its correlative application below this maturity level. Several sample preparation methods were tested on cuttings material and standard deviation values for RBS were minimized by washing, drying, and hand crushing the material to pass through a 40-mesh sieve, although less preparation can still yield reliable results. The high degrees of correlation between whole-rock RBS data and two thermal maturity indicators demonstrate the utility of this approach for generating source rock thermal maturity data from minimally processed, whole-rock samples which could easily be applied in field or laboratory settings.

Late Ediacaran–early Cambrian rifting along the northern margin of Laurentia: constraints from the Yelverton Formation of Ellesmere Island, Canada

Neoproterozoic to early Paleozoic strata exposed along the northeastern margin of North America (Franklinian Basin) record a prolonged history of rifting and passive margin development. An episode of Ediacaran–Cambrian extension is potentially recorded in volcanic and sedimentary rocks of the Yelverton Formation, exposed on northern Ellesmere Island, Nunavut, Canada. Here, we present new whole-rock trace element and isotope geochemistry and hornblende 40Ar/39Ar geochronology from intrusive and extrusive rocks of the Yelverton Formation, along with isotope geochemistry from carbonate rocks underlying the volcanics and detrital zircon U–Pb and Lu–Hf isotopic data from the Yelverton Formation and overlying Grant Land Formation. The carbonate strata yielded an average 87Sr/86Sr value of 0.7076 ( n = 6), constraining the overlying volcanics to the late Ediacaran–early Cambrian (ca. 570–530 Ma). Flows and dikes/sills show three distinct compositions: (1) a depleted, low La/YbPM and Th/Nb group, (2) an enriched, higher La/YbPM and low Th/Nb group, and (3) a low to moderate La/YbPM and high Th/Nb group. One of the high Th/Nb intrusions produced saddle-shaped 40Ar/39Ar hornblende spectra with ca. 482 ± 7 Ma age minima and Silurian–Devonian inverse isochron ages, highlighting a hitherto unknown suite of younger intrusions and volcanic rocks. The trace element geochemistry and 143Nd/144Nd(t) ratios of enriched and depleted volcanic rocks of the indisputable Yelverton Formation are consistent with decompression melting during rifting along a continental margin. We suggest that late Ediacaran–early Cambrian rifting recorded in the Yelverton Formation resulted in the separation of peri-Laurentian terranes, such as the North Slope subterrane and eventual development of the passive margin of northern Laurentia.

Refining the understanding of Cretaceous subduction zone processes in the central Indonesian region: The Bantimala Complex (SW Sulawesi) revisited

A prominent occurrence of Cretaceous metamorphic rocks in the central Indonesian region is the Bantimala Complex, SW Sulawesi, where an eclogite- and blueschist-bearing sequence is part of an accretionary wedge with a complex internal structure. Zircon U–Pb dating of metamafic blocks indicate a significant protolith age range (c. 369–185 Ma) for the HP/LT rock suite. The HP/LT rocks include lawsonite-free and lawsonite-bearing samples. Lawsonite in eclogite is of retrograde origin. Assuming fast exhumation, multipoint Rb–Sr age data (c. 132–125 Ma) are interpreted to broadly approximate the time of peak HP/LT metamorphism. Existing and new K–Ar and Rb–Sr dates can be subdivided into two groups (c. 137–130 Ma and c. 126–113 Ma). Samples with post-eclogitic lawsonite growth belong to the older age group and all lawsonite-free samples are in the younger age group. The mineralogical and age diversity of various blocks indicates detachment of individual fragments from the subducting plate at different depths, followed by return flow in the subduction channel along different P–T–t trajectories. The role of serpentinite during exhumation remains ambiguous. Field observations offer little support for the suggestion that most HP/LT blocks were originally transported in a serpentinite matrix. Subduction-related magmatism is recorded by a U–Pb zircon age of 109.2 ± 1.1 Ma for a quartz dioritic mélange block. Detrital zircon U–Pb data from a sandstone indicate that tectonic stacking of the Bantimala Complex occurred only after partial erosion of already exposed Cretaceous basement.

Contrasting Styles of Lower Crustal Metamorphism from a Granulite Suite of Rocks from Angul, Eastern Ghats Belt, India: Implications for the India–Antarctica Correlation

AbstractThe present work is focussed on a suite of high-grade rocks including mafic granulite, aluminous granulite, khondalite, charnockite, and augen gneiss along with medium-grade rocks like olivine-bearing metanorite, gabbro, and porphyritic granite of the Angul domain at the northern margin of the Proterozoic Eastern Ghats Province (EGP). Based on the petrological and geothermobarometric data, two distinct metamorphic events have been identified. The imprints of the earlier event (MA1) are preserved in the mafic granulite, aluminous granulite, khondalite, augen gneiss, and fine-grained charnockite, but those are best preserved in mafic granulite and aluminous granulite. In mafic granulite, orthopyroxene + clinopyroxene + plagioclase ± garnet+ ilmenite ± quartz assemblage was stabilised at the peak MA1 conditions, whereas the peak MA1 assemblage is represented by Fe3+-garnet + hematite + magnetite + cordierite + K-feldspar + plagioclase + sillimanite + quartz + melt in aluminous granulite. Phase equilibria modelling and thermobarometric data suggest the P–T conditions of &amp;gt;850°C, 7 to 8 kbar for this event. The retrograde metamorphism (MA1R) involved minor decompression (down to ~5 kbar) and subsequent cooling to form biotite- and hornblende-bearing mineral assemblages in aluminous granulite and mafic granulite, respectively. Texturally constrained monazite (U–Th–total Pb) and zircon (U–Pb) data from the former rock suggest ca. 1200 Ma age of the MA1 metamorphism, which was associated with granitic and charnockitic magmatism as constrained from oscillatory-zoned zircon domains in the augen gneiss and fine-grained charnockite. The rock ensemble was affected by a younger metamorphic event (MA2), which is texturally characterised by partial replacement of hornblende (developed during MA1R) to orthopyroxene ± clinopyroxene + plagioclase ± ilmenite + melt assemblage in mafic granulite. Moreover, biotite of aluminous granulite has undergone dehydration melting to produce garnet + cordierite-bearing assemblage. Garnet in the above assemblage did not form as porphyroblastic phase and overgrew the MA1 garnet. The MA2 event followed a counterclockwise P–T trajectory, causing heating (up to 800°C) with associated loading (from 4.0 to 5.8 kbar) along the prograde path. Monazite U–Th–total Pb data from aluminous granulite and khondalite suggest MA2 ages of 987 ± 12 and 975 ± 16 Ma, respectively. Fine-grained charnockite and augen gneiss also recorded the imprints of MA2 event by developing thin to thick sector-zoned overgrowth yielding group ages of 979 ± 12 and 982 ± 29 Ma, respectively. Zircon overgrowth in mafic granulite formed at 962 ± 13 Ma. The MA2 event coincides with the crystallisation of coarse-grained charnockite at 983 ± 22 Ma and porphyritic granite at 960 ± 10 Ma. Geochronological data, thus, indicate that the Angul domain underwent the MA2 metamorphism and associated magmatism at ca. 990 to 960 Ma. The apparent absence of MA1 event (~1200 Ma) in the greater part of the EGP and its dominance in the Angul domain suggest that the latter was possibly an exotic block at ca. 1200 Ma and joined with the rest of the EGP only after ca. 960 Ma. We further suggest that the metamorphic history of the Angul domain is strikingly different from the rest of the EGP, but matches well with that of the Prydz Bay region of the East Antarctica.

Geochemical characteristics of arc fractionated I-type granitoids of eastern Tak Batholith, Thailand

Abstract The Tak Batholith is located in the western part of Thailand’s eastern granitoid belt. To better understand the geochemical characteristics of granitoid and identify their occurrence, rock samples collected from the eastern part of the Tak Batholith were used for petrographical, mineralogical, and geochemical analyses. According to field investigation and lithology, petrography classifies these granitic rocks as medium-grained granodiorite and granodioritic dike. The granodiorites are composed chiefly of plagioclase, orthoclase, quartz, hornblende, and biotite, with tiny amounts of zircon, apatite, and opaque minerals. Meanwhile, the granitic dike shows a highly porphyritic texture, presenting mostly plagioclase phenocrysts, with a small amount of orthoclase and unidentified mafic phenocrysts sitting in a fine-grained groundmass comprising K-feldspar and quartz with a small amount of plagioclase and present granophyric intergrowth. Geochemical data of the amphiboles classified them as Ferro-edenite symbolizing the I-type granite. Whole-rock geochemistry indicates that these rock suites are I-type metaluminous granodiorite, and diorite fractionated from the high K calc-alkaline magma melted from the mafic crust of arc magmatism agreeable with the enrichment of large ion lithophile elements and depletion of highfield-strength elements, and rare earth element patterns.

OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle

Abstract. The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µg g−1 H2O) and grossular-rich garnet (200–1800 µg g−1 H2O) contain at least 1 order of magnitude more H2O than almandine-rich garnet (&lt; 120 µg g−1 H2O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H2O to be calculated. The result shows that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean.