Chemical evolution, petrogenesis, and regional chemical correlations of the flood basalt sequence in the central Deccan Traps, India

The lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed by basalts and basaltic andesites having a significant variation in TiO₂ (from 1.2 to 3.3 wt%), Zr (from 84 to 253 ppm), Nb (from 5 to 16ppm) and Ba (from 63 to 407 ppm), at MgO ranging from 10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in a temperature range from 1220° to 1125°C. These rocks resemble those belonging to the lower-most formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi) as well as those of the Poladpur formation. Samples analyzed for⁸⁷Sr/⁸⁶Sr give a range of initial ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO₂ (1.2–1.5 wt%) and Zr (84–105 ppm) at moderate MgO (5.2–6.2 wt%), matching the composition of low-Ti basalts of Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the north-western outcrops. The mildly enriched high field strength element contents of the samples with TiO₂ > 1.5 wt% make them products of mantle sources broadly similar to those which generated the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized diagrams, and relatively high⁸⁷Sr/⁸⁶Sr, make evident a crustal input with crustally derived materials at less differentiated stages than those represented in this sample set, or even within the sub-Indian lithospheric mantle.     

Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd–Sr–Pb–O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ¹⁸O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (⁸⁷Sr/⁸⁶Sr: 0.7031–0.7035, ε Nd: ca. + 1, δ¹⁸O: 7‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (⁸⁷Sr/⁸⁶Sr: 0.7034, ε Nd: ca. + 1, δ¹⁸O: 6.5‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.59) whereas more differentiated samples have higher ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.714), slightly higher δ¹⁸O values (7.0–7.1‰), less radiogenic ε Nd values (− 1.1 to − 1.4) and more radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.     

Geology, petrochemistry, and genesis of the bimodal lavas of Osham Hill, Saurashtra, northwestern Deccan Traps

The Saurashtra region in the northwestern Deccan continental flood basalt province (India) is notable for compositionally diverse volcano-plutonic complexes and abundant rhyolites and granophyres. A lava flow sequence of rhyolite-pitchstone-basaltic andesite is exposed in Osham Hill in western Saurashtra. The Osham silicic lavas are Ba-poor and with intermediate Zr contents compared to other Deccan rhyolites. The Osham silicic lavas are enriched in the light rare earth elements, and have ε_Nd (t = 65 Ma) values between −3.1 and −6.5 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70709-0.70927. The Osham basaltic andesites have initial ε_Nd values between +2.2 and −1.3, and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70729-0.70887. Large-ion-lithophile element concentrations and Sr isotopic ratios may have been affected somewhat by weathering; notably, the Sr isotopic ratios of the silicic and mafic rocks overlap. However, the Nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas. We suggest that the Osham basaltic andesites were derived by olivine gabbro fractionation from low-Ti picritic rocks of the type found throughout Saurashtra. The isotopic compositions, and the similar Al₂O₃ contents of the Osham silicic and mafic lavas, rule out an origin of the silicic lavas by fractional crystallization of mafic liquids, with or without crustal assimilation. As previously proposed for some Icelandic rhyolites, and supported here by MELTS modelling, the Osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths, due to heating by repetitively injected basalts. The absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of Osham Hill suggests that they reached the surface via separate pathways.     

Clinopyroxene compositions in the Deccan and Rajmahal Traps and their bearing on magma types and evolution

Electron probe analyses of clinopyroxenes from several areas of the Deccan and Rajmahal Traps consisting mostly of subalkalic and alkalic basalts, picritic basalts and a few dolerite dykes have been obtained. Evaluation of the data indicate the absence of pigeonite from subalkalic basalts that occur in close spatial association with mild or strongly alkalic basalts in areas such as Rajpipla, Navagam and central Kachchh. Co-existence of augite and pigeonite, however, has been noticed in subalkalic basalts/dykes and picritic basalts from a number of Deccan localities such as Sagar, Igatpuri, Kalsubai, Triambak, Pavagarh and Girnar besides the one sample from Rajmahal. Diopside, salite, and wollastonite-rich compositions dominate the basanites and foidites of Kachchh whereas chrome-diopside and salite are the main types in the picrite basalt samples from Anila, Botad and Paliyad in Saurashtra akin to those found in contiguous areas in the east from borehole flows at Dhandhuka and Wadhwan studied in detail previously. Compositional variations in zoned clinopyroxenes indicate differentiation of the parental magma and also mixing of different magma types (subalkalic and alkalic) from areas such as Igatpuri, Rajpipla and Kachchh. Based on host-rock chemistry, total alkalis-silica plot, CIPW norms, estimated temperatures of eruption and augite – pigeonite thermometry, it has been inferred that clinopyroxene compositions, especially the incidence of pigeonite, appear to be very sensitive to bulk chemistry of host rocks, especially their Na₂O, K₂O, SiO₂, total iron and TiO₂ contents. Non-quadrilateral cationic components in the clinopyroxenes, such as Al in tetrahedral and octahedral positions together with Si, Na, Ti and Cr abundances have been found to be useful to discriminate clinopyroxenes from alkalic and subalkalic basalt types besides inferences on the ferric iron component in them. Evaluation of host-rock compositions in the ternary olivine–clinopyroxene-quartz plot indicate polybaric conditions of crystallization and evolution especially in samples that are picritic (e.g. Pavagarh, Anila and Kachchh) and which could also breach the olivine–clinopyroxene-plagioclase thermal divide that exists in part between alkalic and subalkalic basalts under atmospheric conditions.     

Geology and geochemistry of Pachmarhi dykes and sills,Satpura Gondwana Basin,central India: problems of dyke-sill-flow correlations in the Deccan Traps

Many tholeiitic dyke-sill intrusions of the Late Cretaceous Deccan Traps continental flood basalt province are exposed in the Satpura Gondwana Basin around Pachmarhi, central India. We present field, petrographic, major and trace element, and Sr–Nd–Pb isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically defined formations in the southwestern Deccan (Western Ghats). Some of these formations have also been identified more recently in the northern and northeastern Deccan. However, the Pachmarhi intrusions are significantly more evolved (lower Mg numbers and higher TiO₂ contents) than many Deccan basalts, with isotopic signatures generally different from those of the chemically similar lava formations, indicating that most are not feeders to previously characterized flows. They appear to be products of mixing between Deccan basalt magmas and partial melts of Precambrian Indian amphibolites, as proposed previously for several Deccan basalt lavas of the lower Western Ghats stratigraphy. Broad chemical and isotopic similarities of several Pachmarhi intrusions to the northern and northeastern Deccan lavas indicate petrogenetic relationships. Distances these lava flows would have had to cover, if they originated in the Pachmarhi area, range from 150 to 350 km. The Pachmarhi data enlarge the hitherto known chemical and isotopic range of the Deccan flood basalt magmas. This study highlights the problems and ambiguities in dyke-sill-flow correlations even with extensive geochemical fingerprinting.     

Isotope hydrogeochemical models for assessing the hydrological processes in a part of the largest continental flood basalts province of India

Continental Flood Basalts (CFB) occupy one fourth of the world’s land area. Hence, it is important to discern the hydrological processes in this complex hydrogeological setup for the sustainable water resources development. A model assisted isotope, geochemical, geospatial and geophysical study was conducted to understand the monsoonal characteristics, recharge processes, renewability and geochemical evolution in one of the largest continental flood basalt provinces of India. HYSPLIT modelling and stable isotopes were used to assess the monsoonal characteristics. Rayleigh distillation model were used to understand the climatic conditions at the time of groundwater recharge. Lumped parameter models (LPM) were employed to quantify the mean transit time (MTT) of groundwater. Statistical and geochemical models were adopted to understand the geochemical evolution along the groundwater flow path. A geophysical model was used to understand the geometry of the aquifer. The back trajectory analysis confirms the isotopic finding that precipitation in this region is caused by orographic uplifting of air masses originating from the Arabian Sea. Stable isotopic data of groundwater showed its meteoric origin and two recharge processes were discerned; (i) quick and direct recharge by precipitation through fractured and weathered basalt, (ii) low infiltration through the clayey black cotton soil and subjected to evaporation prior to the recharge. Tritium data showed that the groundwater is a renewable source and have shorter transit times (from present day to <30 years). The hydrogeochemical study indicated multiple sources/processes such as: the minerals dissolution, silicate weathering, ion exchange, anthropogenic influences etc. control the chemistry of the groundwater. Based on the geo-electrical resistivity survey, the potential zones (weathered and fractured) were delineated for the groundwater development. Thus, the study highlights the usefulness of model assisted isotopic hydrogeochemical techniques for understanding the recharge and geochemical processes in a basaltic aquifer system.     

Review: Groundwater development and management in the Deccan Traps (basalts) of western India

The Deccan Traps or the basalts of western India are the largest exposure of basic lava flows covering about 500,000 km². Groundwater occurrence in the Deccan Traps is in phreatic condition in the weathered zone above the hard rock and in semi-confined condition in the fissures, fractures, joints, cooling cracks, lava flow junctions and in the inter-trappean beds between successive lava flows, within the hard rock. Dug wells, dug-cum-bored wells and boreholes or bore wells are commonly used for obtaining groundwater. The yield is small, usually in the range of 1–100 m³/day. The average land holding per farming family is only around 2 ha. Recently, due to the ever increasing number of dug wells and deep bore wells, the water table has been falling in several watersheds, especially in those lying in the semi-arid region of the traps, so that now the emphasis has shifted from development to sustainable management. Issues like climatic change, poverty mitigation in villages, sustainable development, rapid urbanization of the population, and resource pollution have invited the attention of politicians, policy makers, government agencies and non-governmental organizations towards watershed management, forestation, soil and water conservation, recharge augmentation and, above all, the voluntary control of groundwater abstraction in the Deccan Traps terrain.     

Constraints on the Mantle Sources of the Deccan Traps from the Petrology and Geochemistry of the Basalts of Gujarat State (Western India)

The late Cretaceous-early Tertiary flood basalts in the Gujaratarea of the northwestern Deccan Traps (Kathiawar peninsula,Pavagadh hills and Rajpipla) exhibit a wide range of compositions,from picrite basalts to rhyolites; moreover, the basaltic rockshave clearly distinct TiO₂ contents at any given degree of differentiationand strongly resemble the low-titanium and hightitanium basaltsfound in most of the Gondwana continental flood basalt (CFB)suites. Four magma groups are petrologically and geochemicallydistinguished: (1) A low-Ti group, characterized by rocks with varying SiO²saturation, and with TiO₂ <1•8 wt%, extremely low incompatibletrace element abundances, low Zr/ (av- 3•8), Ti/ V (av.27), and a very slight large ion lithophile element (LJLE) enrichmentover high field strength elements (HFSE). These rocks sharesome features with the Bushe Formation of the Western Ghatsfarther south, but have distinct geochemical characters, inparticular the strong depletion in most incompatible trace elements. (2) A high-Ti group, characterized by a more K-rich characterthan the low-Ti rocks, and with a strong enrichment in incompatibleelements, similar to average ocean island basalt (OIB), e.g.high TiO₂ (>1•8 wt% in picrites), Nb (>19 p.p.m.)Zr/ (av. 6•5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO₂ contents slightly lowerthan the high-Ti rocks at the same degree of evolution, andwith correspondingly lower incompatible trace element contentsand ratios, in particular K₂O, Nb, Ba and Zr/Y (av. 5•2). (4) A potassium-rich group (KT), broadly similar in geochemicalcharacter to the high-Ti group but showing more extreme K, Rband Ba enrichment (av. K₂0/Na₂0l; Ba/Y20). The most primitive low-Ti and high-Ti picrites, when correctedfor low-pressure olivine fractionation, show distinct major(and trace) element geochemistry, in particular for CaO/AI₂O₃,CaO/TiO₂ and Al₂O₃/TiO₂, and moderate but significant variationsin their SiO₂ and Fe₂O_st contents; these characteristics stronglysuggest the involvement of different mantle sources, more depletedfor the low-Ti picrites, and richer in cpxfor the high-Ti picrites,but with broadly the same pressures of equilibration (27–14kbar). This, in turn, suggests a strong lateral heterogeneityin the Gujarat Trap mantle. Low-Ti picrites and related differentiatesin Kathiawar are reported systematically for the first timehere, and suggest the existence of HFSE-depleted mantle in thenorthwestern Deccan Traps, with extension at least to the SeychellesIslands and to the area of the Bushe Formation near Bombay inthe pre-drift position, before the development of the CarlsbergRidge. The absence of correlations between LILE/HFSE ratiosand SiO₂ argues against crustal contamination processes actingon the low-Ti picrites, possibly owing to their probably rapiduprise to the surface. Consequently, the mantle region of thisrock group was probably re-enriched by small amounts of ULE-richmaterials. The substantially higher, trace element enrichmentof the least differentiated high-Ti picrites, relative to thebasalts of the Ambe-noli and Mahableshwar Formations of theWestern Ghats, testifies also to the presence of more incompatibleelement rich, OIB4ike mantle sources in northern and northwesternGujarat. These sources were geochemicaily similar to the present-dayReunion mantle sources. KEY WORDS: Deccan Traps; geochemistry; petrology; picrite basalts; western India ^*Corresponding author, e-mail: mellujo{at}ds.cued.unina.it     

达里干加-阿巴嘎-达里诺尔晚新生代玄武岩特征及成因分析

兴蒙造山带及其毗邻的蒙古国东南缘发育大面积的晚新生代板内玄武岩,其中从蒙古国达里干加到中国内蒙古的阿巴嘎-达里诺尔火山岩断续相连,组成亚洲东部面积较大的晚新生代火山岩区。这些火山岩区的地质和地球化学特征相似,火山岩的形成可能受相同因素制约。研究区火山岩主要形成于中新世-更新世,个别火山岩形成于全新世。火山岩以碱性玄武岩为主,拉斑玄武岩次之,镜下为斑状结构,斑晶以橄榄石为主,辉石含量较少。其中碱性玄武岩含多种地幔捕虏体,包括尖晶石二辉橄榄岩、尖晶石方辉橄榄岩和纯橄岩等。除地质特征相似外,上述三区玄武岩在地球化学特征方面也显示出相近的特点,可能具有较为一致的成因。火山岩的主微量元素、Sr-Nd同位素特征以及含有的地幔包体,表明受地壳混染作用较弱。MgO与Cr、Ni正相关而与Al₂O₃、SiO₂负相关,表明火山岩发生了橄榄石和单斜辉石的分离结晶作用。此外,根据Sr-Nd同位素特征与来自软流圈的火山岩相近,La/Yb（8.57~34.66）和Sm/Yb（2.86~8.85）比值较高,推断火山岩源区为软流圈地幔,辉石岩可能是形成该区岩浆的主要源岩。（Ta/U）_N和（Nb/Th）_N值大于1以及Nb-Ta的正异常,指示其源区有俯冲板片物质的加入;玄武岩富集的重Zn同位素特征（δ⁶⁶Zn,0.47‰~0.50‰）以及Sr-Nd同位素具有沉积物混入的特点,表明源区还加入了沉积物。

     

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths.

The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.     

Highly fractionated I-type granites in NE China (II): isotopic geochemistry and implications for crustal growth in the Phanerozoic

NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.

Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7045±0.0015), positive _Nd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment.

The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic.     

Mixing of asthenospheric and lithospheric mantle-derived basalt magmas as shown by along-arc variation in Sr and Nd isotopic compositions of Early Miocene basalts from back-arc margin of the NE Japan arc

We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO₂ (> 1.5 wt.%; referred to as HT) and lower TiO₂ (< 1.5 wt.%; LT) basalts. HT basalt has higher Na₂O + K₂O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr (SrI) = 0.70389 to 0.70631, initial ¹⁴³Nd/¹⁴⁴Nd(NdI) = 0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na₂O + K₂O and K₂O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea.     

Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China

The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km², composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO₂=69.85–72.35%, K₂O/Na₂O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)_N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb=16.707–17.055, ²⁰⁷Pb/²⁰⁴Pb=15.239–15.326 and ²⁰⁸Pb/²⁰⁴Pb=37.587–37.853, which are similar to that of the continental lower crust. Their _Nd(t) values range from −16 to −20, and depleted-mantle Nd model ages (T_DM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.     

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO₂, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ¹⁸O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ¹⁸O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron (n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes).

The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization ( 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P_H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P_H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their ¹⁸O/¹⁶O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ¹⁸O (Fsp) values point to magmatic origin of fluids.

The stable and radiogenic isotope data [δ¹⁸O (Zrn) = 5.82 ± 0.06‰, I_Sr = 0.7022 ± 0.0064, ε_Nd (T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.     

Hydrogeochemical processes,mixing and isotope tracing in hard rock aquifers and surface waters from the Subarnarekha River Basin, (east Singhbhum District,Jharkhand State,India)

Geochemical observations, including major ion and trace element analysis, and isotopic tracing have been carried out in the Subarnarekha River system (northeastern India) during a surface-water- and groundwater-monitoring program aimed at evaluating impacts of mining. The aquifer is of fracture type. Groundwater flow conditions and pollutant transfer were observed through a network of 69 wells. δ¹⁸O and δ²H results suggest that transfer from rainfall towards groundwater storage through soils and the unsaturated zone is fast, without any major transformation like evaporation. The scatter of ⁸⁷Sr/⁸⁶Sr signatures in surface water and groundwater are explained by three end-members. One is compatible with rainwater inputs. The most mineralised end-member represents anthropogenic inputs (agricultural practices and ore processing). The third end-member, characterised by a high ⁸⁷Sr/⁸⁶Sr signature, is believed to be controlled by natural geochemical processes, although affected by human activities (e.g. drainage of mine waste). Potential flow paths, investigated north of the area, reveal that all groundwater types seem to evolve more in pockets than along a flow path. The limited extent of transfer and the predominance of natural phenomena help to explain the moderate level of groundwater contamination and the characteristics of surface water contamination by mining and the metallurgy industry.     

海南岛晚古生代洋岛玄武岩(OIB型)的发现及地球动力学暗示

海南岛西部的军营-邦溪地区产出一套变质的、晚古生代镁铁质-超镁铁质熔岩系列.根据地质学、岩石学、矿物学和地球化学研究,该套岩石可划分为高镁和低镁两个系列,前者以高镁(Mg# =76.9～81.3)为特征,后者以低镁(Mg#=40.7～48.4)、高钛含量(2.34％ ～3.27％)为特征.这套镁铁质-超镁铁质熔岩具有LREE富集[(La/Yb)N =2.40～7.58]和无明显Eu异常的稀土配分模式,以及无Nb亏损、但略具轻微Ta正异常的微量元素原始地幔标准化曲线;87Sr/86Sr (270Ma)和εNd(270Ma)比值的变化范围分别为0.70645 ～0.70956和+4.7～ +6.5,这些地球化学特征与洋岛玄武岩(OIB)极其相似.一些反映源区特征的比值,如Ta/Hf、Th/Nb、Nb/Zr、La/Ta、La/Sm、(La/Nb)PM、(Th/Ta)PM等均指示其地幔柱成因,是石榴子石二辉橄榄岩地幔低程度部分熔融形成的产物.橄榄石斑晶的矿物化学进一步揭示,岩石的高镁性质是由橄榄石堆晶引起的,而橄榄石斑晶低的Fo(68 ～77)值,暗示其原始岩浆为低镁的玄武质岩浆;低镁系列相对于高镁系列明显低的Cr、Ni含量,说明岩浆随后发生了橄榄石和单斜辉石的分离结晶作用.军营-邦溪地区晚古生代OIB型洋岛玄武岩可能代表了东古特提斯洋在海南岛的又一记录,该认识对深入探讨华南古特提斯洋的演化及其在海南岛的响应提供了新的证据.     

Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China

The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative ε_Hf(t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive ε_Hf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ¹⁸O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ¹⁸O rocks. North Dabie shares the same age of Neoproterozoic low δ¹⁸O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.     

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

We present the results of a comprehensive major element, traceelement and Sr–Nd–Pb–O isotopic study of post-glacialvolcanic rocks from the Neovolcanic zones on Iceland. The rocksstudied range in composition from picrites and tholeiites, whichdominate in the main rift systems, to transitional and alkalicbasalts confined to the off-rift and propagating rift systems.There are good correlations of rock types with geochemical enrichmentparameters, such as La/Sm and La/Yb ratios, and with long-termradiogenic tracers, such as Sr–Nd–Pb isotope ratios,indicating a long-lived enrichment/depletion history of thesource region. ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd defines a negative array.Pb isotopes define well-correlated positive arrays on both ²⁰⁶Pb/²⁰⁴Pbvs ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb diagrams, indicating mixing ofat least two major components: an enriched component representedby the alkali basalts and a depleted component represented bythe picrites. In combined Sr–Nd–Pb isotopic spacethe individual rift systems define coherent mixing arrays withslightly different compositions. The enriched component hasradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb > 19·3) and very similargeochemistry to HIMU-type ocean island basalts (OIB). We ascribethis endmember to recycling of hydrothermally altered upperbasaltic oceanic crust. The depleted component that is sampledby the picrites has unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb < 17·8),but geochemical signatures distinct from that of normal mid-oceanridge basalt (N-MORB). Highly depleted tholeiites and picriteshave positive anomalies in mantle-normalized trace element diagramsfor Ba, Sr, and Eu (and in some cases also for K, Ti and P),negative anomalies for Hf and Zr, and low ¹⁸O_olivine values(4·6–5·0) below the normal mantle range.All of these features are internally correlated, and we, therefore,interpret them to reflect source characteristics and attributethem to recycled lower gabbroic oceanic crust. Regional compositionaldifferences exist for the depleted component. In SW Icelandit has distinctly higher Nb/U (68) and more radiogenic ²⁰⁶Pb/²⁰⁴Pbratios (18·28–18·88) compared with the NErift (Nb/U 47; ²⁰⁶Pb/²⁰⁴Pb = 18·07–18·47).These geochemical differences suggest that different packagesof recycled oceanic lithosphere exist beneath each rift. A thirdand minor component with relatively high ⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pbis found in a single volcano in SE Iceland (Öræfajökullvolcano), indicating the involvement of recycled sediments inthe source locally. The three plume components form an integralpart of ancient recycled oceanic lithosphere. The slope in theuranogenic Pb diagram indicates a recycling age of about 1·5Ga with time-integrated Th/U ratios of 3·01. Surprisingly,there is little evidence for the involvement of North AtlanticN-MORB source mantle, as would be expected from the interactionof the Iceland plume and the surrounding asthenosphere in formof plume–ridge interaction. The preferential samplingof the enriched and depleted components in the off-rift andmain rift systems, respectively, can be explained by differencesin the geometry of the melting regions. In the off-rift areas,melting columns are truncated deeper and thus are shorter, whichleads to preferential melting of the enriched component, asthis starts melting deeper than the depleted component. In contrast,melting proceeds to shallower depths beneath the main rifts.The longer melting columns also produce significant amountsof melt from the more refractory (lower crustal/lithospheric)component. KEY WORDS: basalts; trace element and Sr, Nd, Pb, O isotope geochemistry; Iceland plume; isotope ratios; oceanic crustal recycling; partial melting; plume–ridge interaction     

Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U–Pb ages, oxygen isotopes and REE data

Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ¹⁸O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma.     

Water-rock-tailings interactions and sources of sulfur and metals in the subtropical mining region of Taxco,Guerrero (southern Mexico): A multi-isotopic approach

Multi-isotope (H, O, S, Sr, Pb) systems coupled with conventional (major and trace element) hydrogeochemical analysis were applied to determine the origin of water, to model water-rock-tailings interactions and for source apportionment of sulfur and associated toxic metals in the mining region of Taxco, Guerrero in southern Mexico. Oxygen and H isotopes indicate that meteoric water in the zone is rainwater undergoing varying degrees of isotopic fractionation by atmospheric evaporation whereas Sr isotopes trace the interaction of pristine water from volcanics of the regional recharge zone and subsequently flowing through sandstone and shale to spring points. Leachates form from two distinctive sources (spring water and surface water) having differential interactions with bedrocks prior to entering the tailings. Compared to pristine water, leachates are enriched in sulfate, metals (e.g. Fe, Mn, Pb and Zn) and metalloids (e.g. As). The sulfur isotopic composition of ore-sulfides, leachates, secondary precipitates, regional surface water and hypogenic sulfates is described in terms of a two-component mixing model with shale of Mexcala and limestone of Morelos formations representing the light and heavy end-members, respectively, whereas Sr isotopic composition is bracketed combining three lithogenic (Mexcala/Morelos, Tilzapotla and Taxco Schist) sources. Finally, leachates have a mixture of lead from ore-sulfides and Taxco Schist Formation (Family I) or from ore-sulfides alone (Family II). The application of multiple environmental isotopic techniques is an outstanding tool for elucidating complex interactions of water with bedrocks and tailings and for determining the source of sulfur and toxic metal from mining and other metal polluted environments.