M2 Metamorphism Research Articles

Multiple P–T–d–t paths reveal the evolution of the final Nuna assembly in northeast Australia

AbstractThe final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi‐method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70–1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S‐type granites. Garnet Lu–Hf and monazite U–Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono‐metamorphic domains are distinguished: (a) the western domain, with S1 defined by low‐P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium‐P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low‐P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP–medium‐T (MT) metamorphism (M1) developed within the staurolite–garnet stability field, with conditions ranging from 530–550°C at 6–7 kbar (garnet cores) to 620–650°C at 8–9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP–high‐T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P–T conditions ranged from 600 to 680°C and 4–6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn‐ to post‐ S2, at 730–770°C and 6–8 kbar, and at 750–790°C and 6 kbar, respectively. The pressure–temperature–deformation–time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium‐P and medium‐T conditions in the central domain. This event was followed by the regional 1.56–1.54 Ga low‐P and high‐T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two‐stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.

EPMA monazite geochronology of the granulites from Daltonganj, eastern India and its correlation with the Rodinia supercontinent

We report the monazite dates of the granulites from Daltonganj (Palamau), Chhotanagpur granite–gneiss complex (CGGC) which covers the significant part of the granulite blocks in central India by using an electron micro probe analyser dating. The monazite grain varies between 70 and 80 μm and shows the distribution of U, Th and Pb in all monazite grains of both samples. Two different dates were obtained from different monazite grains; the first age suggests that the granulite from CGGC preserves the first remnant of the protolith of the Mesoproterozoic era at ~1424 Ma and second one at ~972 Ma which provides evidence of metamorphism of the protolith. The CGGC rocks preserve four regional metamorphic events, namely M1, M2, M3 and M4. But in this work, two different ages from the Daltonganj granulites were obtained which are similar to the M2 (<1500 Ma, i.e., the age of protolith of the granulitic gneiss) and M3 (1200–930 Ma) metamorphic events as reported in the CGGC. The M3 metamorphism attained its average P–T condition at ~7.35 kbar/792°C, and it represents the prograde metamorphic event. The M3 metamorphic event supported the Grenville-orogeny, and it was responsible for the metamorphism of the magmatic protolith of granulitic gneiss from the CGGC at the time of amalgamation of the Rodinia supercontinent. The Rodinia assembly had occurred through the global Grenville-orogenic events between 1100 and 900 Ma, with continental blocks which exist at that time.

Tectono‐Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana

AbstractRecent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold‐and‐thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold‐and‐thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate‐P metamorphism (M1) at circa 620 Ma and a low‐P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2‐related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right‐lateral strike‐slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.

The geochemistry and geochronology of the upper granulite facies Kliprand dome: Comparison of the southern and northern parts of the Bushmanland Domain of the Namaqua Metamorphic Province, southern Africa and clues to its evolution

The timing of tectonomagmatic events and petrogenesis of ortho- and paragneisses within the high-grade southern Bushmanland Domain (SBD) of the Namaqua Metamorphic Province is, largely, poorly constrained. U-Pb age dating and whole rock lithogeochemistry of the rocks of the upper granulite facies Kliprand dome provide constraints with regards to the evolution of the SBD during the 1.2–1.0 Ga Namaquan Orogeny.The SBD has a greater juvenile component, particularly added between ∼1.1 and 1.02 Ga, compared to the northern Bushmanland Domain (NBD). This is reflected in marginally younger TDM model ages (1.60–1.95 Ga) than for the NBD (∼1.9–2.4 Ga), and higher εNd(t) values for magmatic rocks, particularly for the late- to post-tectonic Spektakel Suite (SS), concentrated in the SBD. The ∼1.06 Ga Klein Lieslap Charnockite of the SS was derived from reworking of radiogenic Paleoproterozoic arc crust, but has a large juvenile depleted component compared to that of the ∼1.19 Ga Grootberg Gneiss of the syn-tectonic Little Namaqualand Suite.SBD mafic magmas were derived from low degrees of partial melting of subduction-influenced enriched subcontinental lithospheric mantle. The pre-tectonic ∼1.17 Ga Oorkraal Suite shows limited crustal contamination of mantle-derived melts. The post-tectonic ∼1.05 Ga Koperberg Suite was derived from deeper mantle melting than that in the NBD, but shows less crustal contamination. The supracrustal Kamiesberg Group was derived from mixed, reworked Meso- to Paleoproterozoic crust that showed a more dominant depleted component compared to that in the NBD. Incongruent anatexis of the paragneisses gave rise to crosscutting garnetiferous quartzofeldspathic segregations, notably the strongly peraluminous ∼1.07 Ga Ibequas Leucogranite.D2 deformation, at ∼1.11–1.09 Ga, resulted in penetrative foliation. D3 deformation, related to the development of a sub-vertical foliation in the limbs of F2 folds, is constrained to ∼1.07–1.06 Ga, between emplacement of the Ibequas Leucogranite and the Koperberg Suite. Peak granulite facies M3 metamorphism occurred at ∼1.05–1.02 Ga. These age constraints are similar to those of the NBD reflecting a synchronous tectonomagmatic and kinematic history. The SBD represents a deeper crustal level with a greater mantle influence than the NBD.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Two successive phases of ultrahigh temperature metamorphism in Rogaland, S. Norway: Evidence from Y‐in‐monazite thermometry

AbstractIn Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.

Polyphase structural deformation of low- to medium-grade metamorphic rocks of the Liaohe Group in the Jiao-Liao-Ji Orogenic Belt, North China Craton: Correlations with tectonic evolution

The Paleoproterozoic Jiao-Liao-Ji Orogenic Belt in the eastern North China Craton provides evidence of the evolution from the early rifting and ocean opening to the final collision of the Longgang and Nangrim blocks. However, it is still open to debate, and thus controversial, as to whether the Jiao-Liao-Ji Orogenic Belt formed by closure of a rift without subduction or from an ocean with subduction. Based on detailed field mapping and structural analysis on a micro/meso-scale of the Liaohe Group in the center of the Orogenic Belt, we define at least four phases of deformation and metamorphism. S0 is bedding formed during deposition of the lower Liaohe sediments in the Liaohe back-arc basin. During D1 deformation an early greenschist facies event (M1) was associated with a bedding-parallel S1 metamorphic foliation, penetrative axial planar foliation, micro- to meso-scale folds (F1), and an L1 lineation, all formed during accretion, probably in a trench-subduction setting. An M2 metamorphism took place during medium-pressure Barrovian-type amphibolite facies conditions and associated D2 deformation that gave rise to major thrusts, open-isoclinal folds and a well-developed crenulation cleavage during exhumation of high-pressure and high-temperature rocks and collision of the Longgang and Nangrim blocks. M3 metamorphism at sillimanite (M31) and cordierite (M32) grade was associated with D3 deformation that produced regional gentle re-folds related to post-collisional exhumation of the Orogenic Belt. An M4 retrograde metamorphism and D4 deformation gave rise to a weak foliation that can be correlated with late Cretaceous extension that affected the whole eastern North China Craton. We present new isotopic ages (LA-ICPMS U-Pb zircon) of a granitic vein (2162Ma), a pegmatite (1840Ma), and a very late granite (114Ma) that are integrated with relevant published data. Accordingly, the formation age of deformation and metamorphism was set up, and at last, a modified tectonic model was presented.

DEFORMATION HISTORY AND CORRELATION OF PAIKON AND TZENA TERRANES (AXIOS ZONE, CENTRAL MACEDONIA, GREECE)

Paikon and Tzena terranes are situated in the centre part of Axios zone, between Almopia and Paionia ophiolitic belts. Tectonostratigraphic data reveal that both have been affected by the same polyphase deformation and metamorphism, as well that they have the same lithostratigraphic column. The first deformation phase took place during the Middle to Late Jurassic and is associated with ophiolite obduction, nappe- stacking, terrane accretion and crustal thickening (D1). Metamorphism does not exceed greenschist facies (M1). Relict HP-LT metamorphic assemblages predating M1 metamorphism are possibly developed during subduction processes and overloading of the obducted ophiolites on the continental margin, characterized the initial stages of deformation. Compressional tectonics and intense thrusting with the same kinematics continued in Lower Cretaceous time, affected all pre-Upper Cretaceous units and the obducted ophiolites (D2). This phase is associated with low-greenschist metamorphism (M2). The first main extensional event occurs in the Late Cretaceous, related to basin formation and sedimentation (D3). During Paleocene to Eocene, D4 intense imbrication of all tectonic units towards mainly SW takes place again. Nappes collapse and finally crustal exhumation taken place during Oligocene to Miocene, associated with low - angle normal faults, with a main top to the SW sense of movement (D5). In Miocene to recent times, high - angle normal and strike-slip faults are formed in an extensional to transtensional strain regime (D6), associated with Neogene to Quaternary basin formation and terrane dispersion. The basement rocks of both terranes are of Pelagonian origin, exhumed as a multiple tectonic window.

Marble-hosted ruby deposits of the Morogoro Region, Tanzania

The ruby deposits of the Uluguru and Mahenge Mts, Morogoro Region, are related to marbles which represent the cover sequence of the Eastern Granulites in Tanzania. In both localities the cover sequences define a tectonic unit which is present as a nappe structure thrusted onto the gneissic basement in a north-western direction. Based on structural geological observations the ruby deposits are bound to mica-rich boudins in fold hinges where fluids interacted with the marble-host rock in zones of higher permeability. Petrographic observations revealed that the Uluguru Mts deposits occur within calcite-dominated marbles whereas deposits in the Mahenge Mts are found in dolomite-dominated marbles. The mineral assemblage describing the marble-hosted ruby deposit in the Uluguru Mts is characterised by corundum-dolomite-phlogopite ± spinel, calcite, pargasite, scapolite, plagioclase, margarite, chlorite, tourmaline whereas the assemblage corundum-calcite-plagioclase-phlogopite ± dolomite, pargasite, sapphirine, titanite, tourmaline is present in samples from the Mahenge Mts. Although slightly different in mineral assemblage it was possible to draw a similar ruby formation history for both localities. Two ruby forming events were distinguished by textural differences, which could also be modeled by thermodynamic T–XCO2 calculations using non-ideal mixing models of essential minerals. A first formation of ruby appears to have taken place during the prograde path (M1) either by the breakdown of diaspore which was present in the original sedimentary precursor rock or by the breakdown of margarite to corundum and plagioclase. The conditions for M1 metamorphism was estimated at ∼750 °C at 10 kbar, which represents granulite facies conditions. A change in fluid composition towards a CO2 dominated fluid triggered a second ruby generation to form. Subsequently, the examined units underwent a late greenschist facies overprint. In the framework of the East African Orogen we assume that the prograde ruby formation occurred at the commonly observed metamorphic event around 620 Ma. At the peak or during beginning of retrogression the fluid composition changed triggering a second ruby generation. The late stage greenschist facies overprint could have occurred at the waning stage of this metamorphic episode which is in the range of ∼580 Ma.

Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry

The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North‐Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post‐tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K‐feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P‐T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Geochronological constraints on magmatism and polyphase deformation and metamorphism in the southern Omineca Belt, British Columbia

The Omineca Belt between Nelson and Creston in southeastern British Columbia was affected by overlapping pulses of Mesozoic magmatism, metamorphism, and deformation. U–Pb geochronological data from zircon and monazite were collected by laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) to constrain the timing of these events. The Porcupine Creek stock (162.3 ± 1.3 Ma) intruded across folds and fabrics associated with the earliest phase of regional deformation and metamorphism (D1M1), restricting it to the Early–Middle Jurassic. The Jurassic structures are overprinted northwards by Early Cretaceous deformation and metamorphism (D2M2). The Baldy pluton (117.8 ± 1.2 Ma) crosscuts the regional 144–134 Ma M2 isograds, yet was pervasively affected by the D2 deformation, indicating that D2 deformation outlasted M2 metamorphism but had ceased by 111 Ma, the age of an undeformed pluton. Monazite dates from a kyanite-bearing rock in the contact aureole of the Middle Jurassic Wall stock overlap with the age of the intrusion (167 Ma), indicating a contact rather than regional origin for the kyanite. In the southeast part of the study area, three samples from the regional sillimanite zone contain monazite intergrown with sillimanite that yield dates between 80 and 69 Ma, indicating an episode of Late Cretaceous (M3) Barrovian metamorphism and deformation (D3). To the north of this domain, in an area characterized by the older D2M2 deformation, a sillimanite zone schist contains two main monazite age populations, suggestive of overlapping effects of Early Cretaceous and Late Cretaceous metamorphic episodes.

Ediacaran (∼620 Ma) high-grade regional metamorphism in the northern Arabian Nubian Shield: U-Th-Pb monazite ages of the Elat schist

Ediacaran times witnessed a hemisphere-scale orogenesis, forming the extensive Pan-African mountain ranges and resulting in the final assembly of the Gondwana supercontinent. The Elat metamorphic association (S Israel), located at the northernmost tip of the Arabian Nubian Shield (ANS), comprises amphibolite-facies pelitic and psammitic schists and orthogneisses and was presumably shaped by Ediacaran East African orogenesis. However, the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are inconclusive and a major emphasis was previously given to pre-Ediacaran island-arc related tectonics. Here U-Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat association.In-situ analysis of metamorphic monazites by LASS-ICPMS (Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry) yields two concordant age clusters at ∼710 and 620Ma, recording two regional metamorphic events. The corresponding REE patterns of the dated monazite grains indicate that garnet porphyroblast growth took place only during the younger event (M2). Likewise, the regional south-dipping penetrative foliation (S2), common to the Elat schist and the surrounding orthogneisses, formed during the Ediacaran event (M2). The peak M2 staurolite-garnet-sillimanite assemblage was overprinted by retrograde andalusite and cordierite during decompression, followed by the intrusion of andesitic dykes that were immediately metamorphosed at greenschist-facies conditions. This metamorphic P-T-d-t path corresponds to the collision of the already accreted arc terranes of the ANS with the Saharan Metacraton, as also suggested by the large geochronological database of 610–580Ma post collisional granitoid batholiths throughout NE Africa and Arabia. The Cryogenian M1 metamorphism therefore records arc-arc collision during major convergence forming the proto-ANS to the south.

The evolution of a Precambrian arc-related granulite facies gold deposit: Evidence from the Glenburgh deposit, Western Australia

Gold deposits are rare in upper-amphibolite to granulite facies environments. Known examples commonly attract debate about whether they formed under these conditions or instead represent metamorphosed, metasomatic, or superimposed (retrograde) mineralization. The Glenburgh gold deposit is located in the Paleoproterozoic upper-amphibolite to granulite facies Glenburgh Terrane in the southern Gascoyne Province of Western Australia. Gold at the Glenburgh deposit is free and disseminated within quartz–biotite–garnet gneiss, amphibolite, and (post-gold) quartz–chlorite veins. No clear association with a specific host lithology has been identified and mineralization does not have a visually distinct proximal alteration assemblage.The rocks hosting the deposit represent a distinct sedimentary package that was deposited, mineralized, buried, and metamorphosed, all during arc magmatism. Features within the internal structure of gold grains, such as high-purity gold veinlets, incoherent twinning, and low silver content, suggest the gold has been through post-depositional processes such as metamorphism and deformation. Abundant sulfide minerals are interpreted to have formed by sulfidation of the host rock contemporaneously with gold mineralization, and the presence of rounded sulfide inclusions within garnet porphyroblasts illustrates the presence of a sulfide phase prior to peak metamorphism. Geochronology of zircon and monazite constrains the timing of mineralization to be younger than c.2035Ma—the maximum depositional age of the metasedimentary host rocks—but older than c.1991Ma—the peak of M1 metamorphism during the Glenburgh Orogeny; these events were synchronous with arc magmatism. Rocks at the Glenburgh deposit were likely deposited in a fore-arc or accretionary wedge, a favourable setting for porphyry Cu–Mo–Au, epithermal Au, polymetallic (Sn, W) skarn, and orogenic Au mineralization. Phase equilibria modelling of a pelitic migmatite constrains peak P–T conditions to be 865–885°C, 6.8–7.6kbar, consistent with elevated thermal gradients within the arc, followed by conductive cooling of arc magmas. Partial melting during peak M1 metamorphism possible caused gold remobilization. The lack of an alteration assemblage further suggests that the alteration assemblage and mineralization were recrystallized during deformation and metamorphism. However, increases in Ca and K abundance and magnetic susceptibility decreases toward mineralization, suggesting that they may constitute ore vectors.

Metamorphic record of the NW Himalayan orogeny between the Indian plate-Kohistan Ladakh Arc and Asia: Revelations from foliation intersection axis (FIA) controlled P–T–t–d paths

Our recent Foliation Intersection Axes (FIAs) data from porphyroblasts in a metapelitic sequence, exposed south of the Indus suture of the NW Himalaya, revealed a fascinating relationship between the trends of three FIA sets versus the plate motion of India since its collision with the Kohistan–Ladakh Arc. The established FIA trend sequence, ESE–WNW (set 1), E–W (set 2) and NNE–SSW (set 3), matches successive vectors of relative India–Asia plate movement between magnetic anomalies 25–22 (55–50Ma), 21–20 (48–44Ma) and 13–8 (36–29Ma), on the premise that FIAs develop normal to the direction of bulk crustal shortening. The correlation allows us to precisely model the pressure (P)–temperature (T) path followed by this portion of the NW Himalayan orogen from 55 to 29Ma. Representative samples preserving FIA sets in garnet porphyroblasts are modeled in the chemical system MnNCKFMASH using THERMOCALC. A steep continuous P–T path is obtained from garnet porphyroblasts for sample preserving FIA set 1 that brackets metamorphic conditions during early NNE–SSW shortening (anomalies 25–22) between 0.4–0.87GPa and 495–535°C (M1 metamorphism). Garnets preserving FIA set 2 revealed 0.46–0.8GPa and 507–565°C (M2 metamorphism) during N–S (anomalies 21–20) shortening. Subsequent ESE–WNW shortening parallel to the orogenic arc (anomalies 13–8) peaked at 0.9 to 1GPa and 595±19 to 618±28°C (M3 metamorphism) and was followed by high-temperature/low-pressure conditions of 0.8±0.2GPa at 747±32°C that accompanied north-side-down decompression (M4 metamorphism) after 29Ma. The M3 and M4 P–T estimates are based on average P–T conditions of garnet inner rim with the matrix mineral phases, which we suspect may not be in equilibrium due to later reactivation of the matrix foliation. The orthogonal orientation of FIA 1 and the NNE movement of India between anomalies 25 and 22 (55–50Ma) suggest that the Indian plate collided with the Kohistan–Ladakh Arc around this time.

EPMA monazite geochronology of the basement and supracrustal rocks within the Pur-Banera basin, Rajasthan: Evidence of Columbia breakup in Northwestern India

We deduce the timing of the rift-induced collisional event between two orogenies in the central Aravalli–Delhi Fold Belt using monazite chemical dating and metamorphic P–T estimates from metapelites of the Mangalwar Complex (MC), and the overlying Pur-Banera (PB) supracrustals. The MC rocks preserve evidence of three regional metamorphic events, while the PB rocks record the last event. The M1 metamorphism attained its peak P–T at ∼5.5kbar and 520–550°C in the MC rocks at ∼1.82Ga, followed by the M2 event with peak P–T of ∼7.5kbar and 580–660°C at ∼1.35Ga. The youngest high-pressure M3 metamorphism attained a peak P–T of ∼8.0kbar and 590–640°C at ∼0.99Ga. Thermobarometry coupled with ages of included monazites in chemically zoned garnet from the MC metapelites indicate preservation of ages spanning between ∼1.82Ga and ∼0.99Ga from different zones (i.e., core to rim), implying episodic garnet growth during supercontinent cycle. The PB metapelites constitute two prominent ages of ∼1.37Ga and ∼1.05Ga. The youngest high-pressure metamorphism (M3) in the PB rocks with maximum P–T of ∼8.0kbar, and 580–670°C during the Neoproterozoic has overprinted their earlier metamorphic records. Based on monazite geochronology, we assign the ∼1.82Ga and ∼1.37–1.35Ga ages to the amalgamation and breakup of the Columbia supercontinent respectively. The youngest age record of ∼1.05–0.99Ga indicates evidence of Rodinia formation in and around the Pur-Banera basin.

Multiple metamorphic episodes recorded in the Paleozoic Pyeongan Supergroup on the northeastern margin of the Yeongnam massif, South Korea: Implications for the Songrim (Indosinian) orogeny

Metasedimentary rocks of the Pyeongan Supergroup, which occur as roof pendants on the northeastern margin of the Yeongnam massif, South Korea, preserve a record of the impact of the Late Permian–Triassic Songrim (Indosinian) orogeny in the eastern Eurasia margin. Metamorphic results of this paper have revealed that the metasedimentary rocks were affected by three metamorphic events during the Middle Permian to Middle Mesozoic. Representative mineral assemblages from three metamorphic zones I, II and III are Cld+And±Ky±St, Grt+St+Ky+Bt±And, and And+Sill+Bt±Grt±St, respectively. Kyanite and sillimanite paramorphs after andalusite occur in zones I and III, respectively. Metamorphic P–T conditions and paths were determined from conventional geothermobarometers and compositional isopleths of chloritoid, staurolite, and garnet on P–T pseudosections in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) and KFMASH–T (TiO2)–O (Fe2O3) systems. In the study area, the Pyeongan Supergroup experienced medium pressure regional metamorphism (M1 and M2) belonging to greenschist to amphibolite facies followed by thermal metamorphism (M3). The M1 metamorphism occurred at 450–500°C and 2–4kbar and produced the kyanite paramorph after andalusite. The M2 metamorphism occurred at 520–600°C and 4–6kbar, and was associated with the growth of garnet, staurolite, and kyanite. The later thermal metamorphism occurred at ca. 580°C and ca. 4kbar, and was caused by intrusion of Jurassic granitoids. The medium pressure regional metamorphism (M1 and M2) have occurred around 270Ma during the Songrim orogeny based on the SHRIMP U–Pb zircon ages.

Structural style and metamorphic conditions of the Jinshajiang metamorphic belt: Nature of the Paleo-Jinshajiang orogenic belt in the eastern Tibetan Plateau

The Jinshajiang metamorphic belt is a Barrovian sequence distributed within the Jinshajiang Suture Zone (JSZ), which was formed in the thickened Paleo-Jinshajiang orogenic belt after closure of the Jinshajiang Ocean. We have identified three metamorphic phases, M1, M2 and M3, corresponding to deformation stages D1, D2 and D3, in the Barrovian sequences. The metamorphic belt exhibits a metamorphic field gradient from chlorite to biotite, garnet, staurolite–kyanite and sillimanite grades. Inclusions in garnet and staurolite (chlorite, mica, quartz, feldspar, ilmenite and graphite) indicate that M1 reflects greenschist facies metamorphism. Pervasive M2 metamorphism formed a dominant S1 schistosity within the Barrovian sequence. Peak metamorphic conditions for metapelites of the garnet–staurolite and staurolite–kyanite grade were ∼580°C and ∼0.65GPa according to petrogenetic grids. Peak metamorphism was in conditions of ∼635°C and 0.50GPa for the metapelites and ∼650°C and ∼0.61GPa for amphibolites in sillimanite grade. Greenschist facies retrograde metamorphism, M3, followed D2 deformation, a top-down-to-southeast shear in the JSZ. The D3 deformation is characterized by well-developed brittle faults with fault gouge and breccia. Zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims. However, the metamorphic rims are too narrow to analyze. Laser ablation-inductively coupled plasma mass-spectrometry (LA-ICPMS) analyses of the igneous cores of zircons yielded a crystallization age of 242Ma. 40Ar/39Ar dating of white mica from a garnet-schist gave a plateau age of 224Ma. The peak metamorphism is thus limited to be between 242 and 224Ma. We thus suggest a tectonic shift from collision to extension of for the Permian to Triassic Paleo-Jinshajiang orogenic belt.

The Cimmerian accretionary wedge of Anarak, Central Iran

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered “ophiolites” which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the “ophiolites” within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the “ophiolites” were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Leaching of S, Cu, and Fe from disseminated Ni-(Fe)-(Cu) sulphide ore during serpentinization of dunite host rocks at Mount Keith, Agnew-Wiluna belt, Western Australia

Komatiite-hosted disseminated Ni sulphide deposits in the Agnew-Wiluna greenstone belt occur both above and below the olivine isograd that was imposed on the greenstone sequence during the M2 metamorphic/deformation event. Deposits in the northern and central part of the belt and that are located below the isograd (Mount Keith, Honeymoon Well and West Jordan) have complex sulphide mineralogy and strongly zoned sulphide assemblages. These range from least-altered assemblages of pentlandite-pyrrhotite-chalcopyrite±pyrite to altered assemblages of pentlandite±chalcopyrite, pentlandite-heazlewoodite (or millerite), heazlewoodite (or millerite), and rarely to heazlewoodite-native Ni. Deposits to the south and that are above of the olivine isograd (Six Mile, Goliath North) are dominated by less complex magmatic assemblages with a lower proportion of weakly altered pentlandite±chalcopyrite assemblages. More altered assemblages are uncommon in these deposits and occur as isolated patches around the periphery of the deposits. The sulphide zonation is reflected by whole-rock reductions in S, Cu, Fe and Zn, whereas Ni, Pt and Pd and, with some exceptions, Co are conservative. The leaching of S, Cu, Fe and Zn from sulphide assemblages and the whole rock was initiated by highly reduced conditions that were produced during low fluid/rock ratio serpentinization. Consumption of H2O resulted in Cl, a component of the fluid, being concentrated sufficiently to stabilise iowaite as part of lizardite-rich assemblages. Once the rate of olivine hydration reactions declined and during and after expansion and associated fracturing of the ultramafic sequence allowed higher fluid access, a more fluid-dominated environment formed and new carbonate-bearing fluid gained access to varying extents to the ultramafic rock sequence. This drove Cl from iowaite (to form pyroaurite) and caused the sulphide assemblages to be altered from the original magmatic assemblages and compositions to those stable at the prevailing fO2 and fS2 conditions. Mass transfer was made possible via metal chloride complexes and H2S with fluids driven by deformation associated with the M2 metamorphism. Disseminated deposits in higher metamorphic grade terrains where olivine was stable during peak metamorphism did not undergo the metasomatism seen in the deposits in areas of lower metamorphic grade. Some minor leaching of S, Fe and Cu occurred around the periphery of the deposits during early, pre-M2 peak metamorphism, but once olivine stability was reached the driving force for the series of leaching reactions was exhausted. The effect of this process on the original magmatic sulphides is to induce significant variability in texture, mineralogy and bulk composition and to markedly reduce the Fe and S contents of the sulphide fraction (in extreme cases to zero for both elements), and to reduce the volume of the sulphide fraction per unit of Ni. These changes impact unfavourably on Ni sulphide recoveries and metallurgical characteristics of these Ni ores.

Paleoproterozoic magmatic and metamorphic events in the Hongcheon area, southern margin of the Northern Gyeonggi Massif in the Korean Peninsula, and their links to the Paleoproterozoic orogeny in the North China Craton

The Hongcheon area is located at the northern part of Gyeonggi Massif (NGM) in the Korean Peninsula. The metamorphic rocks of the NGM at the Hongcheon area are composed of the Paleoproterozoic Yongduri Gneiss Complex (YGC), Euiam Group (EG) and the Euiam Gneiss Complex (EGC). The quartz-feldspathic gneisses in the northeastern part of the YGC record peak metamorphic temperature–pressure condition of 790–840°C and 7.2–8.9kbar, whereas the granitic and garnet gneisses in the western part of the YGC record peak metamorphic temperature–pressure conditions of 690–720°C and 6.1–6.9kbar, and 640–660°C and 5.0–5.4kbar, respectively. The peak metamorphic conditions represent a regional low-P/T metamorphic event (M2) in which metamorphic temperature and pressures increased towards the east. SHRIMP zircon U–Pb age dating indicates that the M2 metamorphic stage occurred at ca. 1886–1870Ma. The presence of relict kyanite in the gneisses within the YGC suggests that the intermediate-P/T metamorphic stage (M1) occurred in ca. 1925Ma. Whole-rock geochemistry of the metagranitoid in the EGC indicates that they are originally post-collision granitoid. SHRIMP zircon U–Pb age dating indicates that the metagranitoid formed at ca. 1885–1869Ma. Considering the similarity in age between the Paleoproterozoic intrusion and the M2 low-P/T metamorphism, the M2 metamorphism also occurred in a post-collision tectonic setting and the M1 intermediate-P/T metamorphism with kyanite represents collision event. Further, we suggest that the Paleoproterozoic orogeny in the Hongcheon area is closely linked to the Paleoproterozoic orogeny in North Korea and the Jiao-Liao-Ji belt in the North China Craton.