Margin Of Pluton Research Articles

Visible-Near Infrared–Short-Wave Infrared Spectroscopy and Mineral Mapping of Hydrothermal Alteration Zones at the Brejuí W-Mo Skarn Deposit, Seridó Mobile Belt, Borborema Province, Brazil

Abstract The Brejuí W-Mo skarn deposit is the main scheelite deposit of the Seridó tungsten province (Borborema province, NE Brazil). It constitutes the largest Brazilian W ore reserve. The orebodies are hosted in the metasedimentary Jucurutu Formation of the Seridó Group (650–610 Ma), close to the margin of the Neoproterozoic Acari pluton (Brasiliano orogeny). Skarns include prograde and retrograde mineral assemblages. Infrared spectral analysis shows that the skarns mainly reflect retrograde mineral assemblages, formed in three different hydrothermal alteration stages with overprinting. Mineral phases spectrally detected include (1) vesuvianite, actinolite, and phlogopite (alteration stage 1), (2) epidote, prehnite, and illite (alteration stage 2), and (3) laumontite, montmorillonite, chlorite, and gypsum (alteration stage 3; main ore zone). Phlogopite chloritization and actinolite recrystallization were observed in the main W-Mo skarn orebodies. Chloritization is marked by a displacement in the Fe-OH–related absorption wavelength (2,246.5–2,250.5 nm). A spectral index using spectral mixtures of laumontite + montmorillonite + actinolite is proposed here to map mineralized skarn layers (WO3 + Mo ≥ 0.1%) in drill cores. It was used to vector to richer mineralized bodies of the Brejuí deposit successfully and may be applied to similar skarn deposits with the same aim.

Magmatic and Geochemical Studies of Early Neoproterozoic Koraput Anorthosite Massif, Eastern Ghats Mobile Belt and their Implications for Rodinia Assembly

Abstract This paper reports new mineralogical and geochemical information on the Neoproterozoic Koraput Anorthosite pluton in domain-2 of the Eastern Ghats Mobile Belt. The lenticular-shaped, NE-SW trending small Koraput Anorthosite Pluton is dominated by gabbro-anorthosite, anorthosite sensu-stricto, and norite-diorite. Isolated patches of ferrodiorite (residual melt following crystallization of anorthosite) occur at the pluton margin. The estimated magmatic P-T conditions for anorthosite emplacement are ~1000°C and ~1.1GPa. In addition, the intermediate Anorthite (48-52 mole %) presence in plagioclase of gabbro-anorthosite and anorthosite indicates plagioclase crystallisation and accumulation at 30-40 km (circa 1 GPa) depth near the crust-mantle boundary. Furthermore, the geochemical traces suggest Koraput anorthosite pluton crystallized from a less fractionated basaltic magma. The REE and trace element systematics of anorthosite and norite-diorite exhibit enriched LILEs over depleted HFSE with negative Nb, Ta, Th, and Zr anomalies. Gabbro-anorthosite and ferrodiorite, on the other hand, exhibit positive Nb, Ti, and negative Zr and Th anomalies. Nb/Th versus Nb/La diagram implies lithodemic units in Koraput Anorthosite pluton are variably contaminated by crustal sources. Integration of geochemical data with available monazite age suggests that the Koraput anorthosite pluton in isotopic domain-2 of the Eastern Ghats Mobile Belt was emplaced during the collision between proto-India and Antarctica during the Rodinia assembly.

HFSE and REE mineralization at Machinga prospect in the Chilwa alkaline province, southern Malawi

Peralkaline rocks from Machinga prospect, situated at the north-eastern margin of Zomba-Malosa syeno-granitic pluton, were investigated to characterize high field strength elements (HFSE) and rare earth elements (REE) mineralization and elucidate the genesis of HFSE- and REE-bearing rocks based on mineralogical and geochemical methods. The investigation utilized two sets of samples: (1) quartz syenite, granite and granitic pegmatite and (2) a second set that includes eudialyte-bearing granite and pegmatite.A range of REE and HFSE minerals was observed in the rocks including monazite, bastnäsite, synchysite, pyrochlore, zircon and eudialyte. Pseudomorphs after eudialyte, mainly comprising zircon and quartz, occur abundantly in the pegmatite and to a lesser extent in alkaline granite. HFSE and REE are variably distributed in the three rocks types (quartz syenite, granite and granitic pegmatite). However, the most elevated concentration of total REE (between 1.0 wt % and 5.2 wt %) corresponds to highest Nb + Ta + Zr concentration (between 1.5 wt % and 7.5 wt %). The elevated concentration largely occurs in metasomatized granitic pegmatite and metasomatized quartz syenite (?) (intruded by granitic pegmatite in parts) but also corresponds to relatively fresh eudialyte bearing alkaline granite.Using geochemistryand field mapping results; the study demonstrates that the rocks comprising the Machinga prospect are a product of extended fractionation of silica saturated alkaline magma. As the magma fractionation proceeded, alkaline granite was emplaced adjacent to the quartz syenite as an outer ring. Later associated granitic pegmatite lenses crystalized as final stage of emplacement of the pluton.Based on petrography and mineral chemistry, three mineral paragenetic stages associated with HFSE and REE minerals were distinguished. The first magmatic stage crystallized zircon and other igneous minerals. The second stage is characterized by precipitation of eudialyte, and the third by decomposition and replacement of eudialyte by zircon and quartz and other secondary HFSE and REE repositories. These last two stages occurred during late magmatic to early metasomatic-hydrothermal activities, which were closely associated and likely occurred concurrently.

Interdisciplinary fracture network characterization in the crystalline basement: a case study from the Southern Odenwald, SW Germany

Abstract. The crystalline basement is considered a ubiquitous and almost inexhaustible source of geothermal energy in the Upper Rhine Graben (URG) and other regions worldwide. The hydraulic properties of the basement, which are one of the key factors in the productivity of geothermal power plants, are primarily controlled by hydraulically active faults and fractures. While the most accurate in situ information about the general fracture network is obtained from image logs of deep boreholes, such data are generally sparse and costly and thus often not openly accessible. To circumvent this problem, an outcrop analogue study was conducted with interdisciplinary geoscientific methods in the Tromm Granite, located in the southern Odenwald at the northeastern margin of the URG. Using light detection and ranging (lidar) scanning, the key characteristics of the fracture network were extracted in a total of five outcrops; these were additionally complemented by lineament analysis of two different digital elevation models (DEMs). Based on this, discrete fracture network (DFN) models were developed to calculate equivalent permeability tensors under assumed reservoir conditions. The influences of different parameters, such as fracture orientation, density, aperture and mineralization, were investigated. In addition, extensive gravity and radon measurements were carried out in the study area, allowing fault zones with naturally increased porosity and permeability to be mapped. Gravity anomalies served as input data for a stochastic density inversion, through which areas of potentially increased open porosity were identified. A laterally heterogeneous fracture network characterizes the Tromm Granite, with the highest natural permeabilities expected at the pluton margin, due to the influence of large shear and fault zones.

Porosity induced by dislocation dynamics in quartz-rich shear bands of granitic rocks

The production of micro-pores is a driving mechanism for fluids to interact with deep environment and influence rock properties. Yet, such a porosity still remains misunderstood to occur in viscous rocks and may be attributed to either grain boundary sliding (GBS), dissolution effects or sub-grain rotation. Here we focus on quartz-rich shear bands across the Naxos western granite (Aegean Sea, Greece), where we document sub-micron pores at quartz boundaries. While most of these pores are observed along grain boundaries, some of them occur at intra-grain boundaries, which excludes dissolution or GBS to produce them, but instead involves the dynamic of dislocations. We then confirm that quartz is dominated by dislocation creep with evidence of a moderate to strong lattice-preferred orientation (LPO) and numerous tilt/twist boundaries, including at the pluton margin where rocks embrittled. These features coincide with (1) randomly oriented ‘inclusion’ quartz grains along tilt/twist boundaries and (2) a partial dependency of the LPO strength on grain size. Our findings suggest that pores arise from coalescing dislocations at boundaries of rotating sub-grains, providing nucleation sites for new grains to be precipitated during plastic flow. Fluid infiltration, rock embrittlement and related implications are also expected through pores accumulation with increasing strain.

Geology, alteration geochemistry, and exploration geochemical mapping of the Ertsberg Cu-Au-Mo district in Papua, Indonesia

The Ertsberg-Grasberg district hosts the world's largest Cu-Au(Mo) skarn system. The mineralization is related to the Pliocene Ertsberg- and Grasberg intrusive complexes, which intruded the Jurassic-Cretaceous Kembelangan Group siliciclastics and Cenozoic New Guinea Limestone Group. Despite previous studies, elemental migration across the porphyry-skarn system and across the different ore stages is still unclear. Here, we attempt to tackle this issue by integrating ore deposit geology, alteration/mineralization paragenesis, whole-rock geochemistry of the Ertsberg granitoids and carbonate wallrocks, together with spatial mapping of hydrothermal minerals and target/pathfinder trace elements at the Deep Mill Level Zone of Ertsberg. Ores at Ertsberg are mainly skarn-hosted, whose formation comprises five stages: (I) prograde skarn; (II) retrograde alteration; (III) massive anhydrite replacement; (IV) late hydrothermal veining; (V) supergene alteration. Massive magnetite and main CuAu mineralization occurred in Stage II and III, respectively, although minor Cu mineralization persisted to Stage IV.The Ertsberg granitoids have medium SiO2 and high Al2O3 contents, and consist mainly of (quartz-)monzonite and minor (monzo)diorite. Least-altered samples have upper crust-normalized enrichments in certain large ion lithophile elements (LILEs, e.g., U and K) and depletions in all high field strength elements (HFSEs, e.g., Ti, Y, and Yb). The samples are fresh to moderately-altered, featured by garnet-diopside skarn and/or porphyry-style (potassic, propylitic, phyllic) alterations and anhydrite replacement. Elemental mass balance calculation indicates that the porphyry-style alteration zones are featured by feldspar destruction and the accompanying alkali depletions, although the loss of KRb (in potassic and phyllic zones) and Ca-Ba-Sr (in propylitic zone) are balanced by their respective capturing in secondary K-feldspar-mica and actinolite-epidote-anhydrite. Decarbonation of the dolomitic wallrocks likely released Mg-Ca-Sr, which facilitated the subsequent forsterite-diopside exoskarn and anhydrite alteration. The enrichments of various ore-related target/pathfinder elements in these altered samples are consistent with their mineralized nature, e.g., FeMn (magnetite), Cu-Fe-Bi-Se-Te (bornite), MoRe (molybdenite), Au-Ag-As-Bi-Te (auriferous pyrite/arsenopyrite), Zn-Fe-Mn-Sn-In (sphalerite), and Pb-Ag-Bi (galena).Alteration mineral and target/pathfinder elemental distribution patterns show that the ore fluids were likely originated from the porphyry hydrothermal (potassic) centers near the Ertsberg pluton margin. The fluids then spread out from the potassic centers, forming the surrounding propylitic zone, and migrated along the largely NW-SE-oriented intrusive contact and fault zones. This formed the endoskarn in the Ertsberg pluton, the forsterite-diopside exoskarn in the Waripi Formation dolomite, and the garnet-diopside exoskarn in the Ekmai Formation limestone/calcareous sandstone-siltstone. As the hydrothermal system started to wane, the prograde skarn formation is partially overprinted/enveloped by retrograde serpentine and actinolite/tremolite-epidote-chlorite alterations, and then by magnetite mineralization and massive anhydrite replacement in the carbonate wallrocks. The texturally-destructive skarn formation, and the dissolution of carbonate wallrocks and hydrothermal anhydrite were likely pivotal in creating the highly-fractured Broken Zone, which served as a structural trap for the high-grade Cu-Au-Mo ore deposition.

Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: A Late Archean analogy to modern accretionary plate margins?

The Canadian Superior Province has become one of the key test pieces to discuss tectonic processes and mechanisms of crustal growth in the Late Archean. The Province consists of a >2.8 Ga proto-cratonic core intruded by voluminous arc-like plutons and surrounded by a series of narrow, elongate ca. 2.8–2.7 Ga juvenile belts, also referred to as terranes or domains. The terranes seem to wrap around the proto-cratonic core and generally young outward, but the kinematics and geodynamic causes of their assembly remain debated. In this paper, we examine the Radisson pluton in northeastern Québec, which intruded the southern, outer edge of the presumed magmatic arc (Bienville domain) along its ~WNW–ESE-trending tectonic boundary with the proto-cratonic crust (La Grande domain). The pluton, dominated by porphyritic monzogranite to quartz monzonite, was emplaced at around 2712 Ma and exhibits complex internal structure resulting from superposed magmatic to solid-state deformations. An early margin-parallel ~WNW–ESE magmatic foliation containing a steep lineation, recognized by the anisotropy of magnetic susceptibility (AMS), is interpreted as recording vertical stretching and horizontal flattening of highly crystallized magma, either due to emplacement and/or pure shear dominated transpression. More widespread, however, is a horizontal lineation within the same foliation that is interpreted as recording post-emplacement, but still syn-magmatic, tectonic strain (~NNE–SSW shortening and boundary-parallel stretching). Upon cooling, localized dextral S–C mylonite zones accommodated further shortening within the pluton whereas undeformed late-stage felsic dikes cross-cut the solid-state fabric at an angle to the pluton margins. We suggest that this structural succession, also reproduced by numerical fabric modeling, is a local-scale signal of a two-stage assembly of the northeastern Superior Province: the frontal, NNE-directed terrane convergence and attachment to the cratonic nucleus, operating in a ‘hot’ regime with voluminous arc-like plutonism, was followed by more localized dextral shearing parallel to terrane boundaries. The latter phase is recorded at the proto-craton margin but also in the outboard Abitibi greenstone belt virtually at the same time (ca. 2700–2690 Ma). In combination, the two-stage evolution and similar deformation distributed over a broad region resemble modern large hot orogens formed in a plate-tectonic regime.

Genesis and geodynamic setting of the Nanyangtian tungsten deposit, SW China: Constraints from structural deformation, geochronology, and S–O isotope data

Southeastern (SE) Yunnan contains one of the most economically important Mesozoic W–Sn metallogenic belts in South China. The belt comprises three well-known W–Sn ore-concentration areas, namely, the Gejiu, Bainiuchang, and Laojunshan areas from west to east. Numerous skarn W–Sn deposits have been identified in association with Late Cretaceous granites in these areas, although the genesis and geodynamics of the deposits remain uncertain. The Nanyangtian W deposit, which is located in the southern part of Youjiang Basin and the eastern margin of the Laojunshan granitic pluton, consists of skarn and superimposed scheelite–mica–quartz (SMQ) vein mineralization. Multiple stages of structural deformation and mineralization, and geochronological data indicate the Nanyangtian deposit contains important clues to the genesis and geodynamics of W mineralization in SE Yunnan. Structural analysis of the Song Chay dome allows four stages of deformation to be identified from early to late, namely, north-directed detachment (D1), northwest-directed thrusting (D2), dextral transtensional faulting (D3), and late sinistral strike-slip faulting (D4). Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of zircons from plagio-amphibolite and altered granitoid yielded ages of 220.37 ± 0.59 and 162.5 ± 3.0 Ma, respectively, indicating the timings of regional detachment deformation (D1) with related metamorphism, and the emplacement of coeval granite related to skarn W mineralization, respectively. 40Ar–39Ar isotope dating of muscovites from Laochengpo gneissic granite yielded well-defined plateau ages of 224.12 ± 0.35 and 141.2 ± 0.2 Ma, indicating the timing of D1 deformation and Early Cretaceous tectono-thermal event, respectively. 40Ar–39Ar isotope dating of biotite from an SMQ vein within the deposit yielded a well-defined plateau age of 97.81 ± 0.70 Ma, which is interpreted to represent the timing of SMQ vein W mineralization. δ34S values of sulfide in skarn ores range from 7.46‰ to 9.55‰, and δ18O values of skarn-type scheelite vary from 2.9‰ to 4.0‰. However, δ18O values obtained for vein-type scheelite range between 4.4‰ and 5.7‰, indicating a different material source of W mineralization from that of the skarn ores. Based on our analysis of ore-controlling structures, isotopes, and geochronology, we propose that the inter-layer fractures that formed during D2 controlled the Late Jurassic skarn mineralization and associated granitic magmatism, and that Late Cretaceous SMQ vein mineralization resulted from dextral transtension (D3). Integration of the sequence of deformation and the two periods of W mineralization in combination with well-defined geochronological constraints indicates four stages of tectonism and mineralization in SE Yunnan: Late Triassic–Early Jurassic D1 deformation and early Sn mineralization, late Early–Late Jurassic D2 deformation and subsequent skarn mineralization and associated granitic magmatism, Cretaceous D3 deformation and associated W and late Sn polymetallic mineralization within a regional extensional setting, and Cenozoic post-mineralization strike-slip D4 deformation. By combining our new results with other research findings from SE Yunnan and South China, the inferred sequence suggests that the multiple stages of deformation and mineralization were influenced by geodynamic processes involving the Indochina and Paleo-Pacific Plates with the South China Block.

Genetic relations between enclaves and their host granitoids from Doumer Island, northern Antarctic Peninsula: Evidence from mineral chemistry, Sr–Nd and Li isotopes

Dark enclaves in the late Paleocene granitoids of Doumer Island, northern Antarctic Peninsula, were examined by field investigations, zircon U–Pb geochronology, mineral chemistry, whole-rock major- and trace-elements, and Sr-Nd-Li isotopes to elucidate their origins and genetic relationships with host granitoids. The enclaves and host granitoids have similar zircon U–Pb ages of 54.6–55.7 Ma and indistinguishable Nd–Sr isotopes of εNd(t) = 4.7–6.1, ISr = 0.7036–0.7039, indicating a cogenetic relationship. A pyroxene-dioritic enclave (WA16–06-3) is a cognate restite with peritectic phases that resulted from amphibole-dehydration melting at temperatures above 800 °C and at low-P (7–10 kbar). Two other enclaves (WA16–07-2 and WA15–27-3) are fragments of the chilled margins of the conduits entrained in the later insurgent felsic magma. The flat, right-leaning REE patterns are accompanied by positive Eu anomalies (Eu/Eu* = 1.07–1.16) in the dark enclaves and negative Eu anomalies (Eu/Eu* = 0.72–0.88) in the host intrusions. The accumulation of restitic plagioclase and peritectic pyroxene could have induced the positive Eu anomalies in the dark enclaves. The P–T conditions estimated from cummingtonite rims on pyroxene in the pyroxene-dioritic enclave suggest near-isothermal decompression in an environment of fast exhumation. Enclave WA16–07-2 is part of an intense swarm of enclaves near the pluton margin, and crystallized from a less evolved melt that was mechanically eroded by the host magma, indicating a transient state of magma flow. The dioritic enclaves were transported in the form of ductile crystal–melt mushes that were rotated in the insurgent viscous granitoid magma. The enclaves become rounder and sparser towards the interior of the host pluton. Dehydration melting of metabasalt could have produced these melts that are rich in δ7Li while leaving a lighter δ7Li pyroxene-rich residue at the source. The underlying cause of these events was the retreat of the subducting proto-Pacific (Phoenix) oceanic slab beneath the western Antarctic Peninsula that triggered melting of asthenospheric mantle, with basaltic magma underplating and melting a juvenile metabasaltic lower crust.

No ring fracture in Mono Basin, California

AbstractIn Mono Basin, California, USA, a near-circular ring fracture 12 km in diameter was proposed by R.W. Kistler in 1966 to have originated as the protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been reactivated in the middle Pleistocene, and to have influenced the arcuate trend of the chain of 30 young (62–0.7 ka) rhyolite domes called the Mono Craters. In view of the frequency and recency of explosive eruptions along the Mono chain, and because many geophysicists accepted the ring fracture model, we assembled evidence to test its plausibility. The shear zone interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50–400 m wide but is exposed only along a 7-km-long set of four southwesterly outcrops that subtend only a 70° sector of the proposed ring. The southeast end of the exposed shear zone is largely within the older June Lake pluton, and at its northwest end, the contact of the Aeolian Buttes pluton with a much older one crosses the shear zone obliquely. Conflicting attitudes of shear structures are hard to reconcile with intrusive protoclasis. Also inconsistent with the margin of the ovoid intrusion proposed by Kistler, unsheared salients of the pluton extend ∼1 km north of its postulated circular outline at Williams Butte, where there is no fault or other structure to define the northern half of the hypothetical ring. The shear zone may represent regional Cretaceous transpression rather than the margin of a single intrusion. There is no evidence for the Aeolian Buttes pluton along the aqueduct tunnel beneath the Mono chain, nor is there evidence for a fault that could have influenced its vent pattern. The apparently arcuate chain actually consists of three linear segments that reflect Quaternary tectonic influence and not Cretaceous inheritance. A rhyolitic magma reservoir under the central segment of the Mono chain has erupted many times in the late Holocene and as recently as 700 years ago. The ring fracture idea, however, prompted several geophysical investigations that sought a much broader magma body, but none identified a low-density or low-velocity anomaly beneath the purported 12-km-wide ring, which we conclude does not exist.

Petrogenesis and geodynamic setting of the Late Carboniferous granodiorite porphyry in Miaoergou pluton, southern West Junggar

位于西准噶尔南部的庙尔沟岩体主体由碱长花岗岩和少量紫苏花岗岩组成。本文在前人工作基础上，以岩体东南边缘新发现的花岗闪长斑岩为研究对象，开展岩石学、年代学和Hf同位素以及全岩地球化学研究，确定花岗闪长斑岩形成时代、揭示岩石成因类型及源区属性、探讨其与碱长花岗岩和紫苏花岗岩岩浆演化成因联系及其形成的深部动力学过程。锆石U-Pb定年结果显示，花岗闪长斑岩形成于317.4±1.9Ma，为晚石炭世早期岩浆活动的产物，明显早于紫苏花岗岩（~307Ma）和碱长花岗岩（~303Ma）。岩石地球化学数据表明，花岗闪长斑岩具有较高硅、中等铝，贫钙、铁、镁，富集Rb、K、Th、U，强烈亏损Nb、Ta、Ti的特征，为钙碱性弱过铝质I型花岗岩；紫苏花岗岩更多的表现出钙碱性-高钾钙碱性镁质I型紫苏花岗岩特征；碱长花岗岩为碱性准铝质-弱过铝质A型花岗岩。锆石Hf同位素分析结果表明，花岗闪长斑岩、紫苏花岗岩和碱长花岗岩均具有高正的ε_Hf（t）值（+11.6~+15.8）和年轻的二阶段模式年龄（325~600Ma），表明其原始岩浆主要起源于亏损地幔新衍生的年轻地壳物质。综合分析认为，庙尔沟岩体花岗闪长斑岩形成于晚石炭世早期洋壳俯冲背景，由底侵的、受流体交代的幔源基性岩浆与其诱发的年轻下地壳酸性岩浆在深部混合而成。紫苏花岗岩和碱长花岗岩形成于弧后伸展背景，前者是伸展初期继续底侵于下地壳的幔源玄武质岩浆降温释放大量的水和热诱使早期侵位于下地壳的镁铁质岩石再次发生部分熔融的产物，后者是伸展后期大规模软流圈地幔上涌底垫加热年轻中下地壳使其部分熔融而成。;The Miaoergou pluton, located in southern West Junggar, is mainly composed of alkali-feldspar granites with small amount of charnockites. On the basis of previous researches, this paper mainly focuses on the newly discovered granodiorite porphyry in southwestern margin of the Miaoergou pluton. A comprehensive study of petrographical, zircon U-Pb geochronological and in-situ Hf isotopic, whole-rock geochemical analyses were carried out to determine its diagenetic age and reveal the petrogenesis, the nature and composition of the magma source of this porphyry, and furthermore, to discuss the genetic relationships among the granodiorite porphyries, alkali-feldspar granites and charnockites and their geodynamic processes. LA-ICP-MS zircon U-Pb dating for the granodiorite porphyry yielded crystallization age of 317.4±1.9Ma, defining an early Late Carboniferous magmatic event in southern West Junggar, which is significantly earlier than the ages of charnockites (~307Ma) and alkali-feldspar granites (~303Ma). The geochemical characteristics indicate that the studied granodiorite porphyries belong to cal-alkaline weakly peraluminous I-type granite, and are characterized by slightly high silicon, medium aluminum, but low ferrum, calcium and magnesium, and also enrichment in Rb, K, Th, U with pronounced negative Nb, Ta and Ti anomalies. The charnockites possess calc-alkaline to high-K calc-alkaline magnesian I-type charnockite affinities, which were previously considered A-type granites by some geologists, while alkali-feldspar granites belong to alkaline metaluminous to weakly peraluminous A-type granites. In-situ zircon Hf isotope analyses show that these granites have high positive ε_Hf(t) values (+11.6~+15.8) and very young two-stage Hf model ages of 325~600Ma, indicating that the magma was sourced from juvenile crust originated from depleted mantle during the Late Neoproterozoic to Early Carboniferous period. In association with previous analysis, it is proposed that the Miaoergou granodiorite porphyries were formed in an oceanic crust subduction setting during early Late Carboniferous. They are probably the products of the acidic magma formed by partial melting of the juvenile lower crust and mixed with the underplated basaltic magma derived from the fluid metasomatic mantle wedge in the deep crust to some extent. Both the charnockites and alkali-feldspar granites were formed in a back-arc extensional setting related to the northwestward subduction of the Junggar oceanic crust. The charnockites may be generated by re-melting of juvenile lower crust mafic rocks, probably triggered by addition of water and heat released from cooling of underplated depleted mantle-derived basaltic magma during the incipient rifting stage of the back-arc basin, and the alkali-feldspar granites may be generated by partial melting of the juvenile middle-lower crust induced by the large-scale upwelling of asthenosphere during the later stage of the back-arc extension.

Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

Abstract Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplacement and crystallization of the ∼2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subhorizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal pluton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shortening and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary.

The Relationship Between Magmatism and Deformation Along the Intra‐arc Strike‐Slip Atacama Fault System, Northern Chile

AbstractMagmatic arcs may play a major role in the initiation, behavior, and abandonment of intra‐arc strike‐slip systems. Here we present zircon U‐Pb and (U‐Th)/He geochronology/thermochronology with new mapping to relate Coastal Cordillera arc magmatism to sinistral shear along the Atacama fault system (AFS) in northern Chile. New dates from 18 intrusions along the AFS between 24.6°S and 27°S compiled with published data record a minor Early Jurassic magmatic pulse (195–175 Ma), broad latest Jurassic to Early Cretaceous (150–120 Ma) pulse, and a minor younger (120–105 Ma) pulse. Mylonitization occurred only along the margins of Early Cretaceous plutons and surrounding Paleozoic metasedimentary rock, whereas Jurassic plutons and metasedimentary rocks away from Early Cretaceous plutons lack mylonitic fabrics. Early Cretaceous magmatism facilitated AFS deformation by thermally weakening the crust with elevated geothermal gradients that enabled mylonitization to take place at ~5‐ to 7‐km depths and low stresses. Spatial variability of pluton emplacement produced significant rheological heterogeneity, giving rise to a highly segmented fault system that did not originate as a regional‐scale shear zone. Synkinematic dikes (~120–117 Ma) cut mylonitic fabrics, and a postkinematic dike (~110 Ma) records the end stages of slip. The cessation of slip coincided with cooling below ~180 °C at ~116–99 Ma as arc magmatism migrated eastward and geothermal gradients relaxed, coeval with a major reorganization in plate motion and the onset of seafloor spreading in the south Atlantic.

Tectonic and district to deposit-scale structural controls on the Ge’erke orogenic gold deposit within the Dashui-Zhongqu District, West Qinling Belt, China

Ge’erke (>100 t gold) is one of six Mesozoic gold deposits within the Dashui-Zhongqu District, that collectively have a non-JORC resource of about 150 tonnes gold out of a total of >1100 tonnes gold for the host West Qinling Belt. The deposits in the Dashui-Zhonggu District are deeply weathered to depths of up to 400 m, with secondary Fe-oxides having replaced pyrite and other sulfides. This makes traditional ore deposit studies involving mineralogical, geochemical and isotopic analysis difficult. Thus, their association has been controversial with correlations to deposits that include Carlin and intrusion-related types postulated. However, these associations can be discounted in favour of their classification as epizonal orogenic gold deposits. At Ge’erke, gold mineralization was focused in the hangingwall of a broadly south-dipping southern margin of a granite pluton in a wide variety of fault trends. This complexity was caused by heterogeneous stresses developed around the rigid pluton by the broadly NE-SW directed regional principal maximum stress responsible for the sinistral movement along the crustal-scale faults which control the regional alignment of the West Qinling gold deposits. At the orebody and ore shoot scale, gold mineralization is controlled by rheology contrasts, particularly the fault contacts between crystalline limestone units and granite dykes, which are themselves in a variety of orientations because of inhomogeneous strain around the parent granite pluton. In this region the secondary faults that subparallel the Mianlue Suture and associated crustal-scale faults are poorly gold mineralized in the absence of granite plutons. During exploration, such buried plutons and the orientations of their margins can best be detected by gravity surveys, although magnetic surveys may also be useful.

Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late-collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20–40% vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre-Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history (i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, magmatism could result from a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian-Moldanubian continental block). This delamination would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints

Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail. In this study, we attempt to reconstruct the emplacement process of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the ‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic granite plutons.

Fabrics and geochronology of the Taibai ductile shear zone: Implications for tectonic evolution of the Qinling Orogenic Belt, central China

The Taibai ductile shear zone affecting the northern margin of the Taibai granitic pluton which emplaced in the North Qinling Orogenic Belt, is crucial to understand the amalgamation and post-collision history between the North China Block and the South China Block. The shear sense indicators and the kinematic vorticity numbers (0.42–0.59) suggest that the Taibai shear zone is an elongating general shear zone involving both sinistral shear and pure shear components. The quartz lattice preferred orientations mainly indicate the activation of prism 〈a〉 slip system combined with rhomb 〈a〉 slip to prism 〈a〉 slip system. The dynamic recrystallization of quartz is accommodated by sub-grain rotation and bulging recrystallization while plagioclase mainly displays bulging recrystallization. These characteristics suggest that the mylonites have experienced ductile shear deformation under greenschist facies conditions at temperatures of ∼400–550 °C. Zircons from granitic mylonite yield upper and lower intercept ages of 425 ± 4 Ma and 144 ± 1 Ma, respectively, suggesting the protolith of the mylonite crystallized at ca. 425 Ma and was involved into a hydrothermal activity at ca. 144 Ma. Muscovite 40Ar/39Ar dating of the mylonite sample yield a plateau age of 346 ± 5 Ma and an inverse isochron age of 344 ± 4 Ma, constraining the ductile shear deformation occurring during the Early Carboniferous time. We suggest that the Taibai shear zone was formed by NE-SW oblique compression with top-to-the-NW shear sense during the Early Carboniferous time, following the N-S shortening caused by collision between the North China Block and South China Block along the Shangdan suture at ca. 400 Ma.

Deformation-driven emplacement-differentiation in the Closepet pluton, Dharwar Craton, South India: an alternate view

Abstract In the Late Archean north-trending Closepet pluton, trains of euhedral K-feldspar phenocrysts and matrix-supported idiomorphic K-feldspar crystals in the central part of the pluton define oblique-to-pluton margin steep-dipping east/ENE-trending magmatic fabrics. The magmatic fabric is defined by phenocryst-rich and phenocryst-poor layers, with the euhedral porphyries continuous across the layers. The fabrics are near-orthogonal to the gently-dipping gneissic layers in the host gneisses. The fabrics curve adjacent to locally-developed north/NNE-trending melt-hosted dislocations parallel to the axial planes of horizontal/gently-plunging north-trending upright folds in the host gneisses. In the pluton interior, both fabrics in the intrusives formed at supra-solidus conditions, although the volume fraction of melts diminished drastically due to cooling/melt expulsion. At the pluton margin, the north-trending fabric is penetrative and post-dates magma solidification. Within the pluton, the major element oxides, rare earth elements, anorthite contents in plagioclase, and (Mg/Fe + Mg) ratios in biotite decrease with increasing SiO 2 from phenocryst-rich (up to 75% by volume) granodiorite to phenocryst-poor (<15 vol%) granite that broadly correspond to minimum melt composition. The chemical-mineralogical variations in the pluton is attributed to deformation-driven ascent of magma with heterogeneous crystal content, ascending at variable velocities (highest in crystal-poor magma) along oblique-to-pluton margin east/ENE-trending extensional fractures induced by dextral shearing.

Ushuaia pluton: Magma diversification, emplacement and relation with regional tectonics in the southernmost Andes

The Ushuaia pluton (UP) and the Ushuaia Peninsula Andesites (UPA) are intrusive units of a Late Cretaceous rear-arc in the Southernmost Andes. We report new LA-ICP-MS zircon ages from ∼71 to 75 Ma for the UP, and of ∼84 Ma for the UPA, which allow to differentiate these two magmatic events; consistent with their distinct petrography and geochemistry. The UP is a composite, incrementally assembled pluton in upper crustal levels. Successive injections of magma batches define two main sections, each in turn constructed by several smaller pulses: an early ultramafic-mafic section consists mainly of hornblende pyroxenite, hornblendite and gabbro/diorite with reverse concentric zonation, and a later intermediate section built up mainly by monzodiorite. Mingling and layering between mafic and intermediate facies along the contact point to simultaneous emplacement at the end of one main pulse and the beginning of the other. Mineralogy and geochemistry suggest the rock sequence in the UP is cogenetic. In part it resulted by in-situ magmatic processes that included crystal-melt fractionation during injection of magma mushes, mingling, local melt segregation and mixing with crustal partial melts. The country rock is affected by penetrative ductile deformation and regional metamorphism developed during mid-Cretaceous Andean main orogenic phase. It also shows an overimposed pluton-margin parallel foliation and a post-kinematic contact metamorphic aureole. On the other hand, the UP has a concentric igneous foliation parallel to pluton margins interpreted as a primary magmatic feature. The above evidences, along with absence of subsolidus fabrics and metamorphism, indicate the pluton post-date peak regional metamorphism and ductile structures in the country rock. The new zircon ages constrain the top age of early Andean deformation, as well as the magmatic gap registered for the end of the Cretaceous, and confirm an emplacement during a recorded compressive regime that transmitted shortening to the foreland.

K-feldspar megacryst accumulations formed by mechanical instabilities in magma chamber margins, Asha pluton, NW Argentina

The Asha pluton, the oldest unit of the San Blas intrusive complex (Early Carboniferous), exhibits impressive examples of magmatic structures formed by accumulation of K-feldspar megacrysts, enclaves, and schlieren. Almost all recognized structures are meter-scale, vertically elongate bodies of variable shapes defined as fingers, trails, drips, and blobs. They preferentially developed near the external margin of the Asha pluton and generally are superimposed by chamber-wide magmatic fabrics. They mostly have circular or sub-circular transverse sections with an internal fabric defined by margin-parallel, inward-dipping concentric foliation and steeply plunging lineation at upper parts and flat foliation at lower parts. The concentration of megacrysts usually grades from upper sections, where they appear in a proportion similar to the host granite, to highly packed accumulations of K-feldspar along with grouped flattened enclaves at lower ends. These features suggest an origin by downward localized multiphase magmatic flow, narrowing and ‘log jamming’, and gravitational sinking of grouped crystals and enclaves, with compaction and filter pressing as main mechanisms of melt removal. Crystal size distribution analysis supports field observations arguing for a mechanical origin of accumulations. The magmatic structures of the Asha pluton represent mechanical instabilities generated by thermal and compositional convection, probably owing to cooling and crystallization near the pluton margins during early stages of construction of the intrusive complex.