Ga Regional Metamorphism Research Articles

Zircon-monazite geochronology and P-T evolution of the Sargur Greenstone Belt: Implications for the end-Archean tectonics of the Dharwar Craton in southern India

We report petrological and geochronological data for metamafic volcanics and metasediments from the Sargur Greenstone Belt, Western Dharwar Craton (WDC), southern India and discuss the end-Archean metamorphic P–T conditions of the Sargur Greenstone Belt, as well as geotectonic implications of the end-Archean metamorphism in the WDC. The LA-ICP MS zircon U-Pb dating yields a metamorphic age of 2500 ± 20 Ma for the garnet-bearing amphibolite. The garnet-staurolite-kyanite mica schist sample has metamorphic zircon and monazite U-Pb ages of 2443 ± 7 Ma and 2468 ± 10 Ma, respectively, consistent with metamorphic zircon U-Pb ages of 2459 ± 17 Ma and 2455 ± 18 Ma yield by the amphibole-garnet schist samples. The peak mineral assemblage of the garnet-bearing amphibolite is garnet + clinopyroxene + amphibole + plagioclase + quartz + rutile + ilmenite, and that of the metapelitic schist is garnet + staurolite + kyanite + biotite + muscovite + quartz + plagioclase. Phase equilibrium modelling, together with geothermobarometers yield P–T conditions of 7.9–8.9 kbar/681–769 °C and 7.5–9.3 kbar/683–694 °C for the garnet-bearing amphibolite and garnet-staurolite-kyanite mica schist, respectively, corresponding to a medium geothermal gradient of 20–25 °C/km. These data indicate that the Sargur Greenstone Belt has experienced a typical Barrovian-type (amphibolite-facies) metamorphism at ca. 2.50–2.44 Ga. Combined with the previous studies, it is suggested that the Sargur Greenstone Belt was most likely to be formed during 3.35–3.12 Ga and was reworked through four episodes of magmatic-metamorphic events occurred at ca. 3.1 Ga, ca. 3.0 Ga, ca. 2.8–2.6 Ga and ca. 2.5–2.4 Ga. The prominent ca. 2.5–2.4 Ga regional metamorphism is the last stage of the Neoarchean tectono-thermal event in the WDC and thus most possibly indicate accretion-collision of different Archean blocks at ca. 2.5–2.4 Ga and lead to the final cratonization (stabilization) of the whole Dharwar Craton.

U-Pb zircon age, geochemistry and petrogenesis of Mesoarchean anorthositic rocks from the Holenarsipur Greenstone Belt, Western Dharwar Craton: Implications for accretionary tectonics in southern India

The Doddakadanur Anorthosite Complex is a well-preserved sequence of anorthosites, gabbroic anorthosites, and anorthositic gabbros intrusive into amphibolites of the Mesoarchean Holenarsipur Greenstone Belt in the Western Dharwar Craton of southern India. Although the whole sequence is metamorphosed at upper amphibolite to lower granulite facies conditions, igneous cumulus texture is preserved in many of these rocks. The peak P-T conditions corresponding to metamorphic resetting, synchronous with 2.5 Ga regional metamorphism, is estimated to be ~650–700 °C and 7–11 kbar. The major and trace elemental systematics suggest a moderately fractionated parental magma (La/Yb)n = 3.3) with an overall tholeiitic affinity. The trace elemental systematics of these rocks indicates enrichment of the magma source regions by subduction-derived fluids. Uranium-Pb zircon ages yield an emplacement age of 3242 ± 17 Ma and also help to constrain two metamorphic events at 3159 ± 25 Ma and 2538 ± 11 Ma. Considering the intrusive contact relationship between the anorthositic rocks of the Doddakadanur Anorthositic Complex and ultramafics of the Holenarsipur Greenstone Belt, the new U-Pb ages suggest that the mafic-ultramafic rocks of the Holenarsipur Greenstone Belt are at least 3.24 Ga old. Integrating the new age data with available ages for the different magmatic rocks of the region, a spatial and temporal link between anorthosite emplacement and crustal accretion processes in the Western Dharwar Craton can be established. On the basis of whole-rock geochemistry and mineral chemistry, we propose a convergent margin tectonic setting for the rocks of the Doddakadanur Anorthosite Complex.

Metamorphic evolution for the Inyoni shear zone: Investigating the geodynamic evolution of a 3.20 Ga terrane boundary in the Barberton granitoid greenstone terrane, South Africa

Abstract The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.

Latest Paleoproterozoic (ca. 1.8–1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

Abstract Latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79–1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10–15.56, 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, showing affinity to A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 ages (2817–2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 ± 45 Ma and metamorphic age of 317 ± 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39–4.81 wt%) contents and low Nb/Y (0.63–0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85–1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A2-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.

High-Grade Magnetite Mineralization at 1.86 Ga in Neoarchean Banded Iron Formations, Gongchangling, China: In Situ U-Pb Geochronology of Metamorphic-Hydrothermal Zircon and Monazite

Abstract Although less common than hematite ores, high-grade magnetite ores represent a distinct type of iron mineralization hosted by banded iron formations (BIFs). The Gongchangling iron deposit hosted in ~2.55 Ga BIFs in the North China craton represents one of the most economically important iron deposits in China. Located in mining area II, it is a high-grade (>50 wt % Fe) magnetite deposit and one of the largest of its type in the world. However, the lack of reliable age constraints on iron mineralization has hindered the testing of competing genetic models for the formation of the Gongchangling deposit. In situ U-Pb geochronology of monazite and zircon intergrown with garnet from the proximal alteration zone of a high-grade iron orebody yielded an age of 1.86 Ga, which represents the timing of formation of high-grade magnetite mineralization. This age is coeval with a tectonic extension event recorded in the northeastern North China craton. Our results preclude the previously suggested genetic link between high-grade magnetite mineralization and ~2.50 Ga regional metamorphism. Growth of authigenic monazite and zircon is likely related to the breakdown of detrital zircon, which has undergone metamictization. In combination with previously published data, we propose that the development of zoned alteration associated with the deposit, which is characterized by the garnet-amphibole-magnetite assemblage in the proximal zone changing to a chlorite-quartz–dominated assemblage in the distal zone, can be attributed to a gradual decrease in temperature from >550° to ~250°C and to alteration minerals forming from leaching of the BIFs instead of by replacement of the wall rocks. Magnetite mineralization was controlled by the well-developed faults that cut the BIFs and provided conduits for silica-undersaturated alkaline meteoric fluids. Fluid flow likely took place in an extensional tectonic regime, similar to that invoked elsewhere for hematite mineralization but at greater depths.

Multi-stage Cu remobilization of the Huping metamorphic-hydrothermal deposit in the southern North China Craton

The Zhongtiao region is a well-endowed Cu metallogenic province in the southern North China Craton and contains a large number of early Precambrian Cu deposits of different styles, most of which have been subjected to the ca. 1.85 Ga regional metamorphism. However, whether there was any role for the Proterozoic metamorphism on the extensive Cu mineralization is yet to be well understood. This paper presents an integrated investigation on the field geology, mineral chemistry, fluid inclusion and C–H–O–S isotopes of the Huping metamorphic-hydrothermal deposit in the Neoarchean–Paleoproterozoic Zhongtiao metamorphic terranes, and reveals multi-stage Cu remobilization that formed and upgraded the Cu deposits in the region. We identified three stages of syn- to post-metamorphic Cu remobilization/transportation in the Huping Cu deposit, including (I) early mechanical transport, (II) intermediate metamorphic-hydrothermal remobilization and (III) late meteoric fluid incursion. In stage I, sulfides were remobilized and transported internally under lower-greenschist facies metamorphism, forming the veinlet–disseminated ores and chalcopyrite (avg. δ34S = 2.1‰, similar to the chlorite-amphibole schist wall rocks). In stage II, the immiscibility of metamorphic fluids (avg. δ18O = 4.6‰ and δD = −56.6‰) from the metamorphic terranes triggered the precipitation of higher δ34S chalcopyrite (avg. 8.0‰), led by the incorporation of extra sulfur during the fluid migration. In stage III, incursion of meteoric fluids into the waning metamorphic-hydrothermal system formed the barren quartz-calcite veins (avg. δ18O = 0.09‰ and δ13C = −0.88‰). During these Cu remobilization processes, the earlier-formed sulfides were chemically stable during the mechanical transfer by regional metamorphism and deformation. Subsequently, the immiscibility of metamorphic fluids induced significant Cu remobilization and precipitation in the metamorphic-hydrothermal system. We thus propose a model of multi-stage mechanical and chemical Cu remobilization for the Huping Cu deposit, which highlights the crucial impacts of metamorphic-remobilization processes on the regional Cu mineralization in the Zhongtiao region.

Geochemistry of metamorphosed volcanic rocks in the Neoarchean Qingyuan greenstone belt, North China Craton: Implications for geodynamic evolution and VMS mineralization

The Qingyuan greenstone belt (QGB) situated in the northeastern section of the North China Craton (NCC) is considered to be the unique metallogenic province in China that hosts both banded iron formations (BIFs) and volcanogenic massive sulfide (VMS) deposits. Traditionally, the QGB is divided into the Hunbei and Hunnan terranes separated by the Hunhe fault and grouped into the lower Shipengzi, middle Hongtoushan, and upper Nantianmen formations, which have undergone amphibolite- to granulite-facies metamorphism. Exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of BIF, marble, and schist. In this study, detailed field and petrographic observations, lithogeochemical, U-Pb geochronological, as well as whole-rock Sm-Nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks (i.e. plagioclase amphibolite and biotite plagioclase gneiss, respectively) in the QGB in order to discuss implications for tectonic evolution and comitant VMS mineralization.The supracrustal rocks of both Hunbei and Hunnan regions were formed synchronously from 2.57 to 2.53 Ga and then had undergone 2.52–2.48 Ga regional metamorphism. Specifically, the lower section of the Shipengzi Formation is composed dominantly of transitional-tholeiitic basalts with negative Nb anomalies, suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab-derived fluids or melts. In contrast, the lithologic assemblages grade upward into interlayered bimodal volcanic rocks (i.e. N-MORB-type tholeiitic basalts and FI- to FII-type dacites of transitional to calc-alkalic affinities), representing a change in tectonic regime related to subduction from compression to extension. Among them all the basalts are characterized by consistently positive initial Nd compositions (εNd(t) = +2.7 to +3.4) further indicate a depleted mantle source; whereas those dacites possess negative to positive initial Nd values (εNd(t) = −4.6 to +3.0), suggesting they were formed by partial melting of depleted mafic source (likely lower mafic crust) and accompanied with variable addition of old crustal material. Significantly, VMS deposits are genetically linked to this extensional event and closely associated with the FII-type dacites that were generated by higher melting degree at high-temperature conditions and shallow crustal level (likely 10–15 km). In all, geochemical features (e.g., negative Nb anomalies and εNd value), together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks, imply a continental arc setting where the QGB was evolved.

Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation

Detrital zircon ages from quartzite and metaconglomerate in the Tusas and Picuris Mountains in northern New Mexico reveal new information about age and provenance trends within a >1000 km 2 Proterozoic sedimentary basin and provide a critical test of regional correlations. Samples from the Paleoproterozoic Vadito and Hondo groups are dominated by a single detrital zircon population with age probability peaks that range from 1765 to 1704 Ma. Minor Archean and ca. 1850 Ma age probability peaks were also recognized in some samples. Close correspondence between detrital zircon ages and the age of surrounding basement rocks indicates predominately local sources, and we interpret systematic shifts in peak ages with stratigraphic position to represent changes in local sources through time. Similarities of age spectra support previous correlation of stratigraphic units between discontinuous exposures of the Hondo Group. We interpret that these supracrustal rocks were deposited in a single basin that we refer to as the Pilar basin. Two samples of the Marquenas Formation, a pebble to boulder conglomerate previously correlated with the ca. 1700 Ma Vadito Group, are dominated by Paleoproterozoic detrital zircon with age probability peaks at 1707 and 1715 Ma in the middle and upper units, respectively. Unlike the Vadito and Hondo group samples, the Marquenas Formation also contains abundant ca. 1700–1600 Ma zircon derived from Mazatzal-aged sources to the south and Mesoproterozoic zircon with age probability peaks at 1479 and 1457 Ma. Weighted averages of 1477 ± 13 Ma and 1453 ± 10 Ma for the youngest detrital zircon in the middle and upper Marquenas Formation provide new maximum depositional age constraints, indicating that it is not part of the Vadito Group. The minimum age is not well constrained but is interpreted to be ca. 1435 Ma on the basis of the timing of regional metamorphism and deformation previously documented in the Picuris Mountains. These data represent the first evidence of sedimentation directly associated with ca. 1.4 Ga regional metamorphism, plutonism, and deformation in the southwestern United States and provide an important new constraint on the tectonic evolution of southern Laurentia during this time.

40Ar/39Ar thermochronology constraints on the timing of Proterozoic basement exhumation and fault ancestry, southern Sangre de Cristo Range, New Mexico

Proterozoic ancestry is increasingly being proposed for both Ancestral Rocky Mountain (Pennsylvanian-Permian) and Laramide-age (Late Cretaceous–early Tertiary) faults in the southwestern United States. We test this hypothesis by using 40 Ar/ 39 Ar thermochronology on Proterozoic basement rocks in the southern Sangre de Cristo Range, New Mexico. Hornblende and mica yield ages consistent with basement rocks cooling below ∼500 °C after ca. 1.4 Ga regional metamorphism, followed by protracted basement residence at temperatures between 300 and 400 °C until the Neoproterozoic. K-feldspar age spectra are dominated by Neoproterozoic apparent ages and constrain basement exhumation from mid-crustal depths (∼10 km) to within a few kilometers of the surface during the Neoproterozoic. Samples were also collected across the north-striking, steeply west-dipping Montezuma fault, which bounds the eastern Rocky Mountain range front and has known Ancestral Rocky Mountain and Laramide-age movement. Contrasting 40 Ar/ 39 Ar K-feldspar–derived thermal histories from opposing sides of the fault require two episodes of Neoproterozoic fault movement. Overall, rocks to the west record multiple diffusion domain (MDD) thermal histories, which indicate that cooling was caused by 5–7 km of basement exhumation between 1000 and 800 Ma, whereas rocks to the east were exhumed later, between 750 and 600 Ma. Assuming that the present-day fault geometry was established in the Proterozoic, west-side-up throw late in the Grenville orogeny (ca. 1000 Ma) accommodated east-west shortening. West-side-down extensional movement at ca. 750 Ma is inferred to be associated with the incipient rifting of the supercontinent Rodinia.

Geochronology and geochemistry of metamorphic rocks in the Jiaobei terrane: Constraints on its tectonic affinity in the Sulu orogen

Tectonic affinity of tectono-lithological units close to ultrahigh-pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision. This is particularly so for the Jiaobei terrane northeast of the Dabie-Sulu orogenic belt in China. New data from LA-ICPMS zircon U–Pb dating, whole-rock elements and Nd–Sr isotopes, and mineral O isotopes are presented for metamorphic rocks from this terrane. The results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two Triassic collided continents, the North and the South China Blocks. Protolith ages for TTG gneiss, amphibolite and mafic granulite are ∼2.7, ∼2.5 and ∼2.4 Ga, respectively; regional metamorphism took place extensively at ∼1.76 Ga. Protolith of the TTG gneiss was generated by partial melting of mantle-derived rocks at the root of a thickened crust. Protolith of the amphibolite was probably a product of arc-like magmatism; protolith of the mafic granulite was derived from a depleted mantle source. Both of them were locally contaminated by supracrustal materials. Protoliths of paragneiss and schist in the Fenzishan Group were mostly derived from supracrustal sources, but protolith of amphibolite in the Fenzishan Group is of mantle-derived signature. Unlike the UHP metaigneous rocks in the Dabie-Sulu orogenic belt that show unusual 18O-depletion, the Jiaobei metamorphic rocks have basically preserved their original mantle-like O isotope compositions. In general, the nature and timing of geological events recorded in the metamorphic rocks from the Jiaobei terrane are comparable with those from the North China Block rather than the South China Block. Thus, the Jiaobei terrane is concluded to have tectonic affinity to the former, but behave like a micro-continent during the Triassic continental collision. The ∼1.76 Ga regional metamorphism in the Jiaobei terrane is likely related to reworking of the arc-continent collisional orogen in the periphery of the North China Block rather than the ∼1.85 Ga collision event between the eastern and western North China Blocks. The present study lends support to the common assumption that the suture boundary between the North and South China Blocks in the Sulu orogen is located along the Wulian-Yantai fault. Tectonic mingling along the Wulian-Yantai fault is probably related to subduction erosion during the continental collision.

Molybdenite Re–Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019 ± 3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982 ± 3 and 947 ± 3 Ma in the amphibolite-facies domain (three deposits) and between 953 ± 3 and 931 ± 3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.

Alice Springs age shear zones from the southeastern Reynolds Range, central Australia

The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and 40Ar–39Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained 40Ar–39Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.

Timing of metasomatic alteration of the Archean Kidd Creek massive sulfide deposit, Ontario, using 40 Ar- 39 Ar laser dating of single crystals of fuchsite

Laser-generated, step-heated 40 Ar- 39 Ar dates of single grains of fuchsite have been determined from the alteration footwall of the Archean Kidd Creek massive sulfide deposit. The grains are very small (50-200 mu m in diam) yet give age spectra showing excellent plateaus for 90 percent or more of their respective gas releases, with an average integrated age of 2618 + or - 8 Ma (20Sigma ). This result provides a date which is interpreted as relating to a K metasomatic alteration event at Kidd Creek; this is consistent with earlier suggestions, based on a Pb isochron and U-Pb dating, that some alteration was significantly younger than the felsic volcanic rocks (2717 + or - 2 Ma) hosting the massive sulfides. The 40 Ar- 39 Ar laser date may also provide the constraint for the fact that the sericite-fuchsite alteration postdated the 2.67 to 2.69 Ga regional metamorphism in the Abitibi belt. The age of 2618 + or - 8 Ma for K metasomatism is coincident with an age of 2617 + or - 8 Ma for fuchsite from the nearby Hollinger gold mine. Late processes dated by fuchsite and rutile from gold deposits of the Abitibi belt generally fall between 2.58 and 2.63 Ga. This range of ages between the various ore deposits suggests that hydrothermal activity may not have been synchronous at all deposits and/or some ore deposits were affected by more than one thermal episode.