Biotite Dates Research Articles

A genetic link between magnetite mineralization and diorite intrusion at the El Romeral iron oxide-apatite deposit, northern Chile

El Romeral is one of the largest iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile. The Cerro Principal magnetite ore body at El Romeral comprises massive magnetite intergrown with actinolite, with minor apatite, scapolite, and sulfides (pyrite ± chalcopyrite). Several generations of magnetite were identified by using a combination of optical and electron microscopy techniques. The main mineralization event is represented by zoned magnetite grains with inclusion-rich cores and inclusion-poor rims, which form the massive magnetite ore body. This main magnetite stage was followed by two late hydrothermal events that are represented by magnetite veinlets that crosscut the massive ore body and by disseminated magnetite in the andesite host rock and in the Romeral diorite. The sulfur stable isotope signature of the late hydrothermal sulfides indicates a magmatic origin for sulfur (δ34S between − 0.8 and 2.9‰), in agreement with previous δ34S data reported for other Chilean IOA and iron oxide-copper-gold deposits. New 40Ar/39Ar dating of actinolite associated with the main magnetite ore stage yielded ages of ca. 128 Ma, concordant within error with a U-Pb zircon age for the Romeral diorite (129.0 ± 0.9 Ma; mean square weighted deviation = 1.9, n = 28). The late hydrothermal magnetite-biotite mineralization is constrained at ca. 118 Ma by 40Ar/39Ar dating of secondary biotite. This potassic alteration is about 10 Ma younger than the main mineralization episode, and it may be related to post-mineralization dikes that crosscut and remobilize Fe from the main magnetite ore body. These data reveal a clear genetic association between magnetite ore formation, sulfide mineralization, and the diorite intrusion at El Romeral (at ~ 129 Ma), followed by a late and more restricted stage of hydrothermal alteration associated with the emplacement of post-ore dikes at ca. 118 Ma. Therefore, this new evidence supports a magmatic-hydrothermal model for the formation of IOA deposits in the Chilean Iron Belt, where the magnetite mineralization was sourced from intermediate magmas during the first Andean stage. In contrast, the beginning of the second Andean stage is characterized by shallow subduction and a compressive regime, which is represented in the district by the emplacement of the Punta de Piedra granite-granodiorite batholith (100 Ma) and marks the end of iron oxide-apatite deposit formation in the area.

High-pressure granulite-facies metamorphism in central Dronning Maud Land (East Antarctica): Implications for Gondwana assembly

Central Dronning Maud Land (DML; East Antarctica) is located in a key region of the Gondwana supercontinent. The Conradgebirge area (central DML) consists of orthogneisses, derived from both volcanic and plutonic protoliths, and minor metasedimentary rocks, intruded by Cambrian syn- to post-metamorphic plutons and dykes.Mafic-ultramafic boudins in the metavolcanic and metaplutonic gneisses from Conradgebirge consist of amphibolites and high-grade garnet-bearing pyroxene- and amphibole-rich granofels. They occur either as discontinuous levels or as pods boudinaged within highly-strained and strongly-migmatized gneisses. Bulk-rock major and trace-element compositions, together with geochemical discriminant diagrams (e.g., Th/Yb versus Ta/Yb and V versus Ti), suggest derivation from enriched mantle source for the mafic rocks boudinaged in metaplutonic gneisses, whereas a calc-alkaline signature is common for the mafic boudins in metavolcanic rocks. The microstructural study and P–T modelling of an ultramafic metagabbroic rock reveal a prograde metamorphic evolution from amphibolite-facies (ca. 0.5GPa; 500°C) up to high-P granulite-facies conditions (ca. 1.5–1.7GPa; 960–970°C). Partial melting is testified by “nanogranitoid” inclusions enclosed in garnet. An almost isothermal decompression down to ca. 0.4GPa and 750–850°C produced well-developed An+Opx-bearing symplectites around garnet. A final isobaric cooling at nearly 0.4GPa is testified by Grt coronas around high-T symplectites.The above reconstruction traces a clockwise loading-heating P-T evolution with a peak metamorphism at high-P granulite-facies conditions suggesting crustal thickening at nearly 570Ma, followed by a tectonically assisted rapid exhumation, and then, by an isobaric cooling. 40Ar-39Ar dating of amphibole and biotite at ~505–480Ma testify mineral re-equilibration at upper crustal level (T<650°C) during the isobaric cooling.This tectono-metamorphic scenario seems representative of the evolution resulting from the Neoproterozoic/Early Palaeozoic (600–500Ma) collision between parts of East- and West-Gondwana blocks that led to the final assembly of Gondwana.

Rapid cooling and exhumation as a consequence of extension and crustal thinning: Inferences from the Late Miocene to Pliocene Palu Metamorphic Complex, Sulawesi, Indonesia

Metamorphic complexes forming high mountains of 1.5–2km in Western Sulawesi were previously considered to be Mesozoic or older basement of Gondwana crust. However, many of the metamorphic rocks are much younger than previously thought. Some have Eocene sedimentary protoliths. New geothermobarometric and geochronological data from metamorphic rocks of the Palu Metamorphic Complex (PMC) and associated granitoids provide information on the timing and mechanisms of Neogene metamorphism and contemporaneous rapid exhumation. The metamorphic rocks are strongly deformed and some were partially melted to form migmatites. Schists contain relict andalusite, cordierite, staurolite and Mn-rich garnet which are wrapped by a pervasive fabric. 40Ar/39Ar dating of biotite, white mica and amphibole from strongly deformed, mylonitic schists and recrystallised amphibolites reveals cooling occurred in the Early Pliocene (c. 5.3–4.8Ma) in the northern part and during the Late Pliocene (c. 3.1–2.7Ma) in the southern part of the PMC. U-Pb, 40Ar/39Ar and (U-Th)/He analyses of various minerals from PMC metamorphic and S-type magmatic rocks give very similar mid to Late Pliocene ages, indicating very fast cooling and rapid exhumation, and show the high speed at which tectonic processes, including magmatism, exhumation, and reworking into a sediment, must have occurred. The high rates could be unique to this area but we suggest they record the true speed of metamorphic complex exhumation in a very young orogenic belt. Rates in older orogens appear lower because they are averages measured over longer periods of time. Contemporaneous magmatism and deformation are interpreted as a consequence of decompressional melting due to extension and thinning of the crust, promoted by possible detachment faults and normal faulting at the major NW-trending Palu-Koro and Tambarana Faults. In contrast, I-type magmatic rocks, separated from the PMC by the Palu-Koro Fault, were exhumed from upper crustal levels by erosion at moderate rates.

Geology and Genesis of the Cerro la Mina Porphyry-High Sulfidation Au (Cu-Mo) Prospect, Mexico

The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina's stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma (U-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that is host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllic alteration composed of quartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllic alteration. This zoning included a low-temperature ( 120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite, sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.04 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40Ar/39Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite closure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillic-argillic alteration. Sulfur isotope results of sulfides (–2.5 to +4.9‰; mean +0.7‰; n = 20) and a sulfate (barite; +10.5‰; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high CO2 contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fe mine and Campamento deposit represent parts of a porphyry copper system—specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment.

40Ar/39Ar and Rb‐Sr Ages of the Tiegelongnan Porphyry Cu‐(Au) Deposit in the Bangong Co‐Nujiang Metallogenic Belt of Tibet, China: Implication for Generation of Super‐Large Deposit

AbstractThe Tiegelongnan deposit is a newly discovered super‐large porphyry‐epithermal Cu‐(Au) deposit in the western part of the Bangong Co‐Nujiang metallogenic belt, Tibet (China). Field geology and geochronology indicate that the porphyry mineralization was closely related to the Early Cretaceous intermediate‐felsic intrusions (ca. 123–120 Ma). Various epithermal ore and gangue mineral types were discovered in the middle‐shallow part of the orebody, indicating the presence of epithermal mineralization at Tiegelongnan. Potassic, propylitic, phyllic and advanced argillic alteration zones were identified. 40Ar/39Ar dating of hydrothermal biotite (potassic zone), sericite (phyllic zone), and alunite (advanced argillic zone) in/around the ore‐bearing granodiorite porphyry yielded 121.1±0.6 Ma (1σ), 120.8±0.7 Ma (1σ) and 117.9±1.6 Ma (1σ), respectively. Five hydrothermal mineralization stages were identified, of which the Stage IV pyrite was Rb‐Sr dated to be 117.5±1.8 Ma (2σ), representing the end of epithermal mineralization. Field geology and geochronology suggest that both the epithermal and porphyry mineralization belong to the same magmatic‐hydrothermal system. The Tiegelongnan super‐large Cu‐(Au) deposit may have undergone a prolonged magmatic‐hydrothermal evolution, with the major mineralization event occurring at ca.120–117Ma.

Early Jurassic tectonism occurred within the Basu metamorphic complex, eastern central Tibet: Implications for an archipelago-accretion orogenic model

The Basu metamorphic complex, surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the Bangong-Nujiang suture, holds one of the keys to understanding the pre-Cenozoic tectonic evolution of central Tibet. Zircon U-Pb dating of rocks from the Basu metamorphic complex reveals that meta-igneous rocks yield Early Paleozoic crystallization ages of 500–492Ma and an Early Jurassic metamorphic age of ~173Ma, and that undeformed granitoid rocks yield crystallization ages of approximately 186–174Ma. Whole rock geochemical and zircon Lu-Hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources, which may reflect a collisional orogenic setting. 40Ar/39Ar dating of biotite from a sillimanite-garnet-biotite paragneiss shows cooling to 300±50°C at ~165Ma. These data indicate significant Early Jurassic tectonism, during which most of the Basu metamorphic complex was formed. Furthermore, the age data resemble those of the Amdo metamorphic complex located approximately 500km to the west along the Bangong-Nujiang suture. Together, these complexes may represent a “destroyed or unrecognized” block, i.e., the Amdo-Tongka block, which may be the eastern extension of the South Qiangtang terrane. Based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts, we suggest a new archipelago-accretion model that attributes the Early Jurassic tectonism to an arc-continent/micro-continent collision. This model further enables the reconstruction of the eastern Tethyan Ocean and the orogenic processes of central Tibet during the Mesozoic.

Mesoproterozoic fluid events affecting Archean crust in the northern Olympic Cu–Au Province, Gawler Craton: insights from 40Ar/39Ar thermochronology

ABSTRACTThe Olympic Cu–Au Province, Gawler Craton, is host to the Olympic Dam and Prominent Hill iron oxide–copper–gold (IOCG) deposits. Both of these deposits and the region between the two are covered by Neoproterozoic to Cenozoic sediment, making inferences about prospectivity in this portion of the Olympic Domain reliant on geophysical interpretation and sparse drill hole information. We present new U–Pb zircon sensitive high resolution ion microprobe (SHRIMP) dates from two basement intersecting drill holes in the region between Olympic Dam and Prominent Hill that show bimodal volcanism occurred at 2555 ± 5 Ma, and was followed by intrusion of tonalite at 2529 ± 6 Ma. Laser 40Ar/39Ar dating of biotite and muscovite from the tonalite yields ages around ca 2000 Ma, consistent with slow cooling trends observed in Archean rocks elsewhere in the northern Gawler Craton. Step heating experiments on K-feldspar from the same tonalite yields an age spectrum with older ages around 1740 Ma from the highest temperature steps becoming progressively younger to a minimum of 1565 Ma in the lowest temperature heating steps; this is consistent with either Paleoproterozic cooling to final closure of K-feldspar by 1565 Ma or a reheating event at ca 1565 Ma, with the latter more likely, given the evidence for sub-solidus alteration of the K-feldspar. Sericite within hematite–sericite–chlorite altered portions of the tonalite yield a poorly defined age of ca 1.6 Ga. Taken together the 40Ar/39Ar data providing evidence for a fluid event affecting this region between Olympic Dam and Prominent Hill during the early Mesoproterozoic. Low temperature quartz–carbonate–adularia veins occur in <10 cm wide fractures within basalt in one drill hole in this region. Adularia from these veins yields 40Ar/39Ar ages that span from ca 1.3–1.1 Ga. This age range is interpreted to approximate either the timing of adularia formation during a hydrothermal event or the timing of resetting of the 40Ar/39Ar systematics within the adularia as a result of fluid flow in this sample. This is evidence for a mid-Mesoproterozoic fluid event in the Gawler Craton and necessitates a reconsideration of the long-term stability of the craton, as it appears to have been affected, at least locally, by fluid flow related to a much larger event within the Australian continent, the Musgrave Orogeny.

Metamorphic P–T–t path retrieved from metapelites in the southeastern Taihua metamorphic complex, and the Paleoproterozoic tectonic evolution of the southern North China Craton

The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite+plagioclase+quartz+ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite+biotite+plagioclase+quartz+K-feldspar+ilmenite+rutile+pyrite. The retrograde mineral assemblage (M3), biotite+plagioclase+quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P–T conditions of 12.2kbar and 830°C, whereas garnet rims record P–T conditions of 10.2kbar and 840°C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P–T path can be retrieved. High resolution SIMS U−Pb dating and LA−ICP−MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous model in which the Tran-North China Craton (TNCO) was formed through the collision between the East and West blocks.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

The North Cascades orogen (northwestern USA) provides an exceptional natural laboratory with which to evaluate potential temporal and kinematic links between processes operating at a wide range of crustal levels during collapse of a continental arc, and particularly the compatibility of strain between the upper and lower crust. This magmatic arc reached a crustal thickness of ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional transtension was marked by magmatism, migmatization, ductile flow, and exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core coeval with subsidence and rapid deposition in nonmarine basins adjacent to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is the larger of two regions of exhumed deep crust with Eocene cooling ages in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to 53 Ma; the youngest orthogneisses overlap 40 Ar/ 39 Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional domains, and strong stretching parallel to lineation of hinges of mesoscopic folds in the Skagit Gneiss Complex are compatible with transtension linked to dextral-normal displacement of the Ross Lake fault zone, the northeastern boundary of the Cascades core. The other deeply exhumed domain, the 9–12 kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging lineation and gently to moderately dipping foliation, which are associated with top-to-the-north noncoaxial shear. This gneiss is separated from overlying metamorphic rocks by a folded detachment fault. The Eocene Swauk and Chumstick basins flank the southern end of the Cascades core. In the Swauk basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, and then covered by ca. 49 Ma Teanaway basalts and intruded by associated mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin subsided rapidly. Extension directions from these dikes and from Eocene dikes that intruded the Cascades core are dominantly oblique to the overall trend of the orogen (275°–310° versus ∼320°, respectively) and to the northwest-southeast to north-south ductile flow direction in the Skagit and Swakane rocks. This discordance implies that coeval extensional strain was decoupled between the brittle and ductile crust. Strain orientations at all depths in the Cascades core contrast with the approximately east-west extension driven by orogenic collapse in coeval metamorphic core complexes ∼200 km to the east. Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part from dextral shear along the plate margin and from along-strike gradients in crustal thickness and temperature.

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.

Correlation between magnetic fabrics, strain and biotite microstructure with increasing mylonitisation in the pretectonic Wyangala Granite, Australia

The Wyangala Granite is a foliated, porphyritic Silurian granite from the Palaeozoic Circum-Pacific type Eastern Lachlan Orogen (ELO) of Australia. It is a paramagnetic ilmenite-bearing, S/marginal I type two-mica- to mainly biotite–granite with different biotite contents and local chlorite alteration. Very highly strained quartz–epidote bands are present. In this contribution, anisotropy of magnetic susceptibility (AMS) is compared with independently measured intensity and 3D style of strain, biotite microstructure and degree of mylonitisation for low-strain granites with weak S-foliations, through medium-strain protomylonitic granites with moderate S- and C-foliations to a high-strain altered granite with a strong single foliation. The samples are further analysed for possible contributions from sample heterogeneity, magmatic flow and ‘sub-magmatic’ deformation.A good correlation, P′AMS ~1.02+0.04ln P′(e)Qtz, is obtained between site-average degree of AMS (P′AMS) in the granite and degree of finite-strain anisotropy (P′(e)Qtz) from aspect ratios of quartz aggregates in S-foliations in hand specimen and outcrop (P′AMS 1.03–1.14, P′(e)Qtz 1.4–19). The magnetic fabric ellipsoids agree with a kinematic regime between neutral and pure oblate predicted by the March model. The observed quartz strains, however, exceed the AMS March strains and are near neutral, plano-linear character. The geological factors that may have contributed to these differences include intra- and inter-crystalline deformation of biotite and bimodality in S and C.Magmatic fabric is not clearly evident in either the AMS or the biotite data. New data for synkinematic oligoclase, low-titanium biotite and low-sodium K-feldspar show that conditions during deformation were approximately transitional greenschist–amphibolite facies: i.e., well below solidus. This agrees with published age data that put the granite emplacement in an extensional, back-arc setting in already deformed country rocks, and before later convergent regional deformation in Middle Devonian (Tabberabberan event) to Early Carboniferous (Kanimblan event) time.

Thermal history of the giant Qulong Cu–Mo deposit, Gangdese metallogenic belt, Tibet: Constraints on magmatic–hydrothermal evolution and exhumation

A complete thermal history for the Qulong porphyry Cu–Mo deposit, Tibet is presented. Zircon U–Pb geochronology indicates that the mineralization at Qulong resulted from brecciation-veining events associated with the emplacement of a series of intermediate-felsic intrusions. Combined with previously published ages, our results reveal a whole intrusive history of the Qulong composite pluton. Causative porphyries were emplaced at ~16.0Ma as revealed by 40Ar–39Ar dating of hydrothermal biotite (15.7±0.2Ma) and sericite (15.7±0.2Ma). Zircon and apatite (U–Th)/He (ZHe and AHe) dating of Qulong revealed that both followed similar, monotonic thermal trajectories from 900°C (U–Pb ages: 17.5–15.9Ma) to 200°C (ZHe: 15.7–14.0Ma), and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200°C/myr. The Qulong deposit was exhumed between 13.6Ma and 12.4Ma (AHe) at an estimated rate of 0.16–0.24mm/y, which is consistent with previous estimates for other Gangdese Miocene porphyry deposits. Our AHe thermochronology results suggest that neither the Gangdese thrust system, nor the Yadong–Gulu graben affected or accelerated exhumation at the Qulong deposit.

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780°C) pressure up to c. 24kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486±9Ma and 459±9Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.

Equilibrium partitioning and subsequent re-distribution of halogens among apatite–biotite–amphibole assemblages from mantle-derived plutonic rocks: Complexities revealed

The concentration of halogens in apatite, biotite and amphibole is investigated for a large variety of mantle-derived plutonic rocks (gabbros, diorites, monzonites, olivine- and pyroxene-bearing monzonitic to granitic rocks, syenites, carbonatites and a phoscorite). In all rocks studied, apatite occurs as an early magmatic phase, whereas biotite and amphibole may occur either as a late magmatic phase or as late-stage, potentially hydrothermal product replacing precursor olivine, pyroxene and Fe–Ti oxides (ilmenite and magnetite).Based on electron microprobe analyses for F and Cl and detailed textural observations, we test existing models of halogen partitioning between apatite and biotite. Bromine concentration data for apatite, biotite and amphibole are used to further refine our understanding of the geochemical similarities and differences between Cl and Br during magmatic and hydrothermal processes.Our data suggests that F and Cl contents in apatite, biotite and amphibole can indeed be useful monitors of the halogen systematics in magmas, but they may also be subject to post-magmatic changes to variable extents. The relatively small radius and compatible F cation seems to be less prone to post-magmatic alteration and is likely to best reflect the original magmatic halogen abundances — especially in apatite. However, the larger and probably more incompatible Cl anion, is more easily re-mobilized as reflected by strong redistribution of Cl in biotite and amphibole which have been clearly overprinted by hydrothermal fluids. In certain cases, the ability of halogens to re-distribute themselves after magmatic equilibrium partitioning (as emphasized by our data) suggests that observed partitioning (especially between apatite and biotite) may also be used as a very sensitive indicator for post-magmatic hydrothermal processes.

The timing of successive fluid events in the Southern Marginal Zone of the Limpopo Complex, South Africa: Constraints from 40Ar–39Ar geochronology

We have carried out a geochronological study in the high grade metamorphic Southern Marginal Zone of the Limpopo Complex, South Africa, aimed at providing time constraints on episodes of fluid activity. This work chiefly comprises 40Ar/39Ar dating, with accompanying petrography, of amphibole and mica, and further incudes zircon U–Pb geochronology on a post peak metamorphic trondhjemite intrusion. The so-called Zone of Rehydration within 30km N of the Hout River Shear Zone (where the Southern Marginal Zone is thrust over the Kaapvaal Craton) was studied in particular, but sampling also included areas of patchy metasomatism in the Granulite Zone further north, where no post-peak metamorphic rehydration on a regional scale occurred.Amphibole in a mafic granulite and an older generation of zircons in the Petronella trondhjemite yielded dates in the 2700–2730Ma range, corresponding to the published age of peak granulite metamorphism in the Southern Marginal Zone. A cluster of 40Ar/39Ar dates on amphiboles from the Zone of Rehydration indicate an age between 2660 and 2670Ma for this event, in accord with results from previous work on the Hout River Shear Zone. The main population of U–Pb zircon dates from the trondhjemite, dating its intrusion, also falls in this age range, as does the published age of the Matok pluton. These data suggest that rehydration at 2660–2670Ma was a discrete episode. A further 40Ar/39Ar age cluster, defined by dates on both amphiboles and micas, is seen at 2010–2040Ma, which is the age of a major tectonometamorphic event in the Central Zone of the Limpopo Complex, and previously reported from the Southern Marginal Zone as Rb–Sr biotite dates. Events we found in the 2010–2040Ma bracket are strike-slip shearing (dated by fabric forming amphibole), as well as localized potassic metasomatism (dated by both amphiboles and micas). The latter was not confined to the Zone of Rehydration, and appears in one case to be associated with gold mineralization. A number of 40Ar/39Ar dates intermediate between 2600 and 2000Ma were obtained, but these are diverse and can be accounted for by mixing, incomplete resetting and/or excess 40Ar. They do not indicate events in this intermediate age bracket.

Geologic History and Timing of Mineralization at the Haile Gold Mine, South Carolina

New constraints on the geologic history and timing of mineralization at the Haile gold deposit in South Carolina are provided by radiometric dating of igneous and metamorphic samples using Re-Os, U-Pb, and 40Ar/39Ar methods. Seven Re-Os measurements were made on impure mineral separates of fine-grained molybdenite containing variable quantities of pyrite, pyrrhotite, muscovite, and illite. Four measurements produced an isochron at 548.7 ± 2.0 Ma (MSWD = 1.04). An additional age of 541 ± 2 Ma suggests that molybdenite may have been produced over a ~8 Ma timespan (541–549 Ma), but the preferred interpretation is that molybdenite formed primarily at ~549 Ma. Broadly correlative Mo and Au abundances in drill samples, and the common presence of molybdenite in Au-bearing zones, indicate that Au mineralization at Haile probably also occurred at ~549 Ma. Contrasting Re-Os ages produced at 564 ± 2 and 529 ± 2 Ma are interpreted to be inaccurate, due to the effects of impure mineral separates, and possibly resulting from loss of radiogenic Os from fine-grained molybdenite to other sulfide minerals. Some mobilization and loss of Re, possibly under oxidizing conditions during late Paleozoic metamorphism, may explain two relatively old Re-Os ages determined here and by Stein et al. (1997). Laser ablation dating of zircons from a metavolcanic sample at Haile yielded a weighted mean age of 560 ± 8 Ma. This age is similar to the published time window of 551 to 555 Ma for magmatism at Haile, based on U-Pb zircon dating by ion probe (Ayuso et al., 2005). Similar timing of magmatism and Au mineralization indicates that the hydrothermal systems at Haile were likely driven by magmatism and were therefore not the product of collision, orogeny, and metamorphism. These results confirm that magmatism and mineralization at Haile were active at a time when the Carolina terrane was still in a peri-Gondwana location, well prior to its accretion to eastern Laurasia. Ages of some detrital zircons imply an anomalously young depositional age for the metasedimentary unit that hosts the Haile orebodies (<540 Ma). These ages conflict with an older age of 560 ± 8 Ma determined for the overlying metavolcanic unit. This conflict is interpreted to indicate that folding has overturned the geologic units at Haile. A possible Devonian-age metamorphic event at ~390 Ma is documented by 40Ar-39Ar dating of porphyroblastic biotite collected from mineralized rock at Haile. Zircon ages from the Dutchman Creek Gabbro (weighted mean of 314 ± 2 Ma) and 40Ar-39Ar ages on biotite separates from dikes that crosscut the Haile property (311 ± 3, 311 ± 2 Ma) document a pulse of mafic and alkaline magmatism in the late Carboniferous. These late-stage metamorphic and magmatic events appear to be unrelated to mineralization at Haile.

Experimental and geologic evaluation of monazite (U–Th)/He thermochronometry: Catnip Sill, Catalina Core Complex, Tucson, AZ

Monazite is a petrologically important and analytically promising target for (U–Th)/He thermochronology. Previous studies have reported highly variable He diffusion results from monazite from a single sample and demonstrated that composition can significantly affect He diffusion parameters. In this study, we performed incremental heating of single monazite grains to experimentally determine the 4He diffusion properties of reference monazite ‘554’ that occurs within a peraluminous two-mica granite from the Catnip Sill within the Catalina Core complex, Arizona. Assuming that the grain size defines the diffusion geometry, the six experiments yielded Ea values of 212 to 238±5 kJmol−1 (1σ) and Do values of 15.7 to 103 cm2s−1 with one value of 784 cm2s−1. Monazite (U–Th)/He data from five grains yielded closure temperatures of 291 to 262 °C (± c. 15 °C) and ages of 23.8–20.3(±∼1.2;2σ) Ma; the weighted mean age is 21.8±0.73 (MSWD=1.83, n=5) and the weighted mean closure temperature is 282±6°C (MSWD=0.96, n=5;1σ). We tested the accuracy of these results by comparing our monazite thermochronology data with monazite Th/Pb depth profiling results, the 40Ar/39Ar thermal history for the Catnip Sill constrained using coexisting muscovite, biotite, and K-feldspar, and published regional zircon and apatite fission track results. The monazite Th/Pb data indicate emplacement of the Catnip Sill at ∼45 Ma. The 40Ar/39Ar muscovite and biotite data indicate cooling from 460 to 350 °C from 27 to 26 Ma. K-feldspar MDD modeling suggests cooling from 360 to 240 °C from 26 to 24 Ma. Zircon fission track data indicate cooling through 250 °C between 29 and 20 Ma. Additional cooling through 110 °C is recorded by apatite fission track ages of 19–16 Ma. Because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers, our results 1) confirm the accuracy of the 4He diffusion kinetics from monazite ‘554,’ and 2) demonstrate that monazite can be an effective (U–Th)/He thermochronometer, provided that He diffusion properties are either measured or reasonably assumed for each sample.

Geologic position, age, and petrogenesis of plagiogranites in northern Rudny Altai

The geologic position, age, petrologic composition, and petrogenesis of mesoabyssal plagiogranites in northern Rudny Altai, dated earlier at the Early–Middle Devonian, are considered. The Middle Carboniferous (322–318 Ma) age of granitoids has been substantiated by isotope-geochronological data (U–Pb zircon dating and Ar–Ar amphibole and biotite dating). Geologic-structural studies showed that the intrusion of granitoids took place at the time when compression was changed by sinistral faulting. This led to the conclusion that the granitoids formed at the peak of the collision between the Siberian and the Kazakhstan paleocontinents. Geochemical and isotope studies showed that most of the analyzed plagiogranites belong to high-alumina (continental) type and resulted from the deep melting (~15 kbar) of metabasic substrates compositionally similar to N-MORB (judging from results of geochemical modeling and the Nd isotope composition). The presence of plagiogranites of low-alumina (oceanic) type in the postgranite dike series testifies to the melting of the Rudny Altai heterogeneous crust at different depths during its collisional thickening.

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).

Analysis on the Thermal History and Uplift Process of Zijinshan Intrusive Complex in the Eastern Ordos Basin

AbstractThermochronological data of in‐situ zircon U‐Pb, hornblende and biotite 40Ar/39Ar, and zircon and apatite fission track (FT) are presented for studying the thermal history and uplift‐cooling process of Zijinshan (ZJS) intrusive complex at the eastern Ordos basin (OB) next to the central North China Craton (NCC). Alkaline intrusive rocks dominate the complex with the diopside monzonite and syenite, which are used as the dating samples in this work. Zircon U‐Pb dating of the monzonite provides the initial crystallization age of the magmatic emplacement to be ca. 136.7 Ma, and 40Ar/39Ar dating of hornblendes and biotites indicates the late crystallization to solidification age of the emplaced magma to be mainly in a range of 133.1∼130.4 Ma. It is concluded that the ZJS intrusive complex was mainly formed in 136.7∼130.4 Ma of the Early Cretaceous. Based on the comprehensive analyses of the thermochronological T‐t curve and the apatite FT modeling thermal path of the complex and its altered sandstone, it can be further inferred that the ZJS complex as well as its intruded area of the eastern OB has experienced at least three tectono‐thermal stages during magmatic emplacement to later uplift‐cooling process. They are: (1) the magmatic emplacement crystallization to consolidation stage of 136∼120 Ma at an extremely high cooling rate of 52 °C/Ma; (2) the slow uplift‐cooling stage of 120∼30 Ma at a low average cooling rate of 2.5°C/Ma, and (3) the rapid uplift‐cooling stage of ca. 30 Ma at an average cooling rate of 3.6°C/Ma, especially with a more rapid cooling rate of 7°C/Ma since ca. 10 Ma. Further considering the regional tectono‐dynamics of the NCC, ZJS magmatic activity of the eastern OB is an expression of the dynamic transition and its related large‐scale magmatism of NCC during the Early Cretaceous. Then, the later differential uplift process with slow to rapid cooling rate is dynamically associated with the integrated effects of the multi‐direction convergence, toward the central NCC, of the (Paleo‐)Pacific plate and Tethys or Indian plate at variable subduction rates during the Late Cretaceous to Quaternary.