Rock Isochron Age Research Articles

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

AbstractThe 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.

Cryptic excess argon in metamorphic biotite: Anomalously old 40Ar/39Ar plateau dates tested with Rb/Sr thermochronology and Ar diffusion modelling

In 40Ar/39Ar geochronology, excess Ar is 40Ar that does not derive from the in situ radioactive decay of 40K, or from the measurable input of atmospheric Ar, and results in increased daughter-parent ratios that correspond to anomalously old apparent dates without geological age significance. Excess 40Ar is commonly identified by a saddle-shaped 40Ar/39Ar degassing spectrum. However, biotite from the east Albany-Fraser Orogen of Western Australia contains excess 40Ar, but yields well-defined 40Ar/39Ar plateau dates, reproducible upon replicate analysis of biotite from the same sample. This “cryptic excess 40Ar” is inferred where plateau dates (1) are older than existing time constraints on cooling, such as U/Pb zircon ages for amphibolite to granulite facies metamorphism, and (2) vary between multiple samples from the same outcrop that have experienced the same thermal history. Rb/Sr geochronology is used to test the geologic significance of 40Ar/39Ar plateau dates, as the closure temperatures of both chronometers are similar, and the two chronometers are expected to yield similar cooling dates in an undisturbed system.Six Rb/Sr biotite-whole rock isochron ages from three outcrops yield a weighted mean age of 1133 ± 3 Ma (MSWD = 1.13, P = 0.34), and are interpreted to record post-orogenic cooling. 40Ar/39Ar plateau dates from the same samples are 31–394 Ma older, and in six of nine samples cannot be explained by simple monotonic cooling, by low-temperature biotite recrystallisation, or by alteration; instead, biotite contains cryptic excess 40Ar. Diffusion modelling of 40Ar/39Ar step-heating plateaus suggests that Ar, both excess and radiogenic, is homogeneously distributed within the crystal lattice, and was incorporated during biotite crystallisation. Biotite likely crystallised in a rock with a high partial pressure of Ar, possibly due to a high rate of radiogenic 40Ar generation in a K-rich lithology, together with a poorly-connected intergranular fluid network that inhibited Ar loss from the rock volume. This is supported by a homogeneous distribution of Ar at the cm scale, with reproducible biotite 40Ar/39Ar dates from each sample. Patchy fluid circulation during metamorphism led to the pervasive but heterogeneous distribution of excess 40Ar across outcrops.Due to the presence of the 40Ar/39Ar plateau, cryptic excess 40Ar is difficult to identify a priori, without comparison to additional geochronological constraints. Where cryptic excess 40Ar on a regional scale has been identified, such as in the eastern Biranup Zone of the Albany-Fraser Orogen, Rb/Sr geochronology may provide a more robust alternative to constrain the cooling path of rocks from high temperatures.

Electron microprobe monazite ages from a tin placer deposit on Bangka Island, Indonesia

A first attempt of Electron microprobe (EMP) based Th-U-total Pb in situ dating of monazites in tailings from a placer deposit (quartz bearing sands) from Bangka Island, Indonesia, revealed for the first time from a single locality in Indonesia an age cluster around 231 Ma as well as two single grains with ages of 1133 Ma and 1916 Ma. The monazite ages from this study agree with detrital zircon ages and whole rock isochron ages from magmatic intrusions, which were reported from various localities all over the Malaysian peninsula, Myanmar and Thailand but not yet from Bangka Island. Perhaps intrusions in the closer vicinity, which could be the source of the monazite under study, have not been discovered so far or the sedimentary catchment area of the placer deposit is much larger than assumed so far. The existence of the monazite ages investigated in this study may also raise the question of the paleogeographic position of Bangka Island and the relative timing of sedimentation and Miocene rifting. However, the present work is an excellent example for the resistance of monazite and its Th-U-Pb system to sedimentary processes and hence is a useful tool for provenance studies and source of Rare Earth Element (REEs) recovery. An important outcome of this study is that sedimentary monazite relics often provide a wide range of unrealistic ages. Such dates may be wrongly explained in terms of recrystallization along with metasomatic process, instead these ages are only a methodic artefact because monazite relics are either not thick enough for dating or show poor surfaces with numerous cracks.

Zircon U-Pb age and Nd isotope geochemistry of latest Neoproterozoic to early Paleozoic Oeyama ophiolite: Evidence for oldest MORB-type oceanic crust in Japanese accretionary system and its tectonic implications

The Oeyama ophiolite is considered as one of the major ophiolites in the Japanese Islands that forms a part of the “circum-Pacific Phanerozoic multiple ophiolite Belts”. In this study, zircon U-Pb age and their REE geochemistry, and Nd-Sr isotope geochemistry are presented for the gabbroic rocks of the Oeyama ophiolite to determine the precise crystallization age and characterize the proper tectonic setting of this ancient oceanic crust. The zircon grains of the two studied gabbroic samples yield 206Pb/238U ages of 545.4 ± 2.6 Ma and 532.4 ± 3.1 Ma. From five gabbroic rocks, a comparable 566 ± 95 Ma Sm-Nd whole rock isochron age is obtained. The time-corrected εtNd values (t = 545 Ma) for each sample are ranging from +7.2 to +7.7 that suggest their normal mid-oceanic basalt (N-MORB) affinity. Discrimination diagrams using zircon U/Yb versus Hf and Y also indicate their oceanic crustal origin. This is so far known the oldest date of ophiolite in the Japanese accretionary system. On the basis of the present dataset, we have offered a scenario of the potential tectonic development highlighting the journey of this group of oceanic crustal rocks from the mid-oceanic region till their final accretion to the Japanese Islands. The present tectonic model envisages an initial oceanward subduction at ca. 530 Ma switched over to the present continentward subduction at ca. 400 Ma. Moreover, the three different units of the Oeyama ophiolite have been re-classified based on the present age dataset, and the formation and evolution of these units constituting the typical lower part of the ocean plate are explained.

Geochemistry and Sm[sbnd]Nd isotope systematics of mafic-ultramafic rocks from the Babina and Mauranipur greenstone belts, Bundelkhand Craton, India: Implications for tectonic setting and Paleoarchean mantle evolution

The Paleoarchean geodynamical evolution of the Bundelkhand Craton is discussed based on the geochemistry (major, trace and rare earth elements) and SmNd isotope systematic of mafic-ultramafic rocks from volcano-sedimentary succession of the Babina and Mauranipur greenstone belts. The petrography and mineral assemblages of these mafic-ultramafic rocks imply that they were gone through greenschist to amphibolite facies metamorphism. Geochemical analyses indicate that mafic rocks from the Babina are characterized by SiO2 = 43.9–51.2 wt%, MgO = 5.4–11.0 wt%, TiO2 = 0.3–1.14 wt% and Mg# = 44–77, whereas the Mauranipur are characterized by higher silica (51.8–55.6 wt%), MgO = 6.9–9.5 wt% and Mg# = 59–70. The ultramafic rocks from the Babina and Mauranipur contain SiO2 = 46.9–50.3 wt%, MgO = 20.2–21.1 wt%, TiO2 = 0.33–0.46 wt% and Mg# = 77–82. Six samples of mafic-ultramafic rocks from the Babina greenstone belt yield 147Sm143Nd whole rock isochron age of ca. 3.4 Ga. SmNd isotopic studies of mafic rocks from the Babina and Mauranipur reveal that they were derived from a depleted mantle source. Bivariate diagrams such as Cr vs. Ni and V suggest that these magmas might have evolved through fractionation of clinopyroxene and olivine. The mafic rocks from the Babina displayed almost a flat REE and HFSE profile [(La/Yb)PM = 0.87–1.40] with negative Nb (Nb/Nb* = 0.13–0.77) and positive Pb anomalies that could be attributed to metasomatic agents derived from subducting crustal rocks. The Mauranipur mafic rocks show slightly enriched REE [(La/Yb)PM = 1.1–1.7; (La/Sm)PM = 1.1–2.0] with negative Nb anomalies (Nb/Nb* = 0.13–0.77), which are most likely the effects of crustal contamination. The ultramafic rocks from both areas and ultramafic cumulate rock from the Mauranipur display a progressive enrichment from Th, La to Yb, with negative Nb anomalies (Nb/Nb* = 0.40–0.73) with lack of Zr anomalies (Zr/Zr* = 0.10–0.30) that could be attributed to their derivation from shallower mantle. These data collectively suggest that the mafic-ultramafic rocks have been interpreted to be derived from oceanic crust in a subduction-related setting with depleted mantle composition.

Geology and petrogenetic considerations of the Loma Marcelo skarn, Neoproterozoic basement of the Ventania System, Argentina

The Late Precambrian–Early Palaeozoic crystalline basement of the Ventania System is mainly composed of Neoproterozoic S-type granites, Early Cambrian alkaline granites, and Middle Cambrian peralkaline rhyolites. The Neoproterozoic granites (ca. 607–581Ma) crop out in the Cerro Pan de Azúcar-Cerro del Corral area and host the Loma Marcelo Ca/Mg skarn, composed of calc-silicate- and chondrodite-bearing assemblages. Petrographical, geochemical, and geochronological data indicate that the skarn was formed from carbonate xenoliths of sedimentary nature incorporated during the intrusion of the Neoproterozoic granites and that it is the result of three successive metamorphic events. The first one is related to the incorporation of carbonate xenoliths by the intrusion and the associated contact metamorphism. The second event is linked to the crystallization of the Early Cambrian granites and related post-magmatic processes. They are responsible for fluorine influx into the Neoproterozoic basement, which resulted in the formation of high-F minerals in the Loma Marcelo skarn (e.g. vesuvianite, chondrodite). The Sm-Nd garnet-whole rock isochron age of 526.8±12.0Ma in the skarn confirms its relationship with the Early Cambrian magmatism. The last event corresponds to a regional metamorphism assigned to the Late Devonian–Early Carboniferous on the basis of a LA-ICP-MS U-Pb age of 353.4±6.7Ma determined on zircons of the skarn as well as a Rb-Sr quartz-sericite isochron age of 364.4±3.6Ma obtained from a quartz vein that post-tectonically cuts the already mylonitized granites. The different chemical compositions of the carbonate protoliths determined the formation of two types of skarns of calcic and magnesian compositions. A single date of the calcic skarn defined a LA-ICP-MS U-Pb upper intercept age of 648±21Ma, providing the maximum sedimentation age for the calcic protolith of the Loma Marcelo skarn. The minimum sedimentation age is provided by the crystallization age of the S-type granite surrounding the skarn (ca. 607Ma). The Loma Marcelo skarn reveals the occurrence of underground Neoproterozoic carbonate (meta-?) sedimentary rocks as part of the basement of the Ventania System. These rocks could correlate with the carbonate sequences of the neighbouring Claromecó Basin and Tandilia System.

255 生駒山地南部石英閃緑岩の地球化学的性質とそのRb-Sr年代

255 生駒山地南部石英閃緑岩の地球化学的性質とそのRb-Sr年代

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

AbstractThe P–T–t path of high‐P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post‐Pmax heating–decompression and subsequent isothermal decompression following the attainment of peak high‐P and low‐T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.

Geochemical constraints on komatiite volcanism from Sargur Group Nagamangala greenstone belt, western Dharwar craton, southern India: Implications for Mesoarchean mantle evolution and continental growth

We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites (northern extension of Nagamangala belt) and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca. 3.2–3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination. About 1/3 of studied komatiites show Al-depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, (Gd/Yb)N ratios show low CaO/Al2O3 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaO/Al2O3 ratios. The elemental characteristics of Al-depleted komatiites such as higher (Gd/Yb)N (>1.0), CaO/Al2O3 (>1.0), Al2O3/TiO2 (<18) together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet (majorite?) involvement in residue whereas lower (Gd/Yb)N (<1.0), CaO/Al2O3 (<0.9), higher Al2O3/TiO2 (>18) together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE) indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle (depleted to primitive mantle) at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6–3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton (WDC).

Mesoproterozoic magmatic events in the eastern North China Craton and their tectonic implications: Geochronological evidence from detrital zircons in the Shandong Peninsula and North Korea

Whether any Grenvillian magmatic records are preserved in the North China Craton (NCC) is a key issue to understand the Proterozoic tectonic evolution of the NCC and its correlation to the supercontinent Rodinia. Meso- to Neo-proterozoic sedimentary series is well exposed in the NCC, but magmatic events in this period, especially of 1.3–1.0Ga, have seldom been reported. New U–Pb isotopic dating and Hf isotopic composition analyses have been carried out in this study using SIMS and LA–ICP-MS methods on detrital zircons from sandstones of the Tumen Group in the Shandong Peninsula and quartz sandstones of the Sangwon System in the Phyongnam Basin, North Korea. The age populations of the detrital zircons of the Tumen Group are at ~2.5Ga, ~1.85Ga, ~1.7Ga, ~1.58Ga, ~1.5Ga, ~1.36Ga and ~1.2Ga and those of the Sangwon System are at 1.88–1.86Ga, ~1.78Ga, 1.62–1.58Ga, 1.46–1.41Ga, ~1.32Ga, ~1.17Ga and ~980Ma. Most of the age peaks of Neoarchean and Proterozoic correspond to the significant tectonic-magmatic-thermal events previously recognized in the NCC, revealing that the main provenances of the Tumen Group and the Sangwon System are Early Precambrian basement and Late Paleo- to Meso-proterozoic magmatic rocks of the NCC. Furthermore, the youngest detrital zircon ages of ~1.1Ga from the Tumen Group and 984Ma from the Sangwon System, as well as 910Ma Rb–Sr whole rock isochron age of a limestone from the Tumen Group and 899Ma mafic sills intruding the Sangwon System suggest that both groups were deposited in the Neoproterozoic, coevally with the Qingbaikou System in the Yanliao Aulacogen. The common zircon ages of 1.3–1.0Ga from the Tumen Group and the Sangwon System, as well as the contemporaneous Penglai and Yushulazi Group in the eastern margin of the NCC, indicate that during the deposition of these sediments there have been significant contributions from Grenvillian magmatic rocks in the eastern NCC. This may provide clues to understand the possible relationship of the NCC and the supercontinent Rodinia. Moreover, the positive εHf (t) and ~2.8Ga crust model ages of detrital magmatic zircons of 2.8–2.4Ga suggest that there have been significant crustal growth at ~2.8Ga in the eastern margin of the NCC, same as in other areas of the NCC.

Geochronological (Rb-Sr and Sm-Nd) studies on intrusive gabbros and dolerite dykes from parts of Northern and Central Indian cratons: Implications for the age of onset of sedimentation in Bijawar and Chattisgarh basins and uranium mineralisation

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (TDM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial 87Sr/ 86Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛNdi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial 87Sr/86Sr ratio of 0.7098 and ɛNdi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (TDM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial 87Sr/86Sr and low initial 143Nd/144 ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.

Relationship between metamorphism and ore formation at the Sukhoi Log gold deposit hosted in black slates from the data of U-Th-Pb isotopic SHRIMP-dating of accessory minerals

The formation conditions and age of the Sukhoi Log gold deposit are considered on the basis of new isotopic-geochemical data. The U-Pb isotopic study of zircon and monazite from high-grade ore and host black slates at the Sukhoi Log deposit was carried out with SIMS technique using a SHRIMP II instrument. Two generations of monazite are distinguished on the basis of optical and scanning electron microscopy, cathodoluminescence, and micro X-ray spectroscopy. Monazite I is characterized by black opaque porphyroblasts with microinclusions of minerals pertaining to metamorphic slates and structural attributes of pre- and synkinematic formation. Monazite II occurs only within the ore zone as transparent crystals practically free of inclusions and as rims around monazite I. The REE contents are widely variable in both generations. Porphyroblastic monazite I differs in low U and Th (0.01–0.7 wt % ThO2) contents, whereas transparent monazite II contains up to 4 wt % ThO2. The average weighted U-Pb isotopic age of monazite I is 650 ± 8.1 Ma (MSWD = 1.6; n = 9) and marks the time of metamorphism or catagenesis. The U-Pb age estimates of synore monazite II cover the interval of 486 ± 18 to 439 ± 17 Ma. Zircons of several populations from 0.5 to 2.6 Ga in age are contained in the ore. Most detrital zircon grains have porous outer rims composed of zircon and less frequent xenotime with numerous inclusions of minerals derived from slates. The peaks of 206Pb/238U ages in the most abundant zircon populations fall on 570 and 630 Ma and correspond to the age of newly formed metamorphic mineral phases. The discordant isotopic ages indicate that the U-ThPb isotopic system of ancient detrital zircons was disturbed 470–440 Ma ago in agreement with isotopic age of monazite II and the Rb-Sr whole -rock isochron age of black slates (447 ± 6 Ma). The new data confirm the superimposed character of the gold-quartz-sulfide mineralization at the deposit. Black shales of the Khomolkho Formation of the Bodaibo Synclinorium were affected by metamorphism over a long period; the peaks of metamorphism and catagenesis are dated at 570 and 650–630 Ma. The high-temperature ore formation was probably related to a hidden granitic pluton emplaced 450–440 Ma ago, that is, 200 Ma later than the events of greenschist metamorphism. Hercynian granitoid magmatism (320–270 Ma) did not exert a substantial effect on the U-Th-Pb isotopic system in accessory minerals from the ore and could not have been a major source of ore-forming fluids.

Geology of Pulikonda and Dancherla alkaline complexes, Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively. Dancherla syenite body has yielded a six point Rb-Sr whole rock isochron age of 2211±110 Ma with low initial 87Sr/86Sr ratio of 0.7004±0.00046; MSWD = 5.8.The low initial Sr(i) ratio suggests mantle derived juvenile additions to the crust. The Pulikonda syenite body yielded a six point whole rock isochron age of 1500 ±100 Ma with initial 87Sr/86Sr ratio of 0.7085±0.0047 and MSWD = 2%. The higher initial Sr(i) value for this body suggests metasomatised source. Resetting of Rb-Sr systematics has resulted younger age for Pulikonda syenite though it is supposed to be older as per the geological setting and this anomaly is interpreted interms of repeated deformation and reactivation of shear zone, emplacement of alkali granite into Pulikonda syenite and the role of Rudravaram shear zone (basement tectonics) in the formation of the Nallamali fold belt of the Cuddapah Basin. Alkaline magmatism was initially triggered along microplate margin tectonic setting and later confined to the intraplate tectonic setting.

Utility of Rb–Sr geochronology in constraining Miocene and Cretaceous events in the eastern Karakoram, Ladakh, India

Multiple magmatic and deformational events, resulting from accretion of Gondwanan terranes to the northern margin of India and followed by post-accretion tectonics, have been recognized from the eastern Karakoram. The time resolution of these events is important for understanding the geodynamic evolution of the region. To constrain certain major events, conventional Rb–Sr geochronology was applied to the various phases of granitoids and their mineral separates, from mainly within the Pangong transpressional zone of the dextral Karakoram fault (KF), in order to show that this technique can date such young events. Calcalkaline gneissic granodiorite from within the transpression zone and calcalkaline tonalite enclaves within the Karakoram batholith (KB) at Skyangpoche, yield a whole rock date of 118±15 Ma, MSWD 1.5, Sr i of 0.7045±0.0002 and whole rock-biotite age of 115±18 Ma with Sr i of 0.7061±0.0002 and MSWD=1.5, respectively. Four samples of a mesocratic foliated granite within the Pangong migmatite, near the NE strand of the KF, yielded an errorchron corresponding to an age of 72±8 Ma, MSWD of 39 and Sr i of 0.7041±0.0008. Phenocrysts of phlogopite from an ultrapotassic lamprophyre dyke within the Ladakh batholith, near Leh, yield an age of 24.3±2.2 Ma, with Sr i of 0.7083±0.0001. A muscovite-whole rock isochron age of 13.3±0.6 Ma with Sr i of 0.7249±0.0001, and MSWD=1.5 was measured from the Darbuk leucogranite, on the SW strand of the KF. A foliated granite, proximal to Muglib on the NE strand of the KF, yields a whole rock-biotite age of 12.4±0.7 Ma and Sr i of 0.7056±0.0001. The Early Cretaceous ages of the granitoids, comprising early pre-Miocene plutonic components of the KB, are consistent with an U-Pb SHRIMP age of ∼106 Ma, and can be spatially correlated with the K2 gneiss (115–120 Ma) and the Hunza Plutonic Unit (106 Ma) of the KB in northern Pakistan. The Late Cretaceous age of foliated granite along the KF confirms that calcalkaline plutonism extended from Tirit, near the confluence of Nubra and Shyok rivers, to Rutog in western Tibet and was probably related to plutonism in the Ladakh magmatic arc. The earliest Miocene magmatism in SE Ladakh is represented by the ultrapotassic lamprophyre dyke at 24 Ma and spatially represents the westernmost extension of the voluminous ultrapotassic and potassic lavas in SW Tibet at this time (17–25 Ma). Following the intrusion of the main KB-forming leucogranite at 15 Ma (U-Pb SHRIMP crystallization age from zircon rims), the time of shearing on the SW strand of the KF is recorded by the age of muscovite from the Darbuk leucogranite at 13.3 Ma, comparable to that measured by Ar isotopic systematics (13.9 Ma) on mica aggregates from a granodiorite deformed by the KF in the Nubra valley. The biotite cooling age of the foliated granite within the Pangong migmatite is consistent with its cooling following high-grade migmatization, previously measured as having occurred between 17 and 14 Ma.

Dating the geologic history of Oman’s Semail ophiolite: insights from U-Pb geochronology

Eclogites from the deepest structural levels beneath the Semail ophiolite, Oman, record the subduction and later exhumation of the Arabian continental margin. Published ages for this high pressure event reveal large discrepancies between the crystallisation ages of certain eclogite-facies minerals and apparent cooling ages of micas. We present precise U-Pb zircon (78.95 ± 0.13 Ma) and rutile (79.6 ± 1.1 Ma) ages for the eclogites, as well as new U-Pb zircon ages for trondhjemites from the Semail ophiolite (95.3 ± 0.2 Ma) and amphibolites from the metamorphic sole (94.48 ± 0.23 Ma). The new eclogite ages reinforce published U-Pb zircon and Rb-Sr mineral-whole rock isochron ages, yet are inconsistent with published interpretations of older 40Ar/39Ar phengite and Sm-Nd garnet dates. We show that the available U-Pb and Rb-Sr ages, which are in tight agreement, fit better with the available geological evidence, and suggest that peak metamorphism of the continental margin occurred during the later stages of ophiolite emplacement.

Tectonic significance of a Llanvirn age for the Dunn Point volcanic rocks, Avalon terrane, Nova Scotia, Canada: implications for the evolution of the Iapetus and Rheic Oceans

The Paleozoic evolution of Avalonia is crucial to the understanding of the development of the Appalachian orogen and the Iapetus and Rheic Oceans. Paleomagnetic, faunal, structural and isotopic data indicate that Avalonia had accreted to Laurentia by the Early Silurian. In the Avalon terrane of Nova Scotia, Ordovician–Early Devonian rocks consist of bimodal volcanic rocks at the base (Dunn Point Formation) disconformably overlain by a thick sequence of fossiliferous siliciclastics (Arisaig Group) which contain Llandoverian to Lochkovian fossils. Until now, the only published age data for the volcanic rocks yielded an imprecise Rb–Sr whole rock isochron age of 421±15 Ma and the volcanism has been interpreted to reflect local extension and basin development related to the oblique collision between Avalonia and Laurentia after closure of the Iapetus Ocean. We present U–Pb zircon data from a rhyolite which indicate an age of 460.0±3.4 Ma for the Dunn Point Formation, an age that requires a re-interpretation of its tectonic setting. The Dunn Point volcanism probably developed on the Avalonian microcontinent outboard from both Laurentia and Gondwana, possibly in a rifted arc setting, analogous to the modern Taupo Zone in northern New Zealand. A Llanvirn age for these felsic rocks also reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for Avalonia in the Ordovician–Silurian and implies 10° northward movement of Avalonia between 460 and 440 Ma and about 5.5 cm/year for the latitudinal component of the convergence between Avalonia and Laurentia (i.e. the destruction of the Iapetus Ocean) and divergence between Gondwana and Avalonia (development of the Rheic Ocean) during this interval.

Petrochemical and Sr-Nd isotope investigations of Cretaceous intrusive rocks and their enclaves in the Togouchi-Yoshiwa district, northwest Hiroshima prefecture, SW Japan

Petrographic, petrochemical and Sr-Nd isotopic data are presented for granitoids and microdioritic enclaves from two Cretaceous stocks (Togouchi granodiorite and Tateiwayama granite porphyry) from the Togouchi-Yoshiwa district, northwest Hiroshima prefecture, SW Japan. The data are used to examine the genetic relationships between the microdioritic enclaves and their granitoid hosts. The granodiorite, granite porphyry and the microdioritic enclaves are all calc-alkaline in nature, and belong to the I-type ilmenite series. The Togouchi granodiorite has a Rb-Sr whole rock isochron age of 85.6 ± 4.7 Ma with an initial Sr isotope ratio (SrI) of 0.70634 ± 0.00012 (2σ). The Tateiwayama granite porphyry has a slightly younger Rb-Sr whole isochron age (77.4 ± 3.1 Ma) but similar SrI of 0.70653 ± 0.00015, suggesting that both stocks may have been derived from the same source. Despite diverse whole rock chemistry, the microdioritic enclaves in the respective intrusives have quite similar initial Sr and Nd isotope ratios, suggesting that they formed by fractional crystallization of a single magma, and also that the source of the enclaves in both intrusives had similar geochemical characteristics. In both stocks, however, the enclaves have distinctly lower initial Sr isotope ratios than their respective host rocks, indicating that they were derived from a different source than their hosts. In view of the geochemical and Sr-Nd isotope data, we infer that the enclave magmas were derived from a similar LILE- and LREE-enriched source to that of the Cretaceous basalts and gabbroic-dioritic rocks that are sporadically distributed in SW Japan. Such mafic to intermediate magmas were probably derived from the upper mantle, and transferred both heat and material to the lower crust, thus producing granitic magmas by partial melting. Successive mafic magmas or their differentiates could then have been injected into the granitic magma chamber, trapped and quenched, resulting in the formation of microdioritic enclaves in both granitic stocks.

Geologia e geocronologia da suíte metamórfica colorado e suas encaixantes, SE de Rondônia: implicações para a evolução mesoproterozóica do SW do cráton Amazônico

Combined geochronological (U-Pb, 40Ar/39Ar, and Rb-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondônia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: 1) porphyritic monzogranite associated with amphibolite (bimodal magmatism), 2) interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation), 3) muscovite-garnet leucogranite, and 4) mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as undeformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0.18). The porphyritic ortogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 + 45 Ma, and 87Sr/86Sr i = 0.7040 + 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age of 1360 + 13 Ma. The 40Ar/39Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 + 3 Ma (grain 1), and 1303 + 2 Ma/1305 + 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 + 2 Ma. Hornblende from two amphibolite samples yielded 40Ar/39Ar plateau ages of 1313 + 4 Ma; 1313 + 6 Ma and 1312 + 3 Ma (RO-18), and 1325 + 3 Ma; 1326 + 2 Ma; 1330 + 3 Ma (RO-19). The weighted-mean age is 1319 + 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondônia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilândia Terrane (1215 - 1110 Ma). However, the unequivocal characterization of an orogeny refered to the time interval 1.36 - 1.32 Ga is still uncertain due to scarcity of geochronological data in the region. Similar ages are reported for correlatable intrusive rocks which cut the polydeformed basement rocks of central Rondônia. Thus, these ages are compatible with available Mesoproterozoic tectonic models for the SW Amazonian Craton.

Mid-paleozoic collision in the north Qinling: Sm–Nd, Rb–Sr and 40Ar/39Ar ages and their tectonic implications

Different isotopic geochronometers have been used to study the nature of the mid-Paleozoic tectonothermal event in the North Qinling arc system at the southern margin of the North China Block. 40Ar/39Ar age of amphibole from sample QH-3 (426±2Ma), Rb–Sr mineral isochron age (411±5Ma) of sample QSP-10 of the Danfeng Group and Rb–Sr whole rock isochron age (406±22Ma) of a greenschist facies pillow lava sample of the Erlangping Group indicate a widespread Late Silurian tectonothermal event in the North Qinling area. The data also suggest a limited uplift in the North Qinling during the Late Silurian tectonothermal event. This event is restricted to the North Qinling area and the southern margin of North China Block, and not observed in the South Qinling belt and the northern margin of the South China Block. Consequently, it is related to the collision between the North China Block and a North Qinling arc system, which resulted in the closure of the North Qinling back-arc basin(s).

Petrochemistry and geochemistry of HP metabasites from Haiyangsuo in Sulu UHP belt of eastern China

Abundant metabasites occur in highly deformed granitic and migmatitic gneisses as blocks and lenses of tens of meter size around the Haiyangsuo area, northeast part of Sulu UHP belt, eastern China. They comprise garnet-pyroxene granulites, eclogitized granulites and amphibolites. Their protolith compositions were mainly olivine tholeiite and quartz tholeiite, and show variation from Mg-rich to Fe-rich component as tholeiitic cumulates. Pearce’s element ratio slopes suggested that protolith of these rocks were comagmatic, and generated from a primary magma by fractional crystallization of plagioclase, olivine and clinopyroxene. The crystallization differentiation has also been evidenced by trace elements, such as parallel REE patterns, Ni vs Ce variations, Sr increasing depletions, although the large ion lithophile elements (LILE) were modified to different extent during metamorphism. Trace element composition and Nd isotopes indicate a depleted mantle origin for these rocks. But they are not likely to be fragments of ophiolites or tholeiites connected with subduction, they formed probably at intra-continent environment. Sm-Nd whole rock isochron age of 2252±180Ma indicates approximately the formation age of igneous protolith of these rocks, almost 2000Ma earlier than the formation of the Dabie-Sulu UHP collision zone at about 240–220 Ma.