Hornblende Dates Research Articles

Constraints from lamprophyre petrogenesis on the timing of Eocene lithospheric thinning and associated rifting of Borneo and Sulawesi

The petrogenetic history of well characterised magmatic rocks may contain invaluable information of paleo-lithospheric thickness and structure that can inform on tectonic evolution. Petrographically distinctive rocks, such as lamprophyres, provide temporal and spatial constraints to build, assess, and verify tectonic models.An alkaline lamprophyre dyke that intrudes the rift-related sediments of the Eocene Tanjung Formation of the Senakin Peninsula in South Kalimantan, Indonesia has been investigated using petrology, whole rock geochemistry and Ar-Ar geochronology.The Senakin lamprophyre dyke is mostly geochemically homogenous but petrographically stratified with vesicle free monchiquite alkaline lamprophyre mineralogy at the base and an amygdale bearing kaersutite phenocryst free upper zone. The major and trace element chemistry is consistent with an intra-plate Ocean Island Basalt like affinity. 40Ar/39Ar dating of hornblende and groundmass aliquots indicates an emplacement age of 37.45 ± 0.10 Ma (2σ), which is consistent with the Late Eocene age of the Tanjung Formation derived from microfossil analysis.The petrology and geochemistry of the Senakin lamprophyre dyke is consistent with magma genesis at > 2GPa beneath a thick lithosphere followed by fractional crystallisation at Moho depths equivalent to 0.5–0.8 GPa. This indicates a much thicker southeastern Kalimantan paleo-lithosphere. Lamprophyre magmatism was initiated by lithospheric extension associated with the opening of the Makassar Strait and also regional thermal upwelling. This extension continued to thin the lithosphere to its current thickness equivalent to 0.5 GPa.

Torn Between Two Plates: Exhumation of the Cer Massif (Internal Dinarides) as a Far‐Field Effect of Carpathian Slab Rollback Inferred From 40Ar/39Ar Dating and Cross Section Balancing

AbstractExtension across the southern Pannonian Basin and the internal Dinarides is characterized by Oligo‐Miocene metamorphic core complexes (MCCs) exhumed along mylonitic low‐angle extensional shear zones. Cer MCC at the transition between Dinarides and Pannonian Basin occupies a structural position within the distal‐most Adriatic thrust sheet and originates from two different tectonic processes: Late Cretaceous‐Paleogene nappe‐stacking during a continent‐continent collision with Adria in a lower plate position, and exhumation related to Miocene extension driven by the Carpathian slab‐rollback. Structural data and a balanced cross section across the Cer massif show linking of the exhuming shear zone to a breakaway fault, which reactivated the early Late Cretaceous most internal nappe contact. Paleozoic greenschist‐to amphibolite‐grade lithologies surround a polyphase intrusion composed of I‐ and S‐type granites and were exhumed along a shear zone characterized by top‐N transport. Thermobarometric analyses indicate an intrusion depth of 7–8 km of the Oligocene I‐type granite; cooling below ∼500°C occurred at 25.4 ± 0.6 Ma (1σ) yielded by 40Ar/39Ar dating of hornblende. Biotite and white mica from this intrusion as well as from the mylonitic shear zone yield 40Ar/39Ar cooling ages of 17–18 Ma independent of the used techniques (in situ laser ablation, single‐grain total fusion, single‐grain step heating, and multi‐grain step heating). White mica from the S‐type granite yield an 40Ar/39Ar cooling age of 16.7 ± 0.1 Ma (1σ). Associated dikes intruding the shear zone were also affected by N‐S extension resulting in the exhumation of the MCC, which was triggered by the opening of the Pannonian back‐arc basin in response to the Carpathian slab‐rollback.

Paleoproterozoic metamorphism of metaultramafic rocks in the Miyun area, northeastern North China Craton

The Miyun area is located in the northwestern border of the Eastern Block of the North China Craton, where the metaultramafic rocks occur as intrusive bodies, some of which display cumulate features. The metaultramafic rock is (olivine-bearing) amphibole websterite and two generations of mineral assemblage have been identified: the magmatic mineral assemblage -consists- of clinopyroxene + orthopyroxene ± olivine ± spinel ± pyrite, whereas the subsequent metamorphic mineral assemblage comprises of hornblende + serpentine + chlorite + magnetite + zircon ± biotite ± calcite ± chromite ± calcite ± garnet. The metamorphic P-T conditions are estimated to be ~ 600–630 °C/4–6 kbar, which belongs to amphibolite facies and medium-P/T facies series. U-Pb dating of metamorphic zircon and 40Ar/39Ar dating of metamorphic hornblende indicate that the metamorphic event occurred at ~ 1.85–1.82 Ga. These data suggest that the Miyun area involved in the late Paleoproterozoic subduction-collision between the Eastern and Western Blocks, and the metamorphism possibly occurred at upper-middle crustal level with fluids.

40Ar/39Ar dating of hornblende and U-Pb dating of zircon in the Aketashitage orogen, NW China: Constraints on exhumation and cooling in the Paleoproterozoic

The Paleoproterozoic Aketashitage orogenic belt lies in between the two large accretionary orogenic systems, i.e., Paleo-Asian ocean and Paleo-Tethys ocean systems, in northwest China. Previous geochronologic studies reveal widespread Paleoproterozoic orogeny-related magmatism and high-grade metamorphism in this area, determined by the U-Pb dating of zircon and monazite. In this paper, 40Ar/39Ar dating of hornblende of three representative amphibolite samples yielded cooling ages of ∼1.88–1.85 Ga. Comparison of the zircon/monazite U-Pb and hornblende 40Ar/39Ar ages suggests that these amphibolite samples record different exhumation and cooling processes. These new geochronologic data further confirm that the Aketashitage orogenic belt only records Paleoproterozoic metamorphism but no overprint of Paleozoic thermal event. It is thus inferred that the Aketashitage area might act as a continental fragment/terrane which was passively collaged onto the ambilateral Paleozoic orogens but was not involved in the Paleozoic subduction.

New geochronological evidences of late Neoarchean and late Paleoproterozoic tectono-metamorphic events in the Miyun area, North China Craton

The Miyun area is located in the western margin of the Eastern Block of the North China Craton (NCC), and granulite- to amphibolite facies tectono-metamorphic events occurred in the Precambrian. In this contribution, high-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon from garnet-pyroxene amphibolite yielded the metamorphic age of 2507 ± 4 Ma. SIMS U-Pb dating of monazite from garnet-biotite gneiss yielded the metamorphic ages of 2496 ± 16 Ma and 1834 ± 21 Ma, respectively. Furthermore, 40Ar/39Ar dating of hornblende from amphibolite yielded plateau ages of 1817 ± 41–1795 ± 19 Ma, and the biotite yielded a plateau age of 1744 ± 12 Ma. Combined with previous studies and the closure temperatures of the dating minerals, it is inferred that the Miyun area experienced a late Neoarchean granulite-facies metamorphic event occurred at ~2.50 Ga, followed by a late Paleoproterozoic amphibolite-facies metamorphic event occurred at ~1.83–1.74 Ga. These two tectono-metamorphic events are ascribed to the Precambrian orogenic-related environments.

Metamorphic P-T path and geochronology of garnet-bearing amphibolite of the Inyoni Shear Zone, southwestern Barberton Greenstone Belt, South Africa

Geothermobarometry and phase equilibria modeling were conducted for garnet-bearing amphibolite of the Inyoni Shear Zone (ISZ), southwestern Barberton Greenstone Belt, South Africa. The prograde (M1), metamorphic peak (M2) and retrograde (M3) metamorphic P-T conditions were estimated to be ∼560 °C/∼8 kbar, 660–730 °C/7–9.5 kbar and 570–630 °C/5–6 kbar, respectively. The amphibolite records clockwise P-T paths, and the metamorphic peak P-T conditions belong to the medium P/T facies series. The retrograde segment of the P-T path documents near isothermal decompression which may reflect relative fast exhumation. The estimated apparent geotherm is 20–30 °C/km. SIMS U-Pb dating of metamorphic zircon suggests that the metamorphism of the ISZ amphibolite have occurred during ∼3.26–3.22 Ga, and the metamorphic peak might occur at ∼3.24 Ga. Furthermore, 40Ar/39Ar dating of hornblende indicates that the cooling stage might occur at ∼3.20 Ga. It is anticipated that the protolith of the amphibolite of the ISZ may have been involved in a subduction-like tectonic process along with the TTG plutons and was metamorphosed during 3.26–3.22 Ga, then uplifted from within the surrounding TTG plutons at ∼3.20 Ga.

Geochemical and geochronological evidence for a Middle Permian oceanic plateau fragment in the Paleo-Tethyan suture zone of NE Iran

The mafic–ultramafic Fariman complex in northeastern Iran has been interpreted as a Paleo-Tethyan ophiolitic fragment with subduction- and plume-related characteristics as well as a basin deposit on an active continental margin. Contributing to this issue, we present geochemical, geochronological, and mineralogical data for transitional and tholeiitic basalts. Thermodynamic modeling suggests picritic parental magmas with 16–21 wt% MgO formed at plume-like mantle potential temperatures of ca. 1460–1600 °C. Rare pyroxene spinifex textures and skeletal to feather-like clinopyroxene attest to crystallization from undercooled magma and high cooling rates. Chromium numbers and TiO2 concentrations in spinel are similar to those in intraplate basalts. 40Ar–39Ar dating of magmatic hornblende yielded a plateau age of 276 ± 4 Ma (2σ). Transitional basalt with OIB-like trace element characteristics is the predominant rock-type; less frequent are tholeiitic basalts with mildly LREE depleted patterns and picrites with intermediate trace element characteristics. All samples show MORB-OIB like Pb/Ce, Th/La, and Th/Nb ratios which preclude subduction-modified mantle sources and felsic crustal material. Tholeiitic basalts and related olivine cumulate rocks show MORB-like initial eNd values of + 9.4 to + 6.2 which define a mixing line with the data for the transitional basalts (eNd ca. + 2.6). Initial 187Os/188Os ratios of 0.124–0.293 support mixed sources with a high proportion of recycled mafic crust in the transitional basalts. High concentrations of highly siderophile elements are in agreement with the high mantle potential temperatures and inferred high-melting degrees. It is argued that the Fariman complex originated by melting of a mantle plume component as represented by the OIB-like transitional basalt and entrained asthenosphere predominant in the MORB-like tholeiites. Two lines of evidence such as association of the Fariman complex with pelagic to neritic sedimentary rocks and the tectonic position at the boundary of two continental blocks defined by ophiolites and accretionary complexes of different ages suggest formation in an oceanic domain. Thus, we interpret it as a fragment of an oceanic plateau, which escaped subduction and was accreted as exotic block in the Paleo-Tethyan suture zone.

40Ar/39Ar Dating and Characterization of Hornblende from the Nelson Plutonic Suite, Southern Kootenay Arc, SE B.C.

The Kootenay Arc located in SE B.C. has experienced more than one episode of tectonism, metamorphism and plutonism. The Mid-Jurassic to Eocene thermal history of the area has been investigated using K-Ar and 40/39 dating methods of biotite and muscovite; however there are no reliable hornblende dates from this area. This study will investigate two easterly stocks of the Nelson Plutonic Suite, the Mine and Wall stocks. Both of these have U-Pb zircon ages between 171 and 168 Ma but record a wide range of mica cooling and overprinting dates between 166 Ma in the west and 67 Ma in the east. 40Ar/39Ar age spectra for hornblende from 11 rocks in these stocks comprising a transect of the area, will aid in defining the higher temperature part of the thermal history. Previous attempts to prepare bulk hornblende separates were unsuccessful due to overgrowths and intergrowths of biotite, chlorite, plagioclase and K-feldspar. Part of this study involves testing the efficiency of SELFRAG disaggregation, which uses pulses of electrostatic power to break apart the biotite-hornblende-epidote granodiorite along mineral cleavage planes and grain boundaries. This should lead to higher purity mineral separates and better dates. Scanning Electron Microscopy (SEM) and Microprobe analysis analyse the separates looking for K-rich inclusions and the hornblendes variation in chemistry. Ca/K ratios are typical of igneous amphibole. Combined with previous K-Ar and 40/39 results for micas these hornblende dates should provide some insight into the history of the Next Creek fault and the thermotectonic history of the area.

Metamorphic evolution and geochronology of the Dunhuang orogenic belt in the Hongliuxia area, northwestern China

Garnet-bearing mafic granulites and amphibolites from the Hongliuxia area of the southern Dunhuang orogenic belt, northwestern China, commonly occur as lenses or boudinages enclosed within metapelite or marble, which represent the block-in-matrix feature typical of orogenic mélange. Three to four generations of metamorphic mineral assemblages are preserved in these rocks. In the high-pressure amphibolites, prograde mineral assemblages (M1) occur as inclusions (hornblende+plagioclase+quartz±chlorite±epidote±ilmenite) preserved within garnet porphyroblasts, and formed at 550–590°C and 7.7–9.2kbar based on geothermobarometry. The metamorphic peak mineral assemblages (M2) are composed of garnet+hornblende+plagioclase+quartz+clinopyroxene, as well as titanite+zircon+rutile+apatite as accessory minerals in the matrix, and are estimated to have formed at 640–720°C and 14.1–16.0kbar. The first retrograde assemblages (M3) are characterized by “white-eye socket” symplectites (hornblende+plagioclase+quartz±biotite±epidote±magnetite) rimming garnet porphyroblasts, which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under P–T conditions of 610–630°C and 5.6–11.8kbar. The second retrograde assemblages (M4) are intergrowths of actinolite and worm-like quartz produced by the breakdown of the matrix hornblendes, and formed under P–T conditions of ∼490°C and ∼2.8kbar. For the high-pressure mafic granulites, the prograde assemblages (M1) are represented by plagioclase+quartz preserved within the garnet porphyroblasts. The metamorphic peak assemblages (M2) are garnet+matrix minerals (clinopyroxene+plagioclase+quartz+hornblende+rutile+zircon) and were estimated to have formed at ∼680°C and ∼15.4kbar. The retrograde assemblages (M3) are characterized by fine-grained patches of hornblende+plagioclase+quartz rimming the garnet porphyroblasts, as well as hornblende rimming clinopyroxene in the matrix, and were inferred to have formed at ∼620°C and ∼4.2kbar. For the metapelitic gneiss, the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals (biotite+plagioclase+quartz+ilmenite+zircon), which were estimated to have formed at ∼630°C and ∼8.9kbar. The mafic granulites and amphibolites record fairly similar clockwise P–T paths that include nearly isothermal decompression processes, which suggest that they experienced subduction and subsequent rapid tectonic exhumation. SIMS and LA-ICP-MS U–Pb dating of zircons and 40Ar/39Ar dating of hornblende suggest that the metamorphism occurred at ∼430–390Ma. Field occurrences, different protolith ages of the mafic granulites and amphibolites, and the considerable gap in peak P–T conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a mélange accumulation process during the Paleozoic caused by the Silurian–Devonian orogeny, which is possibly associated with the closure of the Liuyuan ocean, a branch of the Paleo-Asian ocean near the southern Central Asian Orogenic Belt.

The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau, Yamaguchi Prefecture, Japan

AbstractThe relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite‐series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO2, but a compositional gap exists between 70.5 to 73.4 wt.% SiO2 (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San‐yo Belt, which is genetically related to the ilmenite‐series granitoids of the Kanmon to Shunan stages.The aplite contains Cl‐rich apatite and REE‐rich monazite‐(Ce), allanite‐(Ce), xenotime and bastnäsite‐(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three‐phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two‐phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.

Remnants of ancient Australia in Vanuatu: Implications for crustal evolution in island arcs and tectonic development of the southwest Pacific

We report new geochemical and geochronological data from igneous rocks of the little studied western belt of the Vanuatu intraoceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place the time of formation of these rocks in the late Eocene to Miocene; therefore, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general matches the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not been found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intraoceanic arc provides an alternative source for the crustal trace element and isotopic signature of island arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intraoceanic arcs.

Combined U-Pb, Lu-Hf, Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane, Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~2.6GPa and 465–503°C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1±3.6Ma and 230.0±4.7Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4±6.4Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~2.6GPa and ~480°C; ca. 230Ma), the Sumdo eclogite (~3.4GPa and ~650°C; ca. 262Ma) and Jiang eclogite (~3.6GPa and ~750°C; ca. 261Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230Ma.

Thermochronological constraints on the Eocene exhumation of the Grand Forks complex, British Columbia, based on 40Ar/39Ar and apatite fission track geochronology

The Grand Forks complex (GFC) is a metamorphic core complex within the composite Shuswap complex in the southern Omineca belt of the Canadian Cordillera. It is juxtaposed against the surrounding low-grade rocks of the pericratonic Quesnel terrane by outward-dipping Eocene normal faults. The GFC attained peak metamorphic conditions of 750–800 °C and 5.5–6.0 kbar (1 kbar = 100 MPa) in the late Paleocene to early Eocene, followed by ∼2.5 kbar of near-isothermal decompression at upper-amphibolite to granulite facies conditions (∼725–750 °C) in the early Eocene. Subsequent low-temperature greenschist-facies exhumation (∼0.7–1.5 kbar) was accommodated by the brittle–ductile Kettle River normal fault (KRF) on the east flank of the complex and the Granby fault (GF) on the west flank. This study presents 16 new 40Ar/39Ar hornblende and biotite dates from the GFC and low-grade rocks in the hanging walls to the KRF and GF. Cooling of the GFC through the closure temperature of hornblende (∼ 530 °C) is constrained to the interval between ∼54 and 51.4 ± 0.5 Ma, whereas cooling through the closure temperature of biotite (∼280 °C) occurred at 51.4 ± 0.2 Ma. In the hanging wall of the KRF, cooling through the closure temperature of hornblende and biotite occurred nearly coevally at 51.7 ± 0.6 Ma and 51.0 ± 1.0 Ma, respectively. Five apatite fission track dates (closure temperature ∼110 °C) from the GFC and adjacent hanging walls are indistinguishable within error, yielding an average age of 34.6 ± 2.0 Ma. The lack of difference in biotite and apatite ages between the GFC and the low-grade hanging wall rocks against which it is juxtaposed suggests no significant movement on the KRF and GF after ca. 51 Ma. Results from this study and a previous study on U–Pb dating of the GFC document rapid cooling of the GFC in excess of 200 °C/Ma in a 4 Ma interval between 55 and 51 Ma (Eocene). This rapid phase of exhumation of the GFC was followed by 15 Ma of slow cooling (∼10 °C/Ma) of the joined GFC and hanging wall between ∼280 °C (biotite closure) and ∼110 °C (apatite closure).

Eocene Granitic Magmatism in NW Anatolia (Turkey) revisited: New implications from comparative zircon SHRIMP U–Pb and 40Ar–39Ar geochronology and isotope geochemistry on magma genesis and emplacement

During the Eocene, the northern Neo-Tethys suture zone and immediately adjacent areas in NW Turkey were loci of emplacement of a number of granitoid plutons with age relations and geochemical signatures indicative of magma generation in association with plate convergence. Discrete granitoid plutons of Eocene age in this region are exposed mainly in two distinct belts within and north of the northern Neo-Tethys suture zone. We report new SHRIMP U–Pb zircon and 40Ar/39Ar geochronology along with Sr–Nd isotope data from the granitoids in order to constrain their melt source(s), cooling history, and tectono-magmatic evolution with respect to the Early Cenozoic collisional tectonics of the region. Ranging in composition from diorite to granite, the plutons are largely represented by medium- to high-K calc-alkaline, I-type granites accompanied by minor amounts of shoshonitic rocks with syenite compositions and mafic microgranular enclaves in some of the plutons. In terms of trace element systematics the granitoids and the associated enclaves from the entire Eocene suite display close similarities to magmas from subduction-related or active continental margin settings, characterized by significant enrichment in LILE/HFSE relative to MORB. The rocks display significantly heterogeneous distributions of radiogenic isotopes with 87Sr/86Sr and 143Nd/144Nd ranging from 0.705824 to 0.708363 and from 0.512384 to 0.512718 respectively, suggesting multi-component melt interaction in their genesis. The granitoids from the southern and northern belts yielded zircon SHRIMP U–Pb ages of 52.8±1.8 to 45.41±0.34Ma and 47.02±0.82 to 36.79±0.67Ma, respectively, indicating overlapping emplacement ages, with some relatively younger ages from the northern belt. 40Ar/39Ar dating of biotite and hornblende also yielded similarly overlapping cooling ages (51.8±0.1 to 44.9±0.2Ma and 45.3±0.1 to 36.0±0.1Ma, respectively). Our combined geochronological data from the co-genetic zircon, and hornblende and biotite separates reveals rapid cooling of the plutons immediately after emplacement, which can readily be attributed to rapid uplift starting at∼53Ma. Unlike the intrusions in the southern belt, some of the granitoids in the northern belt contain inherited zircons of older ages, possibly reflecting more pronounced effects of crustal input in their genesis. On the basis of new geochemical and geochronological data, we infer that melt generation to produce the granitic plutons shortly after subduction waned in the region is most probably related to abrupt thermal weakening of the orogenic lithosphere due either to slab break-off or partial removal of a thermal boundary layer between lithosphere and asthenosphere that led to syn-convergent extension and collapse in the overriding plate, resulting in the onset of the development of the Aegean extensional province.

Syntectonic 1.46 Ga magmatism and rapid cooling of a gneiss dome in the southern Mazatzal Province: Burro Mountains, New Mexico

Large-volume granitic plutons in the Burro Mountains, southwestern New Mexico, cover an area of 1300 km 2 and include biotite leucogranite and biotite-hornblende granodiorite. These intrusions are part of the ca. 1.4 Ga granite and rhyolite province stretching across Laurentia. U-Pb zircon dating of five samples of the biotite leucogranite yielded ages ranging from 1469 ± 12 to 1455 ± 11 Ma (2σ uncertainty). Three samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the zircon ages are within error and together have a weighted mean age of 1462 ± 3 Ma, but the granodiorite is older than the granite based on crosscutting relationships. Pressure and temperature estimates from metapelitic country rock, the first from southern New Mexico, are 4–6.5 kbar and 575–650 °C, consistent with their mineralogy. Decompression textures on garnets indicate exhumation following peak conditions, interpreted to have been reached during magmatic heating at 1460 Ma. Decompression had a magnitude of 1–3 kbar. An amphibolite from the country rock has metamorphic zircons dated at 1458 ± 9 Ma, with low Th/U and soccer-ball morphology enclosed within hornblende; this is the age of amphibolite-facies metamorphism. Electron microprobe monazite U-Pb dates from metapelite country rock mostly range from 1500 to 1400 Ma, with a peak at 1474 Ma. This is interpreted as indicating that the area experienced high-temperature metamorphism associated with pluton emplacement at this time. The 40 Ar/ 39 Ar dates from granite and amphibolite form two main groups. The older group includes four hornblende dates of 1477–1467 Ma and biotite and white mica ages from 1472 to 1459 Ma. These are interpreted as reflecting cooling of the plutons and their country rock soon after intrusion. The younger group of hornblende and biotite ages is at 1228 ± 3 Ma and is close in age to the adjacent Redrock granite, which intruded at 1225 Ma. There is a dominant strong foliation in the country rock gneisses and metasedimentary rocks. Foliated granite and gabbro both yield ca. 1630 Ma U-Pb zircon ages. The Burro Mountain granite is not pervasively deformed, but the granodiorite has strong augen gneissic foliations and mylonitic shear zones with S-C fabrics. This fabric formed prior to intrusion of undeformed dikes of Burro Mountain granite and thus is synmagmatic within geochronologic uncertainty. The fabric is identical to the fabric in the country rock, and xenoliths of country rock are foliated with orientations parallel to the foliation in the granodiorite. The similar orientations and the timing of metamorphism suggest that most of the deformation in the Burro Mountains occurred at 1460 Ma and was synchronous with intrusion of the granodiorite. We interpret the 1460 Ma plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development within an extensional tectonic setting, although a transpressional setting is also permissible.

Structural and geochronological evidence for Early Cretaceous orogen-parallel extension of the ductile lithosphere in the northern Dabie orogenic belt, East China

The WNW-trending North Dabie dome (NDD) in the northern Dabie orogenic belt is bounded by the Xiaotian-Mozitan ductile shear zone (XMSZ) in the north and the Wuhe-Shuihou shear zone (WSSZ) in the south. Fabric geometries and kinematics of both the XMSZ and the WSSZ are similar. They both show a top-to-NW or WNW sense of shear and display similar microstructures which indicate that the deformation temperature was about 600–650 °C. 40Ar– 39Ar dating of hornblende and biotite from the mylonites of the XMSZ suggests that the shear zone was developed prior to 142 Ma. U–Pb zircon LA-ICPMS dating of undeformed granite dikes that cut the XMSZ suggests that the ductile shearing occurred before 130 Ma. The WSSZ was also developed in the Early Cretaceous. The interior of the NDD is dominated by top-to-NW or WNW sense of shear in the migmatitic gneiss and deformed plutons of the earliest Early Cretaceous ages. We suggest that the XMSZ, the WSSZ, and the interior of the NDD were involved in the same Early Cretaceous deformation. The XMSZ and the WSSZ were originally a single connected, more flat-lying zone, which we call the East Dabie Detachment Zone (EDDZ). It developed at a crustal level of over 18 km in depth. Kinematics of the EDDZ suggest that the ductile crust and possibly part of the lithospheric mantle in the eastern Dabie orogen underwent pervasive orogen-parallel and ESE-ward extension at the beginning of the Early Cretaceous. Large-scale magmatic intrusions following this deformation and the resultant increase in buoyancy led to the exhumation of the NDD and the warping of the EDDZ. Erosion separated the EDDZ into two zones (the XMSZ and the WSSZ) as observed today. Many metamorphic core complexes of Early Cretaceous were developed in the eastern North China Craton, most showing top-to-NW or WNW sense of shear. We suggest that such a widespread uniform shear sense reflects a uniform flow direction of the ductile lithosphere in the East China that has also been documented in the Liaonan, Yiwulushan and Yumenshan metamorphic core complexes in the northeastern part of the North China Craton.

Geochemistry and geochronology of the metamorphic sole underlying the Xigaze Ophiolite, Yarlung Zangbo Suture Zone, South Tibet

Strongly foliated amphibolite clasts are found embedded within the ophiolitic mélange underlying the Xigaze Ophiolite near Bainang and Angren, Yarlung Zangbo Suture Zone, Southern Tibet. These high-grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction. They include garnet-clinopyroxene amphibolites, clinopyroxene amphibolites and common amphibolites. Petrographic descriptions, mineral chemistry and thermobarometry of these rocks can be found in a companion paper [Guilmette, C., Hébert, R., Dupuis, C., Wang, C.S., Li, Z.J., 2008. Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic mélange beneath the Yarlung Zangbo Ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32, 423–437.]. The geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust. Rare Earth Elements contents are suggestive of an N-MORB origin. However, enriched LILEs and depleted Nb–Ta–Ti when compared to N-MORBs rather suggest a suprasubduction zone influence. A large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics, suggesting the protoliths of the amphibolites might have crystallized in the same environment as the Xigaze ophiolitic crust, likely in a back-arc basin. 40Ar/39Ar step-heating dating of hornblende concentrates from three samples yielded ages of 123.6±2.9 Ma, 127.7±2.2 Ma and 127.4±2.3 Ma. These cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite. All these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone. The demise of the early subduction circa 130 Ma forced the inception of a new subduction zone at the SSZ spreading axis, burying SSZ mafic rocks underneath a SSZ ophiolitic mantle.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe–reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40 Ar/ 39 Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate “Appalachian” dates (e.g., Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may be related to Carolina zone terranes that accreted during the Alleghanian orogeny.

The Nature and Origin of Gold Deposits of the Tarcoola Goldfield and Implications for the Central Gawler Gold Province, South Australia

The Tarcoola goldfield in central South Australia is hosted by the Paleoproterozoic Paxton Granite and Tarcoola Formation metasedimentary rocks. Both of these units are intruded by narrow dikes of the Lady Jane Diorite, which is considered to be part of the Gawler Range-Hiltaba volcano-plutonic event. Crosscutting relationships and 40Ar/39Ar dating of alteration sericite and primary hornblende have demonstrated that veining, alteration, and mineralization occurred synchronously with intrusion of the diorite dikes at ~1580 Ma. Mineralization at the Perseverance deposit in the Tarcoola goldfield is hosted by quartz veins in both granite and the lower parts of the Tarcoola Formation. Alteration intensity and mineralization are related to fracture density. Very high gold grades (up to 270 ppm) are recorded in carbonaceous siltstones of the Tarcoola Formation. Primary fluid inclusions from mineralized quartz veins at Perseverance are dominated by two-phase aqueous inclusions and three-phase H2O-CO2 (l) - CO2 (v) ± CH4 inclusions. Both inclusion types have salinities of <10 wt percent NaCl equiv, with evidence of other salts being present. Both types homogenize between 150° and ~340°C. They occur together in single quartz grains, which suggests the coexistence of aqueous and carbonic fluids. At the Perseverance deposit the maximum depth of fluid inclusion formation is estimated as 10 km. Processes resulting from pressure fluctuations during brittle fracturing causing phase separation are suggested to be the dominant gold precipitation mechanism, although definitive fluid inclusion evidence for either phase separation or fluid mixing is lacking. Wall-rock sulfidation may have also contributed to gold deposition. Pb isotope compositions of galena from gold-bearing mineralized veins at Perseverance are similar to K-feldspar and whole-rock samples of the Paxton Granite. This is consistent with a common source of Pb and possibly other ore components for both the granite and galena, but not all potential reservoirs were tested. Variably altered and gold mineralized samples that had Paxton Granite precursors show a loss of Nd and a trend from eNd(1580Ma) values of about −6.2 in slightly altered samples toward more positive eNd values of about −3.3 in high-grade gold samples. Of the local lithologic units, the Lady Jane Diorite has eNd(1580Ma) values of about +0.1 and is the most probable source of the more primitive Nd isotope components in mineralized samples. Mineralization in the Tarcoola goldfield has similarities to both orogenic and intrusion-related gold styles. The demonstrated coeval timing of diorite dikes and mineralization at Tarcoola, together with the Nd isotope constraints on metal sources, and the presence of similar mafic dikes in several other known gold prospects in the central Gawler gold province, suggests that the diorite was an essential part of the gold mineralization event throughout this province.

40Ar– 39Ar age, geochemistry and Sr–Nd–Pb isotopes of the Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic complex, SW China

The Lengshuiqing Cu–Ni sulfide deposit of the Yanbian area in Sichuan Province, SW China, is hosted by relatively small mafic–ultramafic bodies that intruded Mesoproterozoic metavolcanic rocks and schists. The mafic–ultramafic complex is mainly composed of the olivine-hornblende pyroxenite and hornblende gabbro units. 40Ar/ 39Ar age, elemental and Sr–Nd–Pb isotopic results of the complex are reported in this paper. 40Ar/ 39Ar dating of hornblende yielded an age of 821 ± 1 Ma for the mafic–ultramafic complex. This age is similar to the SHRIMP U–Pb zircon age of 825 ± 12 Ma for the adjacent Gaojiacun mafic–ultramafic complex. This suggests that the Lengshuiqing mafic–ultramafic complex originated from Neoproterozoic mafic magma. The olivine-hornblende pyroxenite unit is characterized by high MgO contents (24.25–28.08%), Mg # values (74.5–78.0) and compatible elements (Cr = 1179–2336 ppm, Ni = 732–1303 ppm), low ΣREE (19.5–44.6 ppm) and other incompatible element contents. The hornblende gabbro unit has MgO = 7.32–12.51%, Mg # = 55.0–69.3, Cr = 37.7–1131 ppm, Ni = 50.0–243 ppm and ΣREE = 37.2–75.4 ppm. Most of the rocks are moderately enriched in LREE ((La/Yb) N = 2.28–7.67), and significantly depleted in Nb–Ta ((Nb/La) PM = 0.17–0.47; (Ta/La) PM = 0.19–0.58), with slightly moderately positive Eu anomalies ( δ Eu = 0.95–1.65). The trace element feature of the ore-bearing samples is similar to that of the rocks in the complex. The variations in major and trace elements can be well explained by magma crystal fractionation. Isotopically, the rocks in Lengshuiqing mafic–ultramafic complex are characterized by low 87Sr/ 86Sr(i) (0.7039–0.7044), positive ɛ Nd(i) (+2.2 − +4.3) and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.382–17.913, 207Pb/ 204Pb(i) = 15.518–15.578, 208Pb/ 204Pb(i) = 37.227–38.019). The ores also have positive ɛ Nd(i) (+2.4 − +5.4), and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.256–17.733, 207Pb/ 204Pb(i) = 15.484–15.555, 208Pb/ 204Pb(i) = 36.924–37.665). All the samples exhibit positive ɛ Nd(i) values, suggesting that their parental magmas were derived from a long-term depleted asthenospheric mantle. The relatively large variations in 87Sr/ 86Sr(i) of the ores (0.7012–0.7046) could be ascribed to the interaction between magma and country rocks. A good positive correlation between Nb/La and Nb/Th reveals that crustal assimilation was involved in their genesis. This contamination resulted in the observed depletion of Nb and Ta in rocks and ores. The crustal contamination and fractional crystallization, similar to the process of magmatic assimilation and fractional crystallization (AFC), are thus critical for the formation of the Lengshuiqing Cu–Ni ore deposit. The Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic intrusions formed in an intraplate continental rift, which was most likely related to a mantle plume underneath the supercontinent Rodinia.