Palinspastic reconstruction of the Grenville terrane in the Blue Ridge Geologic Province, southern and central Appalachians, U.S.A

Within the southern and central Appalachians, Grenville-age basement rocks are found in major massifs in the Blue Ridge and Sauratown Mountains anticlinoria and in the vicinity of the Grandfather Mountain window. These massifs are, respectively, Pedlar and Lovingston Massifs in the Blue Ridge anticlinorium, Sauras Massif in the Sauratown Mountains anticlinorium, and Watauga, Globe, and Elk River Massifs near the Grandfather Mountain window. In central Virginia the Lovingston Massif is juxtaposed against the Pedlar Massif, and in northwestern North Carolina-southwestern Virginia, the Elk River Massif is thrust over the Globe and Watauga Massifs, all along faults of the Fries fault system, which includes the Rockfish Valley, Fork Ridge, Devil’s Fork, and Linville Falls faults, as well as the Fries fault per se. The Pedlar Massif is a deeper granulite facies country-rock terrane intruded by charnockite plutonic suites. The Lovingston Massif primarily is a shallower granulite/amphibolite facies terrane intruded by biotite dioritoid plutonic suites containing bodies of charnockite. Country rocks of the Watauga Massif were subjected to metamorphic conditions similar to those of the Lovingston Massif, but were intruded by a plutonic suite of biotite dioritoid, biotite granitoid, and granitoid. The Elk River, Globe, and Sauras Massifs all are terranes metamorphosed to amphibolite facies and intruded by granitoid/dioritoid suites containing some porphyritic biotite dioritoid phases. A suite of late Precambrian (post-Grenville) peralkaline granitoid plutons intruded all of the massifs except the Pedlar. These plutons presumably are related to upper Precambrian volcanic rocks that were associated with a rifting environment and that were later metamorphosed and deformed along with overlying sedimentary rocks to form part of the Appalachian orogenic belt.

Mineral assemblages in common sulfide ore deposits are examined together with phase relations to (1) investigate the pressure^ temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals, and (2) place constraints on the amount of melt produced. Deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies. Deposits lacking sulfosalt and/or telluride minerals may begin to melt once P^T conditions reach the upper amphibolite facies, if galena is present, or well into the granulite facies if galena is absent.The result is two broad melting domains: a lowto medium-temperature, low melt volume domain involving melting of volumetrically minor sulfosalt and/or telluride minerals; and a high-temperature, potentially higher melt volume domain involving partial melting of the major sulfide minerals. Epithermal gold deposits, which are especially rich in sulfosalt minerals, are predicted to commence melting at the lowest temperatures of all sulfide deposit types. Massive Pb^Zn (^Cu) deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions, and in the upper amphibolite facies if they do not. Excepting sulfosalt-bearing occurrences, massive Ni^Cu^PGE (platinum group element) deposits will show little to no melting under common crustal metamorphic conditions, whereas disseminated Cu deposits are typically incapable of generating melt until the granulite facies is reached, when partial melting commences in bornite-bearing rocks. The volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals. Even at granulite-facies conditions, this volume is usually less than 0 5%. The exception is massive Pb^Zn deposits, where melt volumes significantly exceeding 0 5 vol. % may be segregated into sulfide magma dykes, allowing mobilization over large distances.

The Lewisian Complex is an Archaean/Proterozoic craton fragment found in NW Scotland and throughout the Outer Hebrides. The 1907 memoir recognized, simply from field relationships and petrographic observation, key features of Lewisian evolution. The bulk of the Lewisian is an old, deformed complex consisting mainly of acid igneous rocks, with some basics, ultrabasics and metasediments. In the Central District of the mainland these are pyroxene bearing (now recognized as granulite facies). The Lewisian Complex was intruded by a suite of basic and ultrabasic dykes which show variable states of later deformation, the intensity of strain being correlated with the development of hornblende schist in the dykes and amphibolite facies assemblages in the country rocks. In the Northern and Southern Districts, this deformation is pervasive and the dykes become concordant hornblende schist sheets. The new foliation with transposed dykes and metasediment sheets is then folded around NW–SE axes. Today there is no single agreed model for the evolution of the complex but an outline is as follows. In the pre-dyke (Scourian) history, subduction led to melting of oceanic crust which provided vast volumes of tonalite-trondhjemite-granodiorite in the period 3100–2700 Ma. Ages show geographic variations but it is not proven whether that implies large displacements between pieces of crust or whether it represents intrusions into other intrusions. The subcontinental lithospheric mantle dates from c . 3000 Ma. K, U and other large ion lithophile elements are depleted in the Central District of the mainland; this is due to depletion in the downgoing oceanic slab which in turn is a result of dehydration prior to melting. Other areas are not depleted in such elements, so various tectonic settings were involved. Remnants of metabasic material in the Lewisian may be relics of oceanic crust. Granulite facies metamorphism with, in places, P >10 kb and T >1000 °C occurred a considerable time after intrusion so is not necessarily linked to igneous events. This ‘Badcallian’ episode affected mainly the Central District and a part of the southern Outer Hebrides; other areas show only amphibolite facies. Zircon dating indicates two high-grade events at 2500 and 2700 Ma. During the ‘Inverian’ episode a series of wide amphibolite-facies shear zones affected the granulite-facies Scourian gneiss prior to the intrusion of the Scourie dykes. The Scourie dykes were intruded from 2400–2000 Ma and are largely quartz tholeiites derived from enriched subcontinental lithospheric mantle; there are some picrites which yield the oldest ages but are also seen to crosscut basic dykes. The dykes trend NW–SE and are steep where not affected by later deformation except where they intrude along, and are controlled by, Inverian fabrics. Post-dyke (Laxfordian) history involves the development of calc-alkaline igneous rocks in the Outer Hebrides and mainland ( c . 1900 Ma). Volcanics associated with sediments younger than 2000 Ma comprise an accretionary complex formed in a subduction setting; they are now intercalated between slabs of Archaean basement indicating that the complex was involved in collision with continental crust. Huge strains transposing dykes and country rocks affected almost all of the Outer Hebrides and the mainland except for the Central District. The NW–SE trending lineation indicates the collision direction; the metasediments on the mainland and the South Harris Igneous Complex may mark a folded suture between two continents. Metamorphism was amphibolite facies almost everywhere; in South Harris it was granulite facies at c . 1880 Ma. At 1750–1675 Ma, a distinct event, called late Laxfordian but much younger than earlier Laxfordian metamorphism and with a distinct tectonic setting, caused folding of the previous structures along NW–SE axes, migmatization and renewed amphibolite facies metamorphism.

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic.

The nature of the mid-crustal velocity discontinuity, identified in seismic refraction experiments in widely-spaced probes of the continents, has been debated for many years; recent reflection profiles have added new constraints. In central Ontario, Canada, the thrustbounded Kapuskasing uplift exposes a continuous oblique cross-section of Archean crust to a depth of some 25 km, including a Conrad-like discontinuity, allowing direct observation of the nature and origin of this complex feature. Lithologically, it represents a gradational change from higher level, homogeneous tonalitic gneiss in the amphibolite facies (Wawa gneiss terrane) to a deeper-level, layered heterogeneous sequence in the upper amphibolite and granulite facies (Kapuskasing structural zone). Accompanying lithological changes are increases in density of the order of 0.1 g.cm-3 and aggregate P-wave velocity of approximately 0.35 km.sec-1. Migmatitic structures in the high-grade Kapuskasing rocks suggest that partial melting during metamorphism was responsible for production of tonalite, which could have coalesced, risen and ponded above the discontinuity in the Wawa terrane. Other effects that contributed to the gross crustal density stratification include the pre-metamorphic emplacement of more mafic intrusions and anorthosites at depth.

The supracrustals occurring in the northern sector of the South Delhi Fold Belt (SDFB; main Delhi Synclinorium of Heron, 1953) between Ajmer-Pushkar valley and Sarnbhar Lake in north-central Rajasthan are classified into Anasagar migmatites and the overlying Ajmer Formation comprising Taragarh quartzites and Kalyanipura arkose-pelite-greywacke sequence. The Anasagar migmatites and paragneisses are intruded by Iatero genic granite(1600 Ma) and post orogenic ultramafic and alkaline rocks. Four folding and two shearing movements characterize these rocks. Three metamorphic zones could be identified in these supracrustals with grade increasing towards west: staurolite-kyanite grade (middle arnphibolite facies) in the eastern sector, sillimanite-muscovite grade (upper amphibolite facies) in the central sector, and orthopyroxene-plagioclase and orthopyroxene-sillimanite grade (granulite facies) in the western sector. The Anasagar and Ajmer supracrustals are deposited in an Early Proterozoic ensialic basin in the Archaean BGC (Banded Gneissic Complex) protocontinent and differ from the other tectono-stratigraphic units of the SDFB in respect of lithological organisation, metamorphism and magmatism besides the latter being decisively of Upper Proterozoic age. The authors, therefore suggest a separate and older stratigraphic status to these supracrustals and delink from the rest of the SDFB stratigraphy.

Layer‐parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D2) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer‐parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low‐grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz‐rich than younger.Mass balance calculations and whole‐rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress‐induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine‐scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.

Paleoproterozoic metamorphism of high-grade granulite facies rocks in the North China Craton: Study advances, questions and new issues

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

The orthopyroxene‐clinopyroxene, garnet‐orthopyroxene and garnet‐clinopyroxene geothermometers, and the garnet‐orthopyroxene‐plagioclase, garnet‐clinopyroxene‐plagioclase and anorthite‐ferrosilite‐grossular‐almandine‐quartz geobarometers are applied to metabasites and the garnetplagioclase‐sillimanite‐quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway. P–T conditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C.This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. Neither P nor T vary significantly across the entire transition zone. The change in ‘grade’parallels the increasing dominance of CO2 over H2O in the fluid phase.LILE‐depletion is not a pre‐condition of granulite facies metamorphism: granulites may have either ‘depleted’or ‘normal’chemistries. The results presented herein show that LILE‐deficiency in granulite facies orthogneisses is not necessarily related to variations in either P or T. The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE‐poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle‐derived CO2‐rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid‐intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin.

U–Pb zircon study of tectonically bounded blocks of 2940–2840 Ma crust with different metamorphic histories, Paamiut region, South-West Greenland: implications for the tectonic assembly of the North Atlantic craton

In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high‐P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ∼200–1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10–14 kbar. Similar P–T conditions are recorded in high‐P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ∼200–1000 m from the Malaspina Pluton retain medium‐P mid‐amphibolite facies assemblages, despite having been subjected to pressures of 10–14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional‐scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3–4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10–14 kbar. Similar high‐P metamorphic events may therefore be more common than presently recognized.

Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia

Recent mapping and radiometric dating in the Grenville Province of western Labrador indicate two major metamorphic events, one Early Proterozoic (circa 1650 Ma) and one Grenvillian (circa 1000 Ma). A threefold tectonic subdivision of the Grenville Orogen is proposed, consisting of an autochthon, a parautochthon and a number of allochthons. The parautochthon in the west consists of a previously unmetamorphosed sedimentary succession, whereas in the east it consists of paragneiss, metamorphosed in the Early Proterozoic, and a granitoid batholith. From the Grenville Front southwards, metasediments of both the autochthon and parautochthon increase in Grenvillian metamorphic grade from greenschist to upper amphibolite facies. These rocks are imbricated by northward-directed thrusts. In the interior Grenville Province, the parautochthonous sequences are tectonically overlain by Lower Proterozoic gneisses of upper amphibolite to granulite facies. These were thrust into place during the Grenvillian orogeny, when they were structurally reworked but only mildly metamorphosed. Upper amphibolite and granulite facies rocks of an Early Proterozoic metamorphism are thus adjacent to those at equivalent and lower grades of a Grenvillian metamorphism.

Systematic mapping of a transect along the well‐exposed shores of Georgian Bay, Ontario, combined with the preliminary results of structural analysis, geochronology and metamorphic petrology, places some constraints on the geological setting of high‐grade metamorphism in this part of the Central Gneiss Belt. Correlations within and between map units (gneiss associations) have allowed us to recognize five tectonic units that differ in various aspects of their lithology, metamorphic and plutonic history, and structural style. The lowest unit, which forms the footwall to a regional decollement, locally preserves relic pre‐Grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the Grenvillian orogeny. Tectonic units above the decollement apparently lack the early granulite facies metamorphism; out‐of‐sequence thrusting in the south produced a duplex‐like structure. Two distinct stages of Grenvillian metamorphism are apparent. The earlier stage (c. 1160–1120 Ma) produced granulite facies assemblages in the Parry Sound domain and upper amphibolite facies assemblages in the Parry Island thrust sheet. The later stage (c. 1040–1020 Ma) involved widespread, dominantly upper amphibolite facies metamorphism within and beneath the duplex. Deformation and metamorphism recently reported from south and east of the Parry Sound domain at c. 1100–1040 Ma have not yet been documented along the Georgian Bay transect. The data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south‐east of the transect area, with further advance to the north‐west during and after the waning stages of this deformation.