Suite Of Rocks Research Articles

The 1126 Ma volcanic event in the Dechang Area, SW Yangtze Block, and its significance

AbstractTraditionally, the strata of the Luonie Valley, Dechang County, SW Sichuan, China, are considered to contain a suite of felsic volcanic rocks (the Huili Group) that erupted after c. 1050 Ma. However, we report here new age constraints, elemental and Lu–Hf isotope geochemistry for a different suite of older basaltic agglomerate lava, basaltic tuff lava and basalt from the same area, which we name the Luonie Formation. New dating results show that the basaltic volcanic suite of the upper part of the Luonie Formation formed at 1126.1 ± 9.9 Ma, significantly earlier than deposition of the Huili Group, but comparable in age to the 1142 ± 16 Ma Laowushan Formation in central Yunnan Province. Granite intrusion into the Luonie Formation dated 1050.7 ± 12.7 Ma provides crucial supporting evidence for this earlier depositional age. We also report a maximum sedimentary age of c. 1158 Ma for the underlying arkose, implying stratigraphic conformity with the basaltic volcanic rock suite.The ϵHf(t) values of the basaltic volcanic rocks are mainly positive, indicating that the rocks are mainly derived from the depleted mantle and slightly stained by crustal materials. The characteristics of P*, Nb* and Zr* anomalies also support this view. The distribution patterns of trace and rare earth elements indicate that the basaltic volcanic rocks formed in an extensional setting. The Zr/4–Y–2Nb and Th–Nb/16–Zr/117 discrimination diagrams also provide evidence for this understanding. Lithofacies analysis shows that basaltic volcanic wrocks with the characteristics of both continental and marine facies should be formed in a littoral–neritic environment. We propose here that the evidence is consistent with a phase of continental extension that preceded the convergence of the SW Yangtze Block to form part of Rodinia.

Reduction of iron with no systematic isotope fractionation during continental subduction observed in metamorphic rocks from the Dabie-Sulu orogen, China

• No resolvable Fe isotope fractionation in metabasites during prograde dehydration. • Eclogites have Fe 3+ /∑Fe systematically lower than the other metabasites. • Fe reduction likely occurred during eclogitization with simultaneous S oxidization. In order to better understand the mobility and charge balance of Fe in subducted slabs, here we report precise Fe isotopic data, supplemented by Fe 3+ /∑Fe and C-N-S elemental contents, in a well-characterized suite of metamorphic rocks from the Dabie-Sulu orogen, China, including mafic to felsic greenschists, amphibolites and eclogites. There is no systematic Fe isotopic difference among the mafic schists (δ 56 Fe = 0.08 ± 0.08‰, 2SD, N = 13), amphibolites (δ 56 Fe = 0.09 ± 0.04‰, 2SD, N = 19) and eclogites (δ 56 Fe = 0.07 ± 0.06‰, 2SD, N = 27), implying limited Fe mobility during prograde metamorphic dehydration. The overall δ 56 Fe variation of the metabasites and felsic schists ranging from 0.02 ± 0.03‰ to 0.20 ± 0.03‰ most likely mirrors heterogeneity in their Neoproterozoic magmatic protoliths, and can be interpreted by derivation from diverse sources and isotope fractionation during magma differentiation. In a contrary to limited Fe mobility, a decrease in Fe 3+ /∑Fe ratios has been identified for metabasites during eclogitization (on average 0.43 for greenschists, 0.42 for amphibolites and 0.24 for eclogites respectively). Reduction of Fe could have been compensated by oxidation of S where a concurrent expulsion of sulfate seems to be necessary. This study thus supports the hypothesis that Fe-S redox coupling in subducted slabs promotes transfer of oxidizing agents into the mantle wedge in subduction zones.

Sikkimite: A New and Rare Suite of Potassic-perpotassic Syenites from South Sikkim, Lesser Himalaya, India

Abstract A new and rare suite of radioactive, plutonic-hypabyssal, nonperalkaline to peralkaline, potassic to perpotassic syenites have been reported as dyke-like and/or lensoid bodies (c. 100 m x 20 m to 5 m x 2 m) within the Buxa and Daling formations (Proterozoic) and Gondwana Group of rocks (Permo-Carboniferous) within the Ranjit tectonic window (RTW) and its peripheral parts in south Sikkim. These are christened in this work as ‘Sikkimites’ considering their unique field occurrences, petrography, mineralogy and geochemistry, unlike any known saturated potassic - perpotassic syenites from the plutonic environment from India or elsewhere. The modal (vol.%) mineralogical variations observed within the potassic syenites include orthoclase (57-65%) with subordinate to minor amounts of brown alkali amphibole (kataphorite, 3-32%), aegirine (1-20%), biotite (13% in one sample), opaque minerals (ilmenite and magnetite, 4.2-24.5%), quartz (traces to 12.9%) and minor to trace amounts (< 3%) of apatite, sphene, zircon, and monazite. Compositionally, the Sikkimite suite range from non-peralkaline to highly peralkaline types (with Mol. Na2O + K2O/Al2O3 varying from<1 to1.2 to 1.5), mildly-perpotassic (Mol. K2O/Al2O3>0.83 to 0.96) and perpotassic (Mol. K2O/Al2O3>1 and Wt.% K2O/Al2O3 = 2.52-23.9) and Wt.% K2O/Na2O(3.05-23.9). The potassic syenites shows wide variations in major, minor and trace elements with significant enrichments in TiO2, Ba, REE, Y, Zr and Nb. The petrogenetic model for these rare potassic syenite suite of rocks (termed here as Sikkimite) invokes a near-primary origin by partial melting of enriched, amphibole- and phlogopite-bearing heterogeneous mantle, having chemical signatures of both non-subduction and subduction-related metasomatism. Ar40-Ar39 dates on them indicate a Cretaceous age (100-90 Ma) suggesting their links to either the widespread igneous manifestation of the Kerguelen or the Marion plume of the Indian Ocean.. These rare potassic syenites were apparently brought to the near surface environments in south Sikkim due to the collision of Greater India with Asia and the ensuing Himalayan thrusts from the north (MCT1 and MCT2) which shifted the crystalline basement and the Gondwana rocks maximum to the south, unlike any other parts of the Himalaya.

Mineral-Melt Equilibria and Geothermobarometry of Campi Flegrei Magmas: Inferences for Magma Storage Conditions

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei’s volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as “magmatic environments”, characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei’s, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods.

Basaltic volcanism of Medvezhia caldera on the Iturup Island of Kurile Isles: impact of regional tectonics on subduction magmatismMartynov Yu.А., Rybin А.V., Chibisova М.V., Ostapenko D.S.,Davydova M.Yu

ABSTRACT We present newly acquired data on the trace element composition and Sr–Nd–Pb-O isotopes of Pliocene – Holocene basalts of the Medvezhiy caldera located on the Iturup Island, Kuril arc. With its unique rhenium mineralization and high-Mg basalt volcanism, it is the largest caldera on the volcanic front of the Kurile Island arc. The calc-alkaline suite of rocks spanning the range from high-Mg basalts to high-Al basalts, has typical arc rock features, such as Nb-Ta depletion and LILE enrichment. The whole set of the caldera geochemical data is divided into two age units, pre- and postcaldera basalts. The precaldera basalts typify low-K, high-Al, basaltic suite of the frontal zones of «cold» subduction. PRIMACALC2 code and Arc Basalt Simulator version 3.1 forward model points to ~21.7% of partial melting of moderately depleted mantle peridotite with relatively high water content (~0.34 wt%) at the pressure of ~1.37 GPa. With similar isotope and trace element signatures, Late Pleistocene–Holocene basalts are marked by the high abundances of MgO (up to 11 wt %), but low Al2O3. The primary magmas were generated from depleted mantle with lower H2O content (~0.13%) and degree of partial melting (~9.5%), but at higher pressure (~1.77 GPa). The decompression scheme reproduces the fractionation trends of postcaldera magnesian basalts for most major elements but fails to explain the composition variations of the precaldera lavas. Their origin has been the matter of some complex processes in the shallow magma chamber. The abrupt change in the type of eruptions and magma composition of the caldera in Late Pleistocene–Holocene were related to a transformation of both the magma-feeding and melt generation systems as the result of geodynamic reorganization of the vast territory adjacent to the Sea of Japan and Kamchatka.

Mafic-ultramafic suite from the Karwar Block, SW India: Implications for Mesoarchean geodynamics

Archean mafic–ultramafic rocks provide important clues on the tectonic and crust building processes in the early Earth. The Western Dharwar Craton (WDC) and the Karwar Block (KB) in southern Peninsular India preserve some of the oldest records of craton building processes in the Archean Earth. The tectonic affinity of the Karwar Block that lies at the western periphery of the WDC remains uncertain. Here we present the results from an integrated study of a mafic–ultramafic rock suite from this block using petrology, whole-rock geochemistry, and zircon U–Pb and Lu-Hf data. These rocks occur as deformed and dismembered units within basement granitic gneiss. The primary igneous minerals such as pyroxene and plagioclase show variable degrees of alteration. The rocks show negative Nb anomalies and prominent Pb enrichment. The uniform enrichment of Th and U over Nb-Ta indicates the influence of metasomatic fluids in the mantle wedge. The geochemical data indicate that these rocks were derived from basaltic magmas generated within a subduction metasomatised mantle wedge in an intra-oceanic arc setting without any significant crustal contamination. U-Pb isotope analysis of zircon grains from the metapyroxenite shows upper intercept ages of 3172 to 3213 Ma and weighted mean ages of 3170 to 3203 Ma. A metagabbro sample yielded upper intercept age of 2792 Ma and the 207Pb/206Pb spot ages in another sample range up to 3202 Ma. Zircon U-Pb data suggest that the mafic–ultramafic suite formed at ca. 3.2 Ga and were overprinted by 2.7 Ga thermal event. The εHf(t) values are all positive and range between 0.73 and 6.02, suggesting that the magma was derived dominantly from Mesoarchean juvenile (depleted mantle) sources. We propose a tectonic model that involves a subduction-accretion stage followed by suturing of the ancient continental nuclei.

Early depositional and magmatic history of the Beardmore-Geraldton Belt: Formation of a transitional accretionary belt along the Wabigoon-Quetico Subprovince boundary in the Archean Superior Craton, Canada

The Beardmore-Geraldton Belt (BGB) is a major gold camp along the boundary between the granite-greenstone eastern Wabigoon subprovince and the metasedimentary Quetico subprovince of the Archean Superior Province, Canada. New detrital zircon geochronology suggests that the metasedimentary rocks of the BGB and northern Quetico subprovince were derived from the erosion of the eastern Wabigoon subprovince that is dominated by ca. 2900–2680 Ma rocks but also comprises crust as old as > 3200 Ma. Uplift of the eastern Wabigoon subprovince as a result of convergence and compression along its southern margin exposed those rocks to erosion and shed sediments in the Quetico basin to the south. Sedimentation in the Quetico basin is bracketed between ca. 2700 Ma, the age of the youngest detrital zircon population, and 2694.0 ± 1.0 Ma, the crystallization age of cross-cutting feldspar-quartz porphyries (FQP). Continued convergence facilitated the thrust-imbrication of the metavolcanic rocks of the eastern Wabigoon subprovince with the metasedimentary rocks of the Quetico basin resulting in the assembly of the BGB. This resulted in crustal thickening, the introduction of hydrated mafic rocks at the base of the crust, and partial melting of these rocks to form TTG suite melts at depths >50 km, as suggested by the geochemical characteristics of the FQP intrusions. Imbrication and thrusting ended with the emplacement of the stitching, 2690 ± 1 Ma Croll Lake Stock, which has a high transition elements and ferromagnesian oxides signature of sanukitoid suite rocks. It formed during delamination of the lower crust or slab break-off from magmas generated by mixing of hot, mantle material with TTG suite melts at depths > 50 km. The early geological evolution of the Geraldton gold camp is broadly similar to that of other major Archean gold camps associated with major faults and/or terrane boundaries, where deposition of fluvial conglomerate and turbiditic sandstones in fault-bounded basins overlying older volcanic rocks is accompanied by magmatic activity and the upward migration of gold-bearing hydrothermal fluids, while differences between the Geraldton and other camps may be explained by craton-scale variations in geodynamic processes.

What drives Fe depletion in calc-alkaline magma differentiation: Insights from Fe isotopes

Abstract The continental crust is strongly depleted in iron relative to mid-oceanic ridge basalt, broadly identical to the calc-alkaline magmas, suggesting that calc-alkaline differentiation is key to continent formation. However, it remains contentious as to what drives Fe depletion during magmatic differentiation in the crust. The two competing hypotheses for calc-alkaline differentiation—magnetite versus garnet (± amphibole) fractionation—predict contrasting Fe isotopic fractionation pathways in evolved melts because magnetite preferentially depletes ferric, isotopically heavy Fe whereas garnet (± amphibole) does the opposite. We report whole-rock Fe isotope data for two suites of igneous rocks from the central Andes, which represent magmas traversing normal and thickened arc crust, respectively. The magmas traversing thickened crust show a strong Fe depletion trend and consistently high δ56Fe values (0.14‰ ± 0.02‰, 1 standard deviation [SD]), while those traversing normal crust are less depleted in Fe and show variable δ56Fe values (0.10‰ ± 0.05‰, 1SD). The two Andean suites are both isotopically heavier than Mariana arc (Pacific Ocean) magmas that differentiate along tholeiitic (Fe-enriching) paths. These results confirm that garnet (± amphibole) fractionation/retention is the primary driver of Fe depletion in calc-alkaline magmas, and highlight a role for crustal thickening in generating calc-alkaline magmas.

Structural features of the Jiangshao Fault Zone inferred from aeromagnetic data for South China and the East China Sea

It is very important to ascertain the trend of the Jiangshao Fault Zone after it enters the East China Sea for the study of the South China Block. In this paper, we intended to solve the problem based on analysis of aeromagnetic data for South China, the East China Sea, and the adjacent areas. We described the distribution and structural features of the Jiangshao Fault Zone, and identified magnetic anomaly features, fault structures, and volcanic rock suites, along with the spatial distribution of igneous bodies with a high magnetic susceptibility. The spatial extent of the Jiangshao Fault Zone onland and offshore was determined through a comparison of the magnetic anomaly features with the geology and inferred structures. The Jiangshao Fault Zone extends from onland to offshore along a NE–SW trend. The formation of the Jiangshao Fault Zone was determined by comparing the stratigraphy and structures of the basement rocks, lithological units, and geological histories of the Yangtze and Cathaysia blocks. The spatial distribution of magmatic bodies with high magnetic susceptibility in the South China Block was revealed by the inversion results of aeromagnetic data. These findings contribute to understanding the tectonic evolution of the Yangtze and Cathaysia blocks, and provide insights into the geological structure of the East China Sea and formation of the East China Sea Shelf Basin.

The Jurassic meta-ophiolitic rocks of Cape Steno, Andros, Greece: a high-pressure/low-temperature m\xe9lange with Pelagonian affinity in the Cycladic Blueschist Unit?

This study aims at clarifying the relationship between the Cape Steno mélange, southern Andros, and the main tectonic units of the Attic-Cycladic Crystalline Belt. Jurassic protolith ages and geochemical characteristics indicate a Pelagonian affinity and point to a correlative relationship with the Tsiknias Ophiolite on Tinos Island. However, jadeitites and high-Si phengite in the gneisses clearly indicate a high-pressure metamorphic overprint that is unknown from the Tsiknias outcrop and other occurrences of the Upper Cycladic Unit. A correlation with the Cycladic Blueschist Unit (CBU) is an obvious assumption, but initially seemed difficult to reconcile with the Cretaceous protolith ages of meta-ophiolitic rocks from the CBU and distinct geochemical characteristics of associated jadeitites. The Jurassic ages of the Cape Steno rock suite either document a broader spectrum of source rocks than previously known from the CBU, or the existence of a distinct tectonic unit. We assume that the geological and tectono-metamorphic evolution of the Cape Steno occurrence is similar to that of the Makrotantalon Unit of NW Andros, which represents a Pelagonian subunit in the nappe stack of the CBU, with abundant slices of serpentinites, rare meta-gabbro and a metamorphic history comprising both Cretaceous and Eocene HP/LT episodes.

Indosinian Supergene Karsts in the Qaidam Basin, NW China: Evidence from Paleo‐tectonics, Paleoclimates and Paleo‐fluids

AbstractIn the Qaidam basin, Northwest China, karstic fissures and caves are developed in many places of the Lower Carboniferous Chengqianggou Fm. and Huaitoutala Fm. and the Upper Carboniferous Keluke Fm., in which are found typical karstic marks. Several suites of transgressive carbonate rocks formed during the Late Paleozoic epoch are preserved as reservoir properties in the Qaidam basin, with paleokarst‐related cave‐fractured diagenetic structures, which provided the basis for cave‐fractured development. Carbonate and clastic samples covering or infilling caves and fractures in Carboniferous strata were collected in the eastern Qaidam basin to focus on petrological and geochemical analyses of karst and infilling materials. Based on this study of the Caledonian diagenetic sequence, and bitumen‐infilled inclusion temperature and burial history, there existed a continuous atmospheric freshwater leaching process, which played a critical role in the construction of the Carboniferous supergene paleokarsts. Investigation and survey of the tectonic setting, plate paleolatitude and river development intensity also proves that there was abundant rainfall during the Indosinian period, after which the strata undergo a shallow burial process. Because Indosinian period is earlier than the key timing of hydrocarbon‐generation of the Carboniferous source rocks, this kind of karst reservoir has potential significance for oil‐gas resources in this area.

Late Cretaceous granitoids of the Sikhote–Alin orogenic belt, southeastern Russia: implications for the Mesozoic geodynamic history of the eastern Asian continental margin

We present new U–Pb age data for granitoids in the Central Sikhote–Alin orogenic belt in SE Russia, which refute the established opinion about the absence of the Late Cretaceous magmatism at the eastern margin of the Palaeo-Asian continent. It was previously thought that a period of magmatic quiescence occurred from 88 to 50 Ma, related to subduction of the Palaeo-Pacific plate under the eastern margin of the Palaeo-Asian continent, although this is inconsistent with evidence from the Sikhote–Alin, Sakhalin and Japan regions. Three suites of plutonic rocks with different ages were identified in this study. The first suite has ages of 105–92 Ma and formed in a syn-orogenic setting. The second (86–83 Ma) and third ( c. 73 Ma) suites formed during the post-orogenic stage of the Sikhote–Alin orogenic belt. The second and third suites were coeval with Late Cretaceous granitoids that formed in a supra-subduction continental arc known as the Eastern Sikhote–Alin volcanic–plutonic belt. However, the studied rocks are located far inland from the Eastern Sikhote–Alin volcanic–plutonic belt. The ages of the studied granitoids coincide with the timing of a change in the angle of convergence between the Palaeo-Pacific plate and eastern margin of the Palaeo-Asian continent. This change in motion of the oceanic plate with respect to the continental plate was probably caused by a rupture in the subducted slab (i.e. a slab tear), followed by asthenospheric upwelling and partial melting of the overlying crust, which ultimately generated post-orogenic intrusive magmatism. Supplementary material: Laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb data of the Bikin–Bol. Ussurka granitoid suites are available at https://doi.org/10.6084/m9.figshare.c.5738616

Uranium-lead isotopic age of monazites of plagiogneyses of the Ingulo-Inguletse series (Ingul megablock of the Ukrainian Shield)

Among the supercrustal formations of the Ingul megablock, Paleoproterozoic rocks, represented by the Ingul-Ingul series, are significantly predominant. In general, the composition of the Ingul-Ingulets series includes (from bottom to top): Zelenorichen, Artemiv, Rodioniv, Kamyano-Kostovatsky (age analogue of Spasivsk) and Roschakhiv (age analogue of Chechelivsk) worlds. The Chechelivsk suite is represented by biotite and garnet-biotite gneisses, places with sillimanite and cordierite, as well as underlying layers of pyroxene gneisses and schists. Suite rocks are widespread within the Ingul Synclinorium, in the framework of the Korsun-Novomyrhorod pluton and in the northern part of the Bratsky Synclinorium. Currently, the age of monazites from plagiogneisses distributed in the Novgorodkiv quarry and south of the village Lavrovka has been determined using the uranium-lead isotope method. The obtained monazite isotopic age values of 2022.9 ± 6.7 Ma and 2031.5 ± 1.5 Ma, respectively, can only serve as the upper age limit for the formation of the Ingul-Ingul series. Analysis of the internal structure of zircon crystals from plagiogneiss distributed south of Lavrivka, showed that the majority of zircons are complex. In almost all crystals there are mainly thin shells of light pink zircon. Light pink zircon cores are found in the middle of the light pink crystals. Crystals with a brown color have a more complex structure. Along with isometric cores of light pink zircon, cores with fine «magmatic» zoning occur, as well as highly fractured uniform brown cores and complex cores.On this basis, it is assumed that this plagiogneiss was formed by clastogenic material from several sources.

PETROGRAPHIC STUDIES AND PHYSICO-MECHANICAL PROPERTIES OF BIRIMIAN GRANITOIDS – A CASE STUDY OF OYOKO GRANITOIDS COMPLEX IN KOFORIDUA

The study investigated the physico-mechanical and petrographic properties of the basin-type granitoids of the Oyoko granitoid complex, to establish their strengths for engineering properties. The results obtained from this work will therefore be necessary in knowing the competences of the rocks for various engineering construction works. The purpose of this research is to determine the physico-mechanical and microstructural features of the granitoid suites of rocks at Oyoko. Thin sections were prepared and microscope used for the petrological studies of the rocks, whilst UCS test was conducted for the strength of the rocks. The Oyoko granitoid complex rocks were petrologically identified to be biotite gneisses, pegmatite and migmatite. The rocks at the study area have received little attention in mechanical and petrographic studies. These granitoid rocks have preserved porphyritic textures and consist of mineral assemblage such as biotite, quartz, plagioclase feldspar, alkali feldspar, amphibole, clino-pyroxene, chlorite, etc. The recrystallization of the plagioclase feldspar imply that these rocks have experienced some level of deformations. The foliated granitic gneisses intruded the older rock units, some of which had been subjected to earlier tectonism. The migmatite defines the area to be closer to a contact zone. The major structural features in the migmatite rocks were ptygmatic folds. The UCS value for the biotite gneiss of 91.6 N/mm2 gives a more representation of the general rock strength of the study area. The biotite gneisses at the study area may be classified to be mechanically strong and recommended for road construction and engineering works.

Petrogenesis of the Early Silurian Renda appinite suite in the southeastern Qilian Orogen, NW China: Implications for the evolution of a Proto-Tethys magmatic arc

ABSTRACT Early Paleozoic subduction beneath peri-Gondwana microcontinents formed a major Proto-Tethys magmatic arc system. Appinite suite rocks constitute important components within that magmatic arc. Herein, we report hornblende 40Ar/39Ar and zircon U–Pb geochronology data, zircon Hf isotope, mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope data for the rocks of hornblende gabbro-diorite-quartz diorite-quartz monzodiorite from the Renda appinite suite and adjacent granite porphyry in the southeastern Qilian Orogen. Hornblende 40Ar/39Ar and zircon U–Pb isotopic data constrain ages of the intrusions of the Renda appinite suite to 439–435 Ma. Rocks in this suite represent the calc-alkalic to alkali-calcic series, and they are enriched in large-ion lithophile elements and depleted in high-field-strength elements. The hornblende gabbros exhibit moderate initial 87Sr/86Sr ratios (0.7046–0.7055) and higher whole-rock εNd(t) (–1.3 to +1.7) and zircon εHf(t) (+7.2 to +11.2) values relative to the other rocks of the suite (0.7047–0.7064, – 7.7 to – 1.1, and – 6.6 to +6.8, respectively) as well as Pb isotopic features similar to EM2 lithospheric mantle. These geochemical characteristics suggest that the Renda appinite suite originates from partial melting of an enriched lithospheric mantle, with subsequent fractional crystallization and hybridization with crustal components. The granite porphyry formed by partial melting of the crust coeval with the emplacement of juvenile magma. This study suggests emplacement occurred during a short-lived collision with slab failure (breakoff) after latest stage (cessation) of Proto-Tethys oceanic subduction along the northern side of the southeastern Qilian Orogen. The heat supplied by the asthenospheric upwelling through a gap in the subducted slab induced melting in lithospheric mantle and deep crust producing Renda appinite suite rocks and granite porphyry.

The gallium isotopic composition of the Moon

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The δ71Ga values of these samples range from -0.10 to +0.66‰ (where δ71Ga is the relative difference between the 71Ga/69Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than δ71Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk δ71Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have δ71Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher δ71Ga values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution.Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Δ71Gaplagioclase-melt of -0.3‰, (where Δ71Gaplagioclase-melt is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have δ71Ga values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP.We estimate that the δ71Ga value of the LMO was ∼0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this δ71Ga value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 ± 0.06‰, Kato et al., 2017) and volatile loss drove the LMO to higher δ71Ga values prior to formation of the lunar crust.

Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535

The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. Reconciling the paradoxical geochemical features of this suite constitutes a feasibility test of lunar differentiation models. Here we present the results of a microanalytical examination of the archetypal specimen, troctolite 76535, previously thought to have cooled slowly from a large magma body. We report a degree of intra-crystalline compositional heterogeneity (phosphorus in olivine and sodium in plagioclase) fundamentally inconsistent with prolonged residence at high temperature. Diffusion chronometry shows these heterogeneities could not have survived magmatic temperatures for >~20 My, i.e., far less than the previous estimated cooling duration of >100 My. Quantitative modeling provides a constraint on the thermal history of the lower lunar crust, and the textural evidence of dissolution and reprecipitation in olivine grains supports reactive melt infiltration as the mechanism by which the magnesian suite formed.

How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

AbstractWe document the mechanical and geochemical processes of fault rock development in the shallow San Andreas fault (Mojave segment), and quantify their importance in shaping the mineralogy, grain size, fabric, and frictional characteristics of gouge. Through a combination of field and laboratory analysis of an extensive suite of shallow (<150 m) drill cores, we show that fault rocks evolved from a granodiorite protolith via three main processes: distributed microfracturing/pulverization; cataclastic flow and incipient fabric development; and production of authigenic illite/smectite during fluid‐rock interaction. The interdependence of these mechanical and geochemical processes results in a diverse suite of fault rocks, and causes significant changes in frictional strength. Spatial variations in the effects of these mechanisms, as manifested in fault‐rock mineralogy and geochemistry, indicate marked variations in their relative contribution to fault‐rock evolution. These data reveal a complex San Andreas fault with multiple principal slip zones and damaged and altered rock hosting numerous interconnected secondary slip surfaces. The resulting picture of the San Andreas Fault zone suggests a substantial departure from the simple structures envisioned for near‐surface seismogenenic faults in numerical models is required, and may inform future efforts to forecast peak ground accelerations during southern California earthquakes.

Petrographic Characteristics and Geochemistry of Volcanic Rocks in the Kyaukmyet Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect lies approximately 5 km ENE of the highsulfidation Kyisintaung copper-gold deposit, Monywa district, central Myanmar. Geologically, the research area is remarked by magmatic extrusion that occurred during the Late Oligocene to Middle Miocene of Magyigon Formation which led to the outcrops of volcanic rocks. Study detailed on petrographical and geochemical of the Kyaukmyet volcanic rocks has not been performed before the present work. The principal aim of this paper is to document the petrographical and geochemical characteristics of volcanic suite rocks exposed in the Kyaukmyet prospect. The results of this data have provided insight into the origin of the rocks and petrogenetic processes during evolution. Petrographically, all the studied volcanic rocks in the research area show that trachytic and porphyritic textures with phenocrysts of quartz, plagioclase, and K-feldspar which are embedded in a fine to medium grained groundmass. The accessory minerals of this rock consist of biotite, chlorite and opaque mineral.Geochemically, these volcanic rocks having calc-alkaline nature and classified as volcanic field (rhyolite) as well as volcanic arc setting. Based on the chondrite normalized spider diagram, LREE has enriched to HREE in this area which indicated negative Eu anomaly and subduction tectonic setting.

Mantle wedge enrichment beneath southern Tibet during the late stage (100–45 Ma) of oceanic subduction: Geochemical constraints from mantle-derived intrusions

The Miocene K-rich mafic rocks in the Lhasa Terrane of the southern Tibetan Plateau are considered to be related to either the subduction of the Neo-Tethyan oceanic slab or Indian continental slab. In order to investigate how subduction of the Neo-Tethyan oceanic slab has geochemically affected the mantle beneath southern Tibet, we obtained geochronological and geochemical data for the late Early Cretaceous Cangguo gabbros (97 Ma) and early Eocene Langka diorites (51 Ma) in southern Tibet. Although both the Cangguo gabbros and Langka diorites are characterized by arc-like trace element patterns, with enrichment in light rare earth and large-ion lithophile elements (e.g., Ba, Pb, and U), depletion in high-field-strength elements (e.g., Nb and Ta), and depleted Sr–Nd–Hf isotopic compositions, there are some other major geochemical differences (e.g., Ba/La and Th/La ratios) between these two suites of rocks. These geochemical characteristics of the Cangguo gabbros and Langka diorites indicate they were produced by melting of the depleted mid-ocean ridge mantle (DMM) that was enriched by oceanic sediment-derived fluids and melts, respectively. Our data and previous studies suggest that dehydration and melting of subducted Neo-Tethyan oceanic sediments beneath the Lhasa Terrane occurred in the late stage (100–45 Ma) of Neo-Tethyan oceanic slab subduction, which may have enriched the mantle beneath the Lhasa Terrane. However, the mantle beneath the Lhasa Terrane that was enriched by the subduction of oceanic slab during 100–45 Ma is still isotopically more depleted than the source of Miocene post-collisional K-rich mafic rocks in the Lhasa Terrane. This indicates that the enriched component in the mantle source of the K-rich mafic rocks cannot be simply attributed to materials derived from the Neo-Tethyan oceanic slab, and other additional materials are required.