Biotite Zone Research Articles

The Snow Lake Deposits in Manitoba, Canada: Formation of Metamorphosed Amphibolite Facies Orogenic Gold Deposits During a Progressive and Prograde Orogenic Event

Abstract The Snow Lake camp is located in the ca. 1.89 Ga Flin Flon-Glennie Complex in the Paleoproterozoic Trans-Hudson orogen, Manitoba, Canada. The Flin Flon-Glennie Complex is bordered by the ca. 1.855 to 1.84 Ga metasedimentary Kisseynew domain to the north and by the Archean Superior craton to the east and is underlain by the Archean Sask microcraton. It hosts several orogenic gold deposits, including the No. 3 zone, Boundary zone, and New Britannia deposit (Toots, Dick, Hogg, Ruttan, and Mine East zones), which formed during thrusting of the Flin Flon-Glennie Complex and Kisseynew domain over the Sask microcraton. The New Britannia deposit produced 1.6 Moz Au and is the largest orogenic gold deposit in the Trans-Hudson orogen in Manitoba. The deposits consist of quartz ± carbonate veins surrounded by alteration zones of biotite, hornblende, plagioclase ± carbonate, diopside, orthoclase, and garnet. The veins and ore zones are folded within the hinge of the synthrusting Nor-Acme anticline, which has an axial plane cleavage defined by biotite and hornblende, but they also cut across the hinge of the anticline and contain foliated fragments of the wall rocks. These mutually overprinting relationships suggest that the veins and ore zones are synfolding. Garnet, diopside, and amphibole porphyroblasts grew during folding because they overgrow the foliation, which also wraps around them. Inclusions of gold and sulfide minerals within the porphyroblasts indicate that mineralization was emplaced early during folding at greenschist or lower amphibolite metamorphic conditions prior to the growth of the porphyroblasts at peak middle amphibolite metamorphic conditions. These observations are corroborated by laser ablation-inductively coupled plasma-mass spectrometry element maps of arsenopyrite grains. These maps reveal a primary internal enrichment of gold in arsenopyrite grains from the Toots, No. 3, and Boundary zones and a lack of primary lattice-bound gold in arsenopyrite grains of the Dick, Ruttan, and Mine East zones. The latter results from the deposition or remobilization of gold during a second hydrothermal event that occurred during shearing of these ore zones along a structure, the Howe Sound fault, which acted as a detachment surface during folding. The Snow Lake deposits are examples of orogenic gold deposits that formed early during a major thrusting and folding event and were later modified and metamorphosed at middle amphibolite facies conditions during the same progressive orogenic event.

Water-Rock Interactions in the Metamorphic Complex of Geothermal Field: Case Study in Wahlua Complex, Buru Island

In Indonesia, surface thermal manifestation is commonly found in a volcanic geological setting. However, hot springs and alteration minerals in study area occur on metamorphic rock. In order to study its correlation with the emergence of the thermal fluid, the objective of this study is to characterize each metamorphic zone based on rock chemistry and the appearance of index minerals. Consequently, the correlation with thermal fluid chemistry can be observed. Data for this study are surface rock samples and thermal manifestation data. Metamorphic zones were determined by integrating geomorphology, petrography, and X-Ray Fluorescence (XRF) analyses. Three metamorphic zones were identified, namely, Garnet Zone, Biotite Zone, and Chlorite Zone. Garnet Zone is characterized by high concentrations of CaO and Al2O3 due to the presence of garnet. The abundance of biotite and muscovite in Biotite Zone may contribute to the highest concentration of K2O than the other two zones. Meanwhile, Chlorite Zone is characterized by the concentrations of SiO2, Fe2O3, and MgO. However, the composition of MgO might be influenced by mineral-fluid reaction. There are two clusters of thermal manifestation, which are western and eastern cluster manifestations. Western cluster manifestations are in the Biotite Zone. Meanwhile, the eastern cluster manifestation is in the Chlorite Zone. However, both clusters’ fluid chemistry shows significant differences in some elements such as SiO2, Na, K, Fe, and Mg concentration. It might suggest that the enrichment and depletion of these constituents in thermal fluid might correlate with each metamorphic zone’s characteristics. An inconsistent pattern between thermal fluid composition and rock chemistry was found in Mg concentration. It is most likely that the Mg concentration was influenced by groundwater dilution rather than the mineral-fluid interaction. The study concludes that the appearance of index minerals shows consistencies with composition variation of some elements in the manifestation fluid, such as SiO2, Na, K, and Fe.

Structure, metamorphism, and mica40Ar/39Ar thermochronology of the southern Purcell anticlinorium and its transition into the central Kootenay arc, Omineca belt, southeastern British Columbia

The Purcell anticlinorium (PA) and the Kootenay arc (KA) are two adjacent Mesozoic tectonic domains in the southeastern Omineca belt of the Canadian Cordillera. Mesoproterozoic rocks of the Belt-Purcell Supergroup (BPSG) core the PA and have open, upward-facing structures. Deeper levels of the BPSG (Aldridge Formation) preserve a Mesoproterozoic phyllitic cleavage (S1) and biotite zone mineral assemblages. The KA, on the west flank of the PA, has a younger Neoproterozoic through Paleozoic stratigraphy. The central part of the KA, the focus of this study, experienced Jurassic recumbent folding and greenschist facies metamorphism that was overprinted by Early Cretaceous ductile deformation (S2schistosity) and Barrovian metamorphism. The interface between the PA and KA is taken as the easternmost occurrence of the S2schistosity. K–Ar and40Ar/39Ar mica dates in the PA range from 1318 to 240 Ma and show no consistent spatial distribution. They are interpreted to reflect partial resetting of Ar following initial cooling in the Mesoproterozoic, indicating that the PA was never sufficiently buried or heated during younger tectonism to uniformly reset the mica cooling ages. In the KA,40Ar/39Ar dates range from 139 to 55 Ma and reflect previously documented periods of Mesozoic and Cenozoic metamorphism, magmatism, and exhumation. The PA–KA interface occurs above a west-dipping ramp in the Laurentian basement. The thick Mesoproterozoic sequence of turbidites and sills in the PA may have acted as a rigid buttress against which the more ductile rocks of the central KA “piled up” during Early Cretaceous compressional orogenesis.

Contribution of deformation and reaction of amphiboles to the weakening of the middle continental crust: A case study from sheared diorites along the Shuiyu shear zone in Northern Beijing

How the deformation of amphiboles and their involvement in metamorphic reactions during shearing affect the rheology of the middle crust remains controversial. Middle crustal fault rocks are exposed along the Shuiyu shear zone (SYSZ) in Yunmeng Shan, Northern Beijing, which was formed by the exhumation of the Yunmeng Shan metamorphic core complex during the Early Cretaceous tectonic extension of the North China Craton. Various deformation structures, optical microstructures, dislocation submicrostructures and fabrics during the transition from brittle to ductile deformation in the middle crust are observed in the sheared diorites. In particularly, two types of amphiboles, i.e., the A-type with brown cores and green mantles as porphyroclasts and the B-type of green-colored fine grains in the matrix, distributed biotite grains and interconnected fine biotite zones, are observed in the deformed diorites. Geothermometer analysis reveals that the amphibole grains in the sheared diorites were crystallized at temperatures over c. 720 °C, deformed at c. 680 °C–600 °C and transformed into biotite at c. 600 °C–520 °C. During these processes, fluid not only enhanced crystal plasticity and governed the deformation of the amphibole grains, but was also involved in their metamorphic transformation into biotite. Our results show that the diorites along the Shuiyu shear zone experienced three stages of middle crustal shearing and weakening, i.e., an early stage of fluid-enhanced crystal plasticity followed by rigid body rotation, a middle stage of transformation from a strong phase (amphibole) to a weak phase (fine-grained biotite and quartz) and a late stage of weak phase interconnection.

Characterization of the inverted metamorphic gradient of the Passos Nappe (SE-Brazil) based on multiple geothermobarometers

Inverted metamorphic gradients are recognized in several Proterozoic to Phanerozoic orogens. However, the mechanisms that are responsible for the formation of these gradients are still a matter of discussion. In the southern Brasília Orogen, an inverted metamorphic gradient is recorded in the Passos Nappe, that comprises a sequence of metasedimentary rocks with minor intercalations of tholeiitic metamafic rocks, metamorphosed under greenschist facies (biotite zone) at the bottom and high-pressure granulite facies at the top. This paper presents a detailed investigation of pressure and temperature conditions along the Passos Nappe by integrating conventional geothermobarometry in metamafic rock and single-trace elements geothermometry (Zr-in-rutile and Ti-in-quartz) in quartzite. Nine lithostratigraphic units are identified in the Passos Nappe, which are informally named A to I, from the bottom to top, adding up to a minimum thickness of 4.56 km. In mafic rocks, the increase of metamorphic conditions is indicated by: (1) chemical variation of amphiboles, from actinolite/edenitic hornblende in Unit C, to pargasitic hornblende in Units E and G, and Fe-pargasite in Units H and I; (2) plagioclase composition varying from An1-13 in Unit C, An15-27 in Unit D to andesine at upper units; and (3) occurrence of clinopyroxene from the top of Unit E upwards. Additional petrographic features in quartzite also corroborate these insights, such as: the increase of grain size of both quartz and rutile, from bottom to top; the morphology of rutile crystals; the mechanism of quartz recrystallization (subgrain rotation dominates in lower units while grain boundary migration dominates in upper units) and the occurrence of rutile needles in quartz in the upper units, which suggest a continuous gradient. Combined application of Zr-in-rutile and Ti-in-quartz geothermometers in quartzite provided the following average temperature and pressure values for each unit: 673 °C and 11.7 kbar for Unit E, 690 °C and 9.5 kbar for Unit G, 747 °C and 11.6 kbar for Unit H and 758 °C and 11.3 kbar for Unit I. Our data indicate that the Zr-in-rutile and Ti-in-quartz systems do not re-equilibrate in metamorphic conditions lower than 600 °C. The extrapolation of these results to the low-grade units indicate: 496 °C and 4.8 kbar at Unit A, 504 °C and 4.8 kbar at Unit B, 502 °C and 4.4 kbar at Unit C. The results obtained with the geothermometer hornblende-plagioclase and the geobarometer garnet-rutile-ilmenite-plagioclase-quartz in metamafic rocks yielded the following average values for each unit: 486 °C and 5.0 kbar at Unit C, 568 °C and 6.0 kbar at Unit D, 622 °C and 8.1 kbar at Unit E, 650 °C and 8.4 kbar at Unit G, 743 °C and 9.4 kbar at Unit H and 723 °C and 11.2 kbar at Unit I. Since the two geothermobarometric approaches are based on independent chemical systems, the results obtained represent robust estimates for the temperature and pressure conditions for the inverted metamorphic gradient of the Passos Nappe. Comparing the inverted metamorphic gradient at the Passos Nappe with classical occurrences at the Himalaya Orogen, it is possible to assume that both origins were similar, which probably involved isotherm inversion due to subduction, channel flow process associated with shear heating and transport of higher pressure rocks over low-pressure rocks by reverse ductile shearing.

Polyphase Archean to Paleoproterozoic deformation along the northwest margin of the Johannesburg Dome, Kaapvaal Craton

Despite extensive gold exploration, the reasons for the dome shape of the Paleoproterozoic Johannesburg Dome (JD) remain elusive. We investigated the structural record of an outlier of Archean to Paleoproterozoic supracrustal rocks (the Zwartkops Hills Outlier, ZHO) overlying the JD to investigate its tectonic evolution. Steep gneissic fabrics in the basement pre-date the intrusion of ∼ 3225 Ma trondhjemite (new U–Pb zircon age). Two Archean deformation events affected West Rand Group rocks and pre-date Platberg Group deposition. To the south of the ZHO, a newly recognized, large-scale, recumbent fold formed during top-to-the-south shearing. In the central part of the ZHO, steeply W-dipping stratification results from E-W shortening. Two more deformation events are Paleoproterozoic in age and affected all units. A shallowly S-SW-dipping cleavage formed in the biotite zone of the greenschist facies is associated with N–S shortening and top-to-the-north shearing. The cleavage transects the large recumbent fold and is observed down to the upper 30 m of the basement. Finally, a weak N–S shortening event is inferred from kink folds and north-dipping axial planar cleavage. These data are consistent with the existing regional models of (i) south- and east-directed fold-and-thrust belts formed during sedimentation of the Archean Central Rand Group, (ii) north-verging schist belt formed during, or after the Transvaal Supergroup deposition, and (iii) impingement of the Bushveld Complex onto the JD during N–S shortening. These findings are integrated with previous regional sedimentological, seismic, and structural studies to suggest that the JD granitoid basement formed a paleo-high during the deposition of the Central Rand and Platberg groups. Once buried in the upper crust, the paleo-high acted as a rigid granitoid promontory that indented weak surrounding cover rocks during Paleoproterozoic N–S contraction events, creating the dome shape of the Johannesburg Dome.

Metamorphism of the Buchan type-area, NE Scotland and its relation to the adjacent Barrovian domain

The metamorphism of the Buchan block in northeast Scotland, and its relation to the Barrovian domain to its west, have been reassessed from consideration of mineral assemblages, microstructures, phase equilibrium modelling and monazite U–Pb geochronology. Zones of increasing metamorphic grade surround a central low-grade domain (biotite zone) and define a northward-opening, U-shaped metamorphic map pattern. The eastern and southern parts of the Buchan domain show the classic Buchan-type prograde sequence of cordierite – andalusite – sillimanite – migmatite/gneiss, representing a metamorphic field gradient of gently increasing pressure between (2.5–3.0 kbar, c. 550°C) and (3.5–4.0 kbar, c. 750°C). A slightly lower pressure metamorphic field gradient (by less than c. 0.5 kbar) is interpreted for the northeastern Buchan domain. The west Buchan domain shows a prograde sequence of staurolite+andalusite – sillimanite – gneiss/migmatite, representing a metamorphic field gradient of higher pressure by c. 1 kbar or less than the classic sequence over a similar temperature range. Unique to the northwestern Buchan domain is a staurolite+cordierite+andalusite domain in which staurolite-bearing schists have been overprinted by cordierite + andalusite assemblages. Monazite U–Pb geochronology of schists and gneisses from the Buchan block, including the Cowhythe and Ellon gneisses, yields ages in the range 470 ± 5 Ma, supporting geological evidence that the gneisses are metamorphosed Dalradian strata rather than older basement gneisses. The metamorphic ages are similar to the ages of mafic igneous intrusions in the Buchan block, even though many of the exposed intrusions post-date the regional metamorphic zones. The Buchan metamorphic zones are truncated to the west by the Portsoy–Duchray Hill Lineament, a ductile shear zone that juxtaposes the Buchan rocks against higher pressure, lower temperature (kyanite-bearing) Barrovian schists to the west ( c . 5.5–7.0 kbar, c. 580–650 °C). A 2–15 km wide corridor of andalusite pseudomorphed by kyanite occurs between the Portsoy–Duchray Hill Lineament and the Keith shear zone to its west. Monazite geochronology of the Barrovian rocks west of Portsoy shows little evidence of the c. 470 Ma signature of the Buchan block, instead yielding a dominant cluster of ages at c. 450 Ma and a more poorly defined grouping at c. 500 Ma. Supplementary materials: Analytical procedures; Thermodynamic modelling; Tables S1 to S13 (mineral assemblages, whole rock compositions, mineral compositions, isotopic analyses); Figures S1 to S5 (supplementary mineral assemblage map, graphs of mineral composition vs. grade, supplementary thermodynamically calculated phase diagrams, back-scattered electron (BSE) images of monazite grains, concordia diagrams for monazite U–Pb results) are available at https://doi.org/10.6084/m9.figshare.c.5536745

Spinifex‐like textured metaperidotites from the Higo Metamorphic Rocks, Japan, a possible high‐pressure dehydration product of antigorite serpentinite

AbstractSpinifex‐like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west‐central Kyushu, Japan, may be formed by high‐pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium‐grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low‐grade metamorphic rocks of the biotite zone, Type I (H) of coarse‐grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high‐grade metamorphic rocks of the garnet‐cordierite zone, and Type II composed of Al‐spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet‐cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite‐olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel‐olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H2O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex‐like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex‐like textured metaperidotites have derived.

Tectonic domains and exhumation history of the Omineca Belt in southeastern British Columbia from 40Ar/39Ar thermochronology

A large geochronological data set comprising 40Ar/39Ar and K–Ar (hornblende, muscovite, biotite, and K-feldspar), Rb–Sr (muscovite), fission track (zircon and apatite) and U–Pb (zircon and monazite) dates has been compiled for the southern Kootenay Arc and western Purcell anticlinorium in the Omineca Belt of the Canadian Cordillera in southeastern British Columbia. New 40Ar/39Ar data for hornblende, muscovite, biotite, and alkali feldspar are presented and combined with data from other studies. We integrate these data with recent advances in the geology of the region to define three partially fault-bounded domains with differing geological and exhumation histories, here termed the western, central, and eastern domains. The western domain is characterized by (1) late synkinematic Jurassic plutons with hornblende, muscovite, and biotite 40Ar/39Ar plateau dates between 170 and 165 Ma, some of which are within error of the U–Pb zircon dates for these plutons, and (2) late Early Cretaceous (118–102 Ma) plutons commonly with concordant mica 40Ar/39Ar plateau dates of a similar age range, indicating rapid cooling following emplacement of both suites. The central domain is bounded by regional-scale normal faults (Gallagher and Midge Creek faults, Blazed Creek/Next Creek faults, and Purcell Trench fault) and contains superposed Early and Late Cretaceous zones of Barrovian metamorphic rocks and several mid- to Late Cretaceous, post-kinematic plutons. The transition from the western domain into the central domain is characterized by 40Ar/39Ar mica age spectra showing a progression of increasing thermal overprinting. Along the north–south length of the central domain, biotite and muscovite yield Paleocene to Eocene K–Ar and 40Ar/39Ar plateau dates between 66 and 40 Ma. The eastern domain consists of (1) a southern portion that occurs in the hanging wall of the Purcell Trench fault, comprising mid-Cretaceous intrusions of the Bayonne magmatic suite emplaced into biotite zone metasedimentary rocks of the Mesoproterozoic Belt-Purcell Supergroup in the western Purcell anticlinorium, and (2) a northern portion that shows a continuous transition with the northern part of the central domain north of the terminus of the Purcell Trench fault. Cretaceous igneous rocks in the southern portion of the eastern and western domains have 40Ar/39Ar mica plateau dates that are <9 Myr younger than U–Pb zircon dates, indicating rapid cooling shortly after emplacement. 40Ar/39Ar step-heating reveals that there was a mid- to Late Cretaceous thermal disturbance in the eastern domain, possibly related to emplacement of younger plutons at deeper crustal levels and the Late Cretaceous Barrovian metamorphic event recorded in rocks of the central domain, such that biotite with dates <ca. 73 Ma yield plateau age spectra but those with older dates are disturbed. The new geochronology, combined with recent mapping and metamorphic studies, leads to the conclusion that the exhumation of the Barrovian metamorphic rocks of the central domain was a multi-stage process. The central domain experienced rapid tectonic decompression and minor pluton emplacement in the Late Cretaceous to early Paleocene (76–61 Ma) when the Cordilleran orogen was under regional contraction during which most of the exhumation occurred. Final exhumation in the footwall of Eocene normal faults was less significant and occurred between 53 and ca. 46 Ma when the Cordilleran orogen had transitioned to regional extension, by which time the three domains had attained a similar crustal level. These episodes of exhumation are similar to those found in other core complexes in the southern Canadian Cordillera and contiguous northern Idaho and Washington. The earlier episode is coincident with regional-scale, Late Cretaceous thrust faulting in the Foreland Belt of the Rocky Mountains. Eocene normal faulting and final exhumation of core complexes in the Omineca Belt mark the end of contraction in the Foreland Belt.

The Facies and Metamorphism Types Determination of Metamorphic Rock in The Part of Mekongga Complex

Lambolemo Village is geomorphologically dominated by hilly slopes morphology. Lithology in the form of metamorphic rocks are mica schists and chlorite schists. The present research aimed to determine the facies and metamorphism types of metamorphic rocks in Lambolemo Village. The research methods engaged were field observation and laboratory observation utilizing petrographic analysis. Observations stage generated texture data and set of mineral both macroscopically and microscopically. The minerals were discovered within metamorphic rock of the research area, namely: muscovite, quartz, biotite, and chlorite. These findings then classified into two zones, there were chlorite zone and biotite zone. These zones were embodied into greenschist facies with chlorite mineral index. The greenschist facies was characterized with chlorite, albit, epidote, and actinolite minerals appearance. The metamorphic rock had a foliation structure. The structure was commonly generated by a regional metamorphic process, which was particularly a feature of the clear alignment, translucent minerals from flat minerals, such as : mica, chlorite and minerals that obtained fibers or scents.
 Keywords: Determination; Facies; Metamorfism Type
 
 Copyright (c) 2020 Geosfera Indonesia Journal and Department of Geography Education, University of Jember
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Geotectonic significance of the Neoproterozoic ophiolitic metagabbros of Muiswirab area, South Eastern Desert, Egypt: constraints from their mineralogical and geochemical characteristics

Petrological and geochemical studies of Neoproterozoic metagabbros were carried out in the Muiswirab area, South Eastern Desert of Egypt. The Muiswirab area comprises of ophiolitic metagabbroic rocks (MOM), which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn- and post-tectonic granitoid rocks. The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros. The geothermobarometric investigation of the analyzed amphiboles from (MOM) revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies (biotite zone) conditions (at a temperature of 450 to 500 °C and pressure of 1–3 kbar). Geochemically, the metagabbros (MOM) show tholeiitic affinity and exhibiting both arc- and MORB- like characters as evidenced by their clinopyroxene compositions and the Ti/V ratios (11.84–31.65), which considered as prominent features of forearc tectonic regime. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE (e.g., Rb, Ba, Sr, Pb) over HFSE (e.g., Ti, Nb, Y, Zr, Hf, Ta). The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB-like components. The ratios of Zn/Fet, La/Sm, Sm/Yb, Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5–30 % partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.

Syn-metamorphic B-bearing fluid infiltrations deduced from tourmaline in the Main Central Thrust zone, Eastern Nepal Himalayas

Mode of occurrence and chemical composition of tourmaline in pelitic schists from the Main Central Thrust (MCT) zone of the Lesser Himalayan Sequences (LHS) are described in detail with the aim of deducing the chemical characteristics of tourmaline formed through B-bearing fluid infiltration and of estimating the composition of the syn-metamorphic fluids. Metasomatic tourmalines from the tourmalinized wall rocks show significant increases in XCa [=Ca/(Ca + Na)] at almost constant XMg [=Mg/(Mg + Fe2+)] from the cores or mantles to the rims. Tourmaline in tourmaline-rich (> 1.0 vol%) pelitic schists from the biotite zone to the kyanite zone also show marked increase in XCa at almost constant XMg, and are interpreted as a product of B-bearing fluid infiltration. Abundant margarite and anorthite formed in the pelitic schists intercalated with the metadolostone layers suggesting that the B-rich fluid became Ca enriched as it interacted with metadolostone layers, and metasomatically introduced Ca into the pelitic schists. Infiltration of such B- and Ca-rich fluids into pelitic schists likely resulted in production of abundant tourmaline with the compositional trend of increasing XCa at almost constant XMg. Most of the tourmaline in tourmaline-rich pelitic schists are in equilibrium with plagioclase, suggesting that the fluid composition was buffered by the pelitic schists. Composition of tourmaline in the pelitic schists with <1 vol% tourmaline mostly show increase in XCa and wider range of XMg values, reflecting its growth during prograde metamorphism. The composition of fluids that coexisted with tourmaline is estimated by applying experimentally-determined fluid/tourmaline chemical relationships to the composition of tourmaline from veins associated with tourmalinization, pelitic schists with <1 vol% tourmaline and tourmaline-rich pelitic schists. Assuming coexisting anion to be Cl, the salinity estimated for these rock types was ∼0.44–0.59 mol/l NaCl + CaCl2 that is similar to or slightly lower than the present-day seawater. Veins associated with tourmalinization and tourmaline-rich pelitic schists are sporadically distributed in the MCT zone of the LHS, suggesting that the syn-metamorphic, B-bearing saline fluid infiltrations took place widely in the MCT zone, and the fluid pathways were localized and channeled. Our observation supports the scenario whereby infiltration of fluid into the High Himalayan Crystallines (HHC) caused vapor-saturated partial melting of the HHC to give tourmaline leucogranite melts contemporaneous with inverted metamorphism in the MCT zone.

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

AbstractThe present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm−1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω1 &gt; 8.5 cm−1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.

Metamorphism of Jhyallaphay-Barpak-Bhachchek area of Gorkha District, Lesser Himalaya and Higher Himalaya, Central Nepal

The study is focused on geological mapping, petrography and metamorphism of Jhyallaphat–Barpak–Bhachchek area, a part of Gorkha District, Central Nepal using a base of 1:25000 scale covering an area of 139.80 sq. km. The rocks of the study area can be broadly divided into two tectonic zones; the Lesser Himalaya consisting of fie lithological units, and the Higher Himalaya consisting of Formation I of the Tibetan Slab. Three metamorphic zones can be distinguished in the study area; biotite zone, garnet zone and kyanite zone. The biotite zone of the mineral assemblage in pelitic rocks consists of biotite+muscovite+chlorite+quartz, in psammitic rocks comprises of biotite+muscovite+chlorite+feldspar+quartz and in carbonate rocks comprises of biotite+muscovite+calcite/dolomite+feldspar+quartz, respectively. These mineral assemblages show that the area belongs to the greenschist facies. The mineral assemblage of the garnet zone in pelitic rocks constitutes garnet+biotite+muscovite+chlorite+quartz, and in psammitic rocks constitutes of garnet+biotite+muscovite+feldspar+quartz. The minerals assemblages found within the biotite and garnet zones represent the well-known inverted metamorphism in the Lesser Himalaya. Mineral assemblage of the kyanite zones constitutes of kyanite+garnet+biotite+muscovite+feldspar+quartz. The mineral assemblages of the both garnet and kyanite zones show that the area belongs to the epidote amphibolite facies.&#x0D; The bedding and foliation planes are almost parallel, showing that isograds also cut across the foliation. Therefore, the main metamorphic event should have followed development of foliation in the area. The rocks of the area show at least two metamorphic events: syntectonic prograde and post-tectonic retrograde. Syn-tectonic prograde metamorphism (M1), which has grown during a single phase of deformation and most frequently encountered garnet prophyroblast. Metamorphic deformation is represented by the presence of metamorphic foliation, stretching lineation, and S-C fabric. Post-tectonic retrograde metamorphism (M2), which is followed by retrograde mineral formation changing its P-T condition from high to low grade minerals, such as the formation of the biotite and chlorite minerals around the rims of the garnet porphyroblasts.

Provenance of the Ediacaran Salinas Formation (Araçuaí Orogen, Brazil): Clues from lithochemical data and zircon U-Pb (SHRIMP) ages of volcanic clasts

Abstract Salinas Formation occurs in a large region of the Northern Aracuai orogen, Southeastern Brazil. It includes turbiditic wackes (> 10% matrix) to arenites (< 10% matrix), pelites and clast-supported conglomerates, metamorphosed from the biotite zone of the greenschist facies to the sillimanite zone of the amphibolite facies. Salinas Formation lies unconformably on the top of or in tectonic contact with the Macaubas Group, and hosts Cambrian granitic intrusions dated between 540 and 500 Ma. Aiming to unravel sediment provenances for the Salinas basin, we present a detailed lithochemical (45 samples) study on low-grade rocks preserved from the regional deformation, which are found in the type area of the Salinas Formation. In addition, we compare them with deformed and more metamorphic rocks of similar composition but located in other basin sectors. The lithochemical data indicate limited chemical weathering in the sediment sources, good correlations with the mineralogical compositions in respect to the variable amounts of metamorphic minerals typical of pelitic (micas, garnet, and other peraluminous silicates) and psammitic (feldspars, quartz) fractions. The main provenances of sedimentary protoliths are clearly related to continental magmatic arc and active continental margin environments. U-Pb (SHRIMP) analyses performed on zircon grains from clasts of intermediate to felsic volcanic rocks, extracted from a clast-supported metaconglomerate, yield concordant zircon Pb206 /U238 ages from ca. 579 Ma to ca. 697 Ma, with most of the ages in the interval of 587 to 630 Ma. This indicates that the main primary sediment source is the Rio Doce magmatic arc (630-580 Ma), in very good agreement with the arc-related lithochemical signature. Eleven zircon crystals yield a Concordia age of 620 ± 10 Ma, representing an important Ediacaran volcanic episode in the tectonic evolution of the Aracuai orogen.

К истории минералогических исследований в Забайкалье

The Olimpiada gold-sulfide deposit in the Yenisei Ridge, as per the results of additional exploration of the recent years, has confirmed its uniqueness in terms of reserves (1560 t) and ore extent in the depth (1500 m). Meanwhile, no indicators of thinning out and change in mineralization parameters have been noted. Oxidized ores mined out to date were developed in the upper part of the deposit. About 200 t of metal have been mined from them. The deposit is confined to the silicate and carbonate band of the Lower Riphean Kordinskaya suite clastic stratum. Granitoids separated from the deposit at 1.5 km and above surround the deposit. Introduction of intrusions in the Tatar-Ishimbinskaya tectonic zone assisted in the formation of compensation synformal depressions (Innokentyevskaya and Chirimbinskaya synclines) in the contact zones of intrusions united by the antiformal rock unit (Medvezhinskaya anticline). These connected W-shaped structural elements represent the structure of the ore field. Subhorizontal shifts in the area shaped the magmatogene structure into tectonic syncline and anticline folds. The ore bodies are concentrated in fold curves and rock bends at limbs. The northeastern curve of the Medvezhinskaya anticline has the highest ore content. There, the deposits of the northern and southeastern limbs of the Medvezhinskaya antiform are connected and represent a single ore body sloping to the southeast. No indicators of thinning out at the depth have been revealed. The micaceous-quartz-carbonate matrix of the sulfide-disseminated ores has been boudinaged, milonite-altered and folded into minor folds up to plication. The main ore forming elements, including Au, Ag, Fe and Sb, form dissemination of native minerals, sulfides, sulfosalts and tellurides. In terms of the mineralogical and geochemical composition the ores are divided into gold-arsenic and gold-arsenic-antimony ores separated spatially. The gold-arsenic-antimony ores with the geochemical mercury and tellurium impurity are confined to the northeastern deposit of the ore body. Metamorphism in the tectonic-metamorphic zone of the deposit occurred at the temperature of 400–420оС and the pressure of 3–4 kbar in chlorite and biotite zones, and in the garnet and margarite zone — at 580-605оС and the pressure of 7.2–7.5 kbar. Metasomatosis of silicate and carbonate rocks with skarnoid formation (Act+CZo+Gar+Sph) occurred at 320–480оС and the pressure of 1.3–1.5 kbar, and of acidic metasomatites (silicified and sericitized rocks) at Т=290–380оС and Р=1.0–3.0 kbar. The polystage hydrothermal quartz-gold-sulfide process of aggregate formation occurred in the interval of 460–110оС. The age of the main stages of ore formation is within 817–660 Ma. The industrial ores of gold-arsenic composition have the age of 758–803 Ma, and the gold-arsenic-antimony ores are 660–795 Ma old. The isotope and geochemical studies testify to the participation of the crust and mantle substance in the deposit formation. Aquatic-chloride-carbon dioxide solutions saturated with hydrocarbons participated in the ore substance transportation.

Neoproterozoic deposition and Triassic metamorphism of metasedimentary rocks in the Nam Co Complex, Song Ma Suture Zone, NW Vietnam

The depositional and metamorphic ages, and provenances of metasedimentary rocks from the Nam Co Complex in the Son La area, Song Ma Suture Zone, northwestern Vietnam, were investigated in order to reveal the convergent tectonic history between the Indochina and South China cratons. This complex is structurally characterized by an antiformal metamorphic core consisting of two distinct metamorphic zones: inner garnet and outer biotite zones. The U-Pb isotopic dating on detrital zircon from metasandstones of the Nam Co Complex yielded major population (> 50%) around ~850 Ma with minor population scattered between 1.2 to 3.0 Ga. The age spectra as well as the morphologies, internal structures and geochemical features of detrital zircon are accounted for by magmatic activities prevalent at the convergent plate margin during the Neoproterozoic. In contrast, the UTh- Pb isotopic compositions of monazite from a garnet-zone schist defined a common Pb mixing line with the lower intercept age of 234 ± 10 Ma (2σ) reflecting the Indosinian metamorphic event. The U-Pb age distribution pattern of detrital zircon in the Nam Co Complex is compatible with that of the South China craton rather than that of the Indochina craton. In summary, sedimentary protoliths of the Nam Co Complex were deposited in a convergent-related basin along the southwestern margin of the South China craton during the Neoproterozoic, and then was finally amalgamated to the Indochina craton via the southerly subduction of South China craton during the Indosinian orogeny.

Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA

A comprehensive understanding of the degassing systematics of organic carbon (OC) during regional metamorphism is necessary to evaluate the role that metamorphism plays in the global carbon cycle. In this study, weight percentages and δ13C values of OC were measured in 70 samples of metapelites from the Wepawaug Schist, Connecticut, where classic Barrovian metamorphism occurred and graphitic OC is widespread. Relative to low-grade chlorite + biotite zone rocks, our mass balance analysis shows that OC in the metapelites underwent progressive loss from −14% (−0.06 g OC per 100 g rock) in the garnet zone, through −21% (−0.09 g/100 g) in the staurolite zone, to −26% (−0.11 g/100 g) in the kyanite zone. The average δ13C values in different metamorphic zones (ranging from −14.74‰ to −16.24‰) are all much higher than normal organic material in marine sediments, and increase slightly from the chlorite + biotite zone to the garnet zone and decrease slightly at higher metamorphic grades. Organic carbon degassing in the form of CH4 during the late stage of diagenesis or in the earliest stages of metamorphism could produce this significant 13C enrichment. Under the assumption that the 13C enrichment is caused by graphite degassing during the lowest-grade metamorphism (chlorite zone or lower), the degassing profile of OC during the regional metamorphism is reconstructed by combining the δ13C and OC mass change data. The computed results indicate that graphitic OC in the Wepawaug Schist probably underwent considerable loss at lowest-grade metamorphic conditions, ranging from ~−40% to ~−90% (or from −0.23 g OC per 100 g rock to −2.8 g OC per 100 g rock), and remained relatively inert at higher grades. Based on the mass balance analysis, δ13C systematics, and exploratory modeling results, this study argues that the lowest-grade or pre-metamorphic stages would be the more efficient OC liberators, and that the degassing potential of OC in the major stages of Barrovian metamorphism appears to be much more restricted. Additional independent studies are required to decipher the early degassing of OC after the deposition of organic matter, which could in turn help better constrain the degassing of OC during regional metamorphism.

The Pico do Itapeva Formation: A record of gravitational flow deposits in an Ediacaran intracontinental basin, southern Brasília Orogen, SE Brazil

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasília Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern São Paulo State, Brazil. It is located in the interference zone of the southern Brasília and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupé Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening- and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI- arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII- arkose, pebbly arkose and scarce lutite and; LAIII- conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasília and Ribeira orogenies.

Halogen Chemistry of Hydrothermal Micas: a Possible Geochemical Tool in Vectoring to Ore for Porphyry Copper-Gold Deposit

Porphyry copper-gold deposit commonly exhibits an extensive alteration zone of hydrothermal micas particularly biotite and sericite. This study is aimed to analyze and utilize the chemistry of halogen fluorine and chlorine of biotite and sericite to be a possible tool in vectoring to ore for copper porphyry deposits. To achieve the objectives, several selected altered rock samples were taken crossing the Batu Hijau copper-gold mine from inner to outer of the deposit, and hydrothermal micas contained by the rocks were analyzed petrographically and chemically. Mineral chemistry was detected by electron microprobe analyzer, whilst biotite is petrographically classified as either magmatic or hydrothermal types. Sericite replacing plagioclase occurred as fine-grained mineral and predominantly associated with argillic-related alteration types. Biotites in the Batu Hijau deposit are classified as phlogopite with a relatively low mole fraction magnesium (XMg) (~0.75) compared to the “typical” copper porphyry deposit (~0.82). The relationship between the XMg and halogen contents are generally consistent with “Fe-F and Mg-Cl avoidance rules”. F content in biotite and sericite decrease systematically from inner part of the deposit which is represented by early biotite (potassic) zone where the main copper-gold hosted, to the outer part of the deposit. However, chlorine in both biotite and sericite from each of the alteration zones shows a relative similar concentration, which suggests that it is not suitable to be used in identification of the alteration zones associated with strong copper-gold mineralization. H2O content of the biotite and sericite also exhibits a systematic increase outward which may also provide a possible geochemical vector to ore for the copper porphyry deposits. This is well correlated with fluorine content of biotite in rocks and bulk concentration of copper from the corresponding rocks.