Progressive hybridisation between Late Devonian mafic-intermediate and felsic magmas in the Meguma Zone of Nova Scotia, Canada

Two felsic plutons of Late Devonian (385–370 Ma) age in the Meguma Zone of southwestern Nova Scotia contain three circa 376 Ma synplutonic mafic-intermediate intrusions that collectively record progressive stages of in situ hybridisation. A 5 m wide spessartite dyke in the Port Mouton Pluton probably underwent rapid cooling and crystallisation immediately after intrusion, which heated and coarsened the adjacent tonalite. An 85 m long sheet of pillowed kersantite (also in tonalite of the Port Mouton Pluton) presumably contained residual magma after quenching and obtained K, P, Ba, Rb, more radiogenic Sr, Zr, Nb, and light REE from the tonalite during magma mingling. The third synplutonic body, a >100 m wide diorite sheet, intrudes granodiorite of the adjacent Shelburne Pluton and has a circa 45 m wide gradational contact of metaluminous hornblende-tonalite. This tonalite dominantly records magma mixing by the transfer of Ti, Mg, Fe, Ca, and V in hornblende, biotite, plagioclase, and (at least in part) apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite, probably in response to convection. Scattered data points, unusual “concave-down” variation trends for Al₂O₃, P₂O₅, and Sr, and non-hyperbolic Sr-Nd isotopic characteristics in the tonalite, apparently reflect syn- or post-mixing fractionation and accumulation of xenocrysts from residual magma. Phosphorus may have assisted diffusion of Sr, Zr, Nb, and light REE, and caused premature quenching of the hybrids at Mcleods Cove and Birchtown, during magma mingling and mixing. Received: 1 January 1996 / Accepted: 3 August 1996     

Nitrogen distribution in Lower Palaeozoic slates/phyllites of the Meguma Supergroup, Nova Scotia, Canada: implications for Au and Zn-Pb mineralisation and exploration

Major and trace elements and nitrogen were analysed in 59 Cambrian and Ordovician slates/phyllites of various lithostratigraphic levels within the Meguma Supergroup, 14 Ordovician slates/phyllites of The Ovens gold mineralisation, and 16 Ordovician slates/phyllites of the Eastville Pb-Zn-Cu mineralisation.The fixed nitrogen content and the normative N_fix/K₂O ratio of muscovite separates members of the Goldenville (sandstone-greywacke-quartzite dominated) and Halifax Formation (slate-phyllite dominated). Low fixed nitrogen concentration is related to high manganese concentration. Black slates/phyllites of the saddle-reef-type Au-mineralisation in The Ovens area are characterised by high fixed nitrogen and a high normative N_fix/K₂O ratio of muscovite. The nitrogen is of secondary hydrothermal origin. Therefore, it might be possible to use the fixed nitrogen as an exploration tool for black slates/phyllite hosted gold mineralisation in Nova Scotia. Mineralised slates/phyllites of the Eastville stratabound Pb-Zn mineralisation cannot be distinguished from nonmineralised time equivalent lithologies on the basis of the fixed nitrogen distribution or the N_fix/K₂O ratio of muscovite.     

Geochemical evolution of peraluminous plutons in southern Nova Scotia, Canada—a pegmatite-poor suite

Three southern Nova Scotia plutons crystallized rapidly at 375 Ma from magma containing both mantle and crustal components. Isotopic and chemical data suggest that the crustal contribution included both lower crustal material and Cambro-Ordovician turbidites of the host Meguma Group. Despite local evidence of mixing and mingling of magmas, the bulk of the plutons evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differentiated. Like parental plutons, pegmatites fall into biotite+plagioclase and muscovite+potassium feldspar assemblages. The latter locally contain Mn-rich garnet+biotite, giving calculated P–T conditions of pegmatite crystallisation of 620°C, 0.44 GPa under water-saturated conditions. Host rocks at the time of emplacement experienced P–T conditions varying from <500° to 620°C at 0.44 GPa. Beryl-bearing pegmatites occur only where host rocks were below sillimanite-grade, and pegmatites of any kind are scarce in sillimanite-grade host rocks. Rarity of pegmatites can be ascribed to a combination of insufficient amounts of F, Li, and B in the magma with rapid cooling which prevented extensive fractionation and undercooling. Our data suggest rather that peraluminous suites, produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks, and typified by biotite-rich, cordierite-bearing plutons, are not favorable locales for major rare-metal-enriched pegmatite fields. Such fields appear to require relatively prolonged anatexis resulting from crustal thickening, or an areally extensive mafic underplate,     

Application of LA ICP-MS analysis of arsenopyrite to gold metallogeny of the Meguma Terrane,Nova Scotia,Canada

Investigation of gold metallogeny in the Paleozoic Meguma terrane (Canada) is conducted through LA-ICP-MS analysis of arsenopyrite collected from eight slate-belt style vein gold deposits using a novel approach integrating elemental distribution maps and their derived elemental paragenesis with multi-element binary plots. The data reveal two distinct gold events: 1) an early event characterized by a Co-Ni-Mo-Sb-Se elemental association related to initial growth of arsenopyrite that reflects the presence of invisible gold (>10 ppm); and 2) a second event, spatially associated with late fracture sets, that is characterized by an Al-Ti-V-Mn element association and reflects either remobilization or upgrading of primary invisible gold and is manifest as visible gold.The results of this study indicate a complex and protracted history of gold mineralization which has important ramifications for the Meguma gold deposits, as well as other orogenic gold districts globally. In the case of the Meguma Terrane, it involves an initial gold event that is followed by element mobilization and, in the case of precious metals (Au, Ag), an upgrading through a zone refining process. In addition, the variable coupling and decoupling of elements is only revealed using in-situ derived LA-ICP-MS data.     

Lead isotope and trace element composition of K-feldspars from peraluminous granitoids of the Late Devonian South Mountain Batholith (Nova Scotia, Canada): implications for petrogenesis and tectonic reconstruction

The South Mountain Batholith (SMB) of southwestern Nova Scotia (Canada) is a Late Devonian (~375 Ma) composite intrusion, which crops over an area of about 7,300 km². This peraluminous granitoid body consists of rocks ranging from granodiorite through monzogranite and leucomonzogranite to leucogranite that locally host greisen tin-base metal mineralization. K-feldspar displays large compositional variations of trace elements and Pb isotopic ratios, particularly in the highly fractionated rocks. Many variations are consistent with processes of fractional crystallization, but a distinct enrichment of Rb, Li and Cs accompanied by low K/Rb, Ba/Rb, Eu/Eu* and K/Cs ratios point to the role of fluids during the late stages of magmatic evolution. The correlation of Pb isotopic ratios with the enriched elements and their ratios implies that the isotopic variations are an integral part of the evolution of the SMB. Together with well-defined isochronal relationships of Pb systems in the feldspars, the correlation suggests that fractional crystallization accompanied in the late stages by fluid fractionation led to the formation of Li–F-rich leucogranites. Internally derived U-rich fluids fractionated U/Pb ratios, which in turn produced distinct variations of ²⁰⁶Pb/²⁰⁴Pb and ²³⁸U/²⁰⁴Pb ratios in K-feldspars. This implies that the Pb isotopic values of K-feldspar, which have traditionally been used for tectonic reconstructions, might have been modified in many granitic rocks. Thus, only early magmatic K-feldspars, which show no discernible effects of fluid fractionation yield the initial Pb isotopic compositions of the parental granitic magmas and their sources. The data also show that the geochemical characteristics of the leucogranites are the results of magmatic evolution rather than a distinctive source.     

Stratigraphy and facies development of the marine Late Devonian near the Boulongour Reservoir,northwest Xinjiang,China

Late Devonian to Early Carboniferous stratigraphic units within the ‘Zhulumute’ Formation, Hongguleleng Formation (stratotype), ‘Hebukehe’ Formation and the Heishantou Formation near the Boulongour Reservoir in northwestern Xinjiang are fossil-rich. The Hongguleleng and ‘Hebukehe’ formations are biostratigraphically well constrained by microfossils from the latest Frasnian linguiformis to mid-Famennian trachytera conodont biozones. The Hongguleleng Formation (96.8 m) is characterized by bioclastic argillaceous limestones and marls (the dominant facies) intercalated with green spiculitic calcareous shales. It yields abundant and highly diverse faunas of bryozoans, brachiopods and crinoids with subordinate solitary rugose corals, ostracods, trilobites, conodonts and other fish teeth. The succeeding ‘Hebukehe’ Formation (95.7 m) consists of siltstones, mudstones, arenites and intervals of bioclastic limestone (e.g. ‘Blastoid Hill’) and cherts with radiolarians. A diverse ichnofauna, phacopid trilobites, echinoderms (crinoids and blastoids) together with brachiopods, ostracods, bryozoans and rare cephalopods have been collected from this interval. Analysis of geochemical data, microfacies and especially the distribution of marine organisms, which are not described in detail here, but used for facies analysis, indicate a deepening of the depositional environment at the Boulongour Reservoir section. Results presented here concern mainly the sedimentological and stratigraphical context of the investigated section. Additionally, one Late Devonian palaeo-oceanic and biotic event, the Upper Kellwasser Event is recognized near the section base.     

Chemical and isotopic composition of the lower crust beneath the Meguma Lithotectonic Zone,Nova Scotia: evidence from granulite facies xenoliths

Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinet groups of xenoliths occur. Both groups contain quartz+feldspar+biotite+Fe–Ti oxides+sulfides. In addition, group A contains garnet+graphite±[aluminosilicates+spinel±sapphirine (hight Al₂O₃ subgroups A1 and A2)] or [clinopyroxene+sphene+apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite-facies rocks. LowP-T sedimentary assemblages of quartz-chlorite-clay minerals-calcite-albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO>13 wt%) subgroup with high transition-metal contents, and low-MgO (MgO<9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from Nd_t=-2.56 to-0.80 and⁸⁷Sr/⁸⁶Sr _t =0.7046 to 0.7182 fort=370 Ma. For group B these values are +1.45 to +5.33 and 0.7028 to 0.7048, respectively. Model ages (T_CHUR) are correspondingly low and are tightly constrained (544±52 Ma). These young ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (Avalon), and an older allochthonous Meguma Group upper crust.     

U-Pb geochronology of Late Palaeozoic plutons,Cobequid Highlands,Nova Scotia,Canada: evidence for Late Devonian emplacement adjacent to the Meguma-Avalon terrane boundary in the Canadian Appalachians

The Cobequid Highlands in the Canadian Appalachian orogen lie within Avalonia adjacent to the Meguma Terrane. U-Pb (zircon) data show that the age range of voluminous bimodal plutonism in the highlands is from 358 to 363 Ma (late Devonian). This age range is much narrower than that previously suggested by Rb/Sr geochronology and confirms that the Cobequid Highlands preserve the youngest large-scale plutonic event in the Canadian Appalachians. Late Palaeozoic tectonic history of the Appalachian orogen is profoundly influenced by predominantly dextral motion on the Avalon-Meguma terrane boundary. This age of plutonism is coeval with previously published ⁴⁰Ar/³⁹Ar (muscovite) plateau ages derived from shear zones in the Meguma terrane adjacent to the terrane boundary, which is defined by the Minas fault zone. The NNE trending structural grain of the Appalachian orogen is disturbed in this area by the E-W Minas fault zone and pluton emplacement may have been associated with motion along this terrane boundary.     

扬子地块西缘天全新元古代过铝质花岗岩类成因机制及其构造动力学背景

四川西部天全地区花岗岩属于扬子地块西缘岩浆岩带,是"康滇地轴"北段的重要组成部分。岩石形成年龄为851±15Ma(MSWD=0.7),属于新元古代花岗岩,与扬子地块西缘和北缘大量的中酸性侵入体和火山岩具有相近的形成年龄。火夹沟花岗闪长岩为过铝质、低Si O2、具有相对亏损的Sr-Nd-Pb同位素地球化学组成,结合岩石低的Al2O3/Ti O2和高的Ca O/Na2O比值,其应是在镁铁质岩浆底侵的条件下,成熟度较低的杂砂岩部分熔融形成的过铝质熔体,岩石较低的Si O2含量表明其同化了部分镁铁质熔体。而角脚坪花岗岩具有高的Si O2含量,为过铝质、富Na的熔体,而且具有极度亏损的Sr-Nd同位素组成,表明其应是亏损的玄武质岩石(洋壳或是与地幔柱有关的玄武岩)在H2O饱和条件下发生低程度部分熔融形成的过铝质熔体。结合扬子西缘其它新元古代火成岩的地球化学特征及区域构造资料,我们认为天全地区的Na质花岗闪长岩-花岗岩组合代表在高地温梯度条件下,玄武质岩石在H2O饱和条件下发生部分熔融形成的过铝质花岗岩。     

Trace element geochemistry of scheelite and rutile from metaturbidite-hosted quartz vein gold deposits, Meguma Terrane, Nova Scotia, Canada: genetic implications

Scheelite and rutile from several metaturbidite-hosted gold-bearing quartz vein deposits of the Meguma Terrane of Nova Scotia were analyzed for trace elements including rare earth elements, niobium and tantalum. Scheelites have high concentrations of Sr, Nb, Y and rare earth elements (REE) with bell-shaped chondrite-normalized REE patterns accompanied by both positive and negative Eu anomalies. They also have high Nb/Ta ratios (80–300). Three distinct trace element types of the scheelites are interpreted to reflect chemical differences in the pulses of hydrothermal fluids. Hydrothermal rutiles have high contents of W (up to 4.2 wt.% WO₃), are rich in Ta compared to Nb and have a very low Nb/Ta ratio (~0.3). Hydrothermal fluids which produced both scheelite with a high Nb/Ta and rutile with a low Nb/Ta ratio are an efficient medium for fractionation of this ratio although these two minerals play an important role during the process.     

新疆吾尔喀什尔地区晚泥盆世铁列克提组火山岩LAICP-MS锆石U-Pb年龄、地球化学特征及构造意义

西准噶尔晚泥盆世铁列克提组主要分布于吾尔喀什尔山西部,为一套海陆交互相火山-沉积地层。火山岩地球化学研究表明,Si O2含量为51.18%~62.48%,Ti O2为0.60%~1.49%、全碱(K2O+Na2O)为1.26%~6.28%,相对富钠贫钾。A12O3为15.53%~18.68%、Mg O为2.64%~7.55%,Mg#为41.2~58.5,属于亚碱性系列。稀土元素总量高(∑REE=36.21×10-6~97.10×10-6),轻、重稀土分馏较强((La/Yb)N=2.88~6.72),具轻微负Eu异常(Eu/Eu*=0.74~1.07)。相对富集LILE(Rb、Ba、Th和K),亏损HFSE(Zr、Hf),Nb和Ta强烈亏损,Ti轻微亏损。岩石源区主要受俯冲流体交代,受地壳物质混染较弱,整体具有弧火山岩的地球化学特征。与火山岩伴生的凝灰岩LA ICP-MS锆石U-Pb同位素测年结果为363.3±4Ma。综合分析认为铁列克提组火山岩属于大陆边缘弧火山作用的产物,同时,博什库尔-成吉斯岩浆弧可能至少延续至晚泥盆世。这为进一步认识西准噶尔博什库尔-成吉斯岩浆弧的性质及构造演化提供了证据。     

The role of dextral transpressional faulting in the evolution of an early Carboniferous mafic–felsic plutonic and volcanic complex: Cobequid Highlands, Nova Scotia, Canada

The Wentworth plutonic complex, consisting of gabbro and granite, was emplaced in the earliest Carboniferous in the Cobequid shear zone of the northern Appalachians. The plutonic complex is coeval with a 5-km-thick pile of volcanic rocks. Early alkalic A-type granite correlates with thick felsic pyroclastics and minor basalt, which are overlain by 1.5-km-thick basalts that correlate with a large gabbro pluton that is intruded, in turn, by late granites. The basalt and gabbro are Fe-rich tholeiites. The geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma. The Wentworth plutonic complex lies on the north side of the dextral Rockland Brook fault, near the western tip of wedge-shaped basement block of the Avalon terrane. Field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike–slip faults. Dykes parallel to the mylonitic foliation in the Rockland Brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction. The plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone-bounded blocks, while the sinking of those blocks finally provided the space for mafic magma emplacement. Dyking was active over at least a 10-Ma time period. The overall location of plutonism in the Cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the Magdalen basin and the E–W transpressional contact of the Avalon and Meguma terranes. Magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge-shaped basement blocks, whose movement controlled the localisation of the voluminous magmatic activity.     

Hybridization of mafic microgranular enclaves: mineral and whole-rock chemistry evidence from the Karamadazı Granitoid, Central Turkey

On the Eastern Tauride Belt, the Cretaceous calc-alkaline Karamadazı Granitoid consists of quartz diorite containing mafic microgranular enclaves (MME) and leucocratic granite. The quartz diorite consists of plagioclase (An_8-65), hornblende, biotite, K-feldspar, quartz, epidote and titanite. Subrounded MME in the quartz diorite are holocrystalline, fine-grained, quartz diorite to diorite in composition, and display a similar mineral assemblage to their host. Large crystals in MME and quartz diorite show various disequilibrium microstructures indicative of hybridization. Plagioclase crystals exhibit inverse, normal, and oscillatory zoning with maximum core-to-rim An content increase up to 38% in the enclave and 40% in the quartz diorite. Both hornblende and augite exhibit normal and reverse zoning even in the same sample. The new field, textural, mineral compositional, and geochemical evidence leads to the conclusion that MME could have formed through injection of successive pulses of basic magma into upward mobile magma chambers containing cooler, partially crystalline quartz diorite magma. The quartz diorites show similarity to high-Al TTG (tonalites–trondhjemites–granodiorites), with their high Na₂O, Sr, LREE, and low Mg#, Cr, HREE contents, and are suggested to be produced by extensive interaction between the crustal and mantle-derived melts through mixing at depth. In contrast, leucogranites have geochemical characteristics distinct from the quartz diorites and MME, and are probably not involved in MME genesis.     

An oxygen isotope study of two contrasting orogenic vein gold systems in the Meguma Terrane, Nova Scotia, Canada, with implications for fluid sources and genetic models

Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408?Ma; Dufferin, 380?Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their ??¹⁸O_quartz signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average ??¹⁸O values of +15.2?±?0.9?? (n?=?16) for The Ovens and +15.7?±?0.6?? (n?=?12) for Dufferin. Using an average ??¹⁸O value of +15.4?? for the two deposits, the corresponding ??¹⁸O_H2O values, calculated for 400°C and 350°C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4?±?0.2?? and +10.2?±?0.2??, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30?Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in ??¹⁸O_H2O values for deposits when arranged in their relative stratigraphic position such that ??¹⁸O_H2O values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy.     

Dolomitization of the Middle Devonian Winnipegosis carbonates in south-central Saskatchewan, Canada

The Middle Devonian Winnipegosis carbonate unit in south‐central Saskatchewan is partially to completely dolomitized. Two major types of replacive dolomite are distinguished. Microcrystalline to finely crystalline dolomite (type 1) displays nonplanar‐a to planar‐s textures, mimetically replaces the precursor limestone, accounts for about four‐fifths of dolomite phases volumetrically, and mainly occurs in the Winnipegosis mounds and the Lower Winnipegosis Member directly underlying the mounds. Medium crystalline dolomite (type 2) shows planar‐s to planar‐e textures, commonly occurs in the Lower Winnipegosis and Brightholme members, and decreases upward in abundance. The ⁸⁷Sr/⁸⁶Sr ratios of type 1 dolomite (0·70795 to 0·70807) fall within the estimated Sr‐isotopic range for Middle Devonian marine carbonates. Stratigraphic, petrographic and geochemical data constrain the formation of type 1 dolomite to hypersaline sea water in a near‐surface environment, after marine cementation and sub‐aerial diagenesis and prior to precipitation of the Middle Devonian Leofnard salts. Movement of dolomitizing fluids could be driven by density differences and elevation head. The shift to lower δ¹⁸O values of type 1 dolomite [?7·4 to ?5·1‰ Vienna Pee Dee Belemnite (VPDB)] is interpreted as the result of recrystallization at elevated temperatures during burial. Type 2 dolomite has higher ⁸⁷Sr/⁸⁶Sr ratios (0·70809–0·70928), suggesting that the dolomite probably formed from basinal fluids with an increased richness in the radiogenic Sr isotope. In type 2 dolomite, Sr²⁺ concentrations are lower, and Fe²⁺ and Mn²⁺ concentrations are higher, compared with the associated limestone and type 1 dolomite. Type 2 dolomite is interpreted as having been formed from upward‐migrating basinal fluids during latest Devonian and Carboniferous period.     

Stromatoporoid growth forms and Devonian reef fabrics in the Upper Devonian Alexandra Reef System,Canada – Insight on the challenges of applying Devonian reef facies models

Existing facies models for Devonian reef systems can be divided into high‐energy and low‐energy types. A number of assumptions have been made in the development of these models and, in some cases, criteria that distinguish important aspects of the models are poorly defined. The Upper Devonian Alexandra Reef System contains a variety of reef fabrics from different depositional environments and is ideal for studying the range of environments in which stromatoporoids thrived and the facies from these different environments. A wide variety of stromatoporoid growth forms including laminar, tabular, anastamosing laminar and tabular, domal, bulbous, dendroid, expanding conical, concave‐up whorled‐laminar, concave‐up massive tabular and platy‐multicolumnar are present in the Alexandra Reef System. The whorled‐laminar and massive tabular concave‐up growth forms are virtually undocumented from other Devonian reefs but were common in the reef front of the Alexandra, where they thrived in a low‐energy environment around and below fair‐weather wave base. In contrast, high‐energy parts of the reef margin were dominated by bioclastic rubble deposits with narrow ribbon‐like discontinuous bodies of laminar stromatoporoid framestone. In the lagoon, laminar stromatoporoids formed steep‐sided sediment‐dominated bioherms in response to sea‐level rise and flooding. Relying mostly on the different reef facies in the Alexandra system, a new classification scheme for Devonian reef fabrics has been developed. Devonian reef fabrics can be classified as being: (i) sediment‐laden metazoan dominated; (ii) metazoan–microbial dominated (boundstone); (iii) metazoan dominated (framestone); or (iv) metazoan–marine cement dominated. Distinction of these fabrics carries important sedimentary and palaeoecological implications for reconstructing the depositional environment. With examples from the Alexandra Formation, it is demonstrated that reef facies accumulated in a range of depositional environments and that the simple observation of massive stromatoporoids with or without microbial deposits does not automatically imply a high‐energy reef margin, as otherwise portrayed in a number of the existing facies models for these systems.     

New insights into the origin and migration of brines in deep Devonian aquifers, Alberta, Canada

Analysis of hydraulic heads and chemical compositions of Devonian formation waters in the west central part of the Alberta Basin, Canada, characterizes the origin of formation waters and migration of brines. The Devonian succession in the study area lies 2000–5000 m below the ground surface, and has an approximate total thickness of 1000 m and an average slope of 15 m/km. Four Devonian aquifers are present in the study area, which form two aquifer systems [i.e., a Middle–Upper Devonian aquifer system (MUDAS) consisting of the Elk Point and Woodbend–Beaverhill Lake aquifers, and an Upper Devonian aquifer system (UDAS) consisting of the Winterburn and Wabamun aquifers]. The Ireton is an effective aquitard between these two systems in the eastern parts of the study area. The entire Devonian succession is confined below by efficient aquitards of the underlying Cambrian shales and/or the Precambrian basement, and above by overlying Carboniferous shales of the Exshaw and Lower Banff Formations.The formation water chemistry shows that the Devonian succession contains two distinct brine types: a ‘heavy brine,’ located updip, defined approximately by TDS >200 g/l, and a ‘light brine’ with TDS <200 g/l. Hydraulic head distributions suggest that, presently, the ‘light brine’ attempts to flow updip, thereby pushing the ‘heavy brine’ ahead. The interface between the two brines is lobate and forms large-scale tongues that are due to channeled flow along high-permeability pathways. Geological and hydrogeochemical data suggest that the following processes determined the present composition of the ‘light’ and ‘heavy’ brines: original seawater, evaporation beyond gypsum but below halite saturation, dolomitization, clay dehydration, gypsum dewatering, thermochemical sulfate reduction (TSR), and halite dissolution. The influx of meteoric (from the south) and metamorphic (from the west) waters can be recognized only in the ‘light brine.’ Albitization can be unequivocally identified only in the ‘heavy brine.’ The ‘heavy brine’ may be residual Middle Devonian evaporitic brine from the Williston Basin or the Elk Point Basin, or it may have originated from partial dissolution of thick, laterally extensive Middle Devonian evaporite deposits to the east of the study area. The ‘light brine’ most probably originated from dilution of ‘heavy brine’ in post-Laramide times.     

Geochronology and geochemistry of Early Devonian intrusions in the Qimantagh area,Northwest China: evidence for post-collisional slab break-off

The Qimantagh area of Northwest China lies in the western part of the East Kunlun Orogenic Belt and is characterized by extensive magmatism, particularly in the Triassic. However, recent research has shown that Devonian magmatism was also widespread in this area and has a genetic relationship with mineralization. This article presents a detailed study of three types of Early Devonian intrusions: high-K calc-alkaline granites, A-type granites, and mafic intrusions, all from the Qimantagh region. These rocks were subjected to precise zircon U–Pb dating, major and trace element analyses, and Sr–Nd isotope measurements, focusing on the Lalingzaohuo (eastern Qimantagh) and Yemaquan (central Qimantagh) monzogranites, as well as the coeval Tanbeixuefeng (western Qimantagh) mafic dike swarm. To better understand the Early Devonian igneous activity in the Eastern Kunlun, data for other coeval granitoids were compared with our data. The Yemaquan monzogranite yielded a mean zircon U–Pb age of 400.5 ± 1.4 Ma. These rocks are metaluminous to slightly peraluminous, with Al₂O₃ contents of 13.10–14.16 wt.%, high alkali contents (total K₂O + Na₂O) of 6.89–7.68 wt.%, relatively low Sr contents (79–192 ppm), and high (La/Yb)_N ratios, all of which indicate an I-type granite affinity. The Lalingzaohuo monzogranites yielded mean zircon U–Pb ages of 396.2–402.2 Ma. These rocks have higher SiO₂ and alkali contents than the Yemaquan monzogranite, with high 10,000 Ga/Al ratios, high Zr + Nb + Ce + Y contents, high Fe₂O₃^T/MgO ratios, and high Y contents, indicating an A-type granite affinity. These two monzogranites have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.703–0.706 and εNd(t) values of –0.1 to –0.7. The Sr–Nd isotopic data require a significant input of a mantle component in the petrogenesis of these granites. The Tanbeixuefeng diabase dikes formed at ~396 Ma and have a continental tholeiitic affinity, as evident from small Ti–Nb–Ta anomalies and high contents of light rare earth and large-ion lithophile elements. We propose that post-collisional slab break-off was responsible for the generation of these Early Devonian intrusions in the Qimantagh area.     

Geochemistry and zircon U-Pb geochronology of Aligoodarz granitoid complex, Sanandaj-Sirjan Zone, Iran

The Aligoodarz granitoid complex (AGC) is located in the Sanandaj-Sirjan Zone (SSZ), western Iran and consists of quartz-diorites, granodiorites and subordinate granites. Whole rock major and trace element data mostly define linear trends on Harker diagrams suggesting a cogenetic origin of the different rock types. (⁸⁷Sr/⁸⁶Sr)_i and εNd_t ratios are in the ranges 0.7074-0.7110 and −3.56 to −5.50, respectively. The trace elements and Sr-Nd isotopic composition suggest that the granitoids from the AGC are similar to crustal derived I-type granitoids of continental arcs. The whole rock suite was produced by assimilation and fractional crystallization starting from a melt with intermediate composition likely possessing a mantle component. In situ zircon U-Pb data on the granites with LA-ICP-MS yield a crystallization age of ∼165 Ma. Inherited grains spanning in age from ∼180 Ma up to 2027 Ma were also found and confirm that assimilation of country rock has occurred.Chemical and chronological data on the AGC were compared with those available for other granitoid complexes of the central SSZ (e.g., Dehno, Boroujerd and Alvand). The comparison reveals that in spite of the different origins that have been proposed, all these granitoid complexes are likely genetically related. They share many chemical features and are derived from crustal melts with minor differences. Alvand granites have the most peculiar compositions most likely related to the presence of abundant pelitic component. All these intrusions are coeval and reveal the presence of an extensive magmatic activity in the central sector of the SSZ during middle Jurassic.