Initial Strontium Isotope Ratios Research Articles

Geochronological and Geochemical Constraints on the Petrogenesis and Geodynamic Setting of the Daheishan Porphyry Mo Deposit, Northeast China

AbstractThe Daheishan Mo deposit of the Lesser Xing'an–Zhangguangcai Range metallogenic belt in northeast China is a super‐large molybdenum deposit with Mo reserves of 1.09 Mt. The Mo mineralization occurs mainly in a granodiorite porphyry. Zircon SIMS U–Pb dating yields a crystallization age of 168.3 ± 1.4 Ma for the granodiorite porphyry. Molybdenite Re–Os dating indicates that Mo mineralization occurred at 169.2 ± 1.2 Ma. These geochronological data indicate that these magmatic and hydrothermal activities occurred during the Middle Jurassic. The granodiorite porphyry can be classified as high‐K calc‐alkaline series, and the rare earth elements (REE) are characterized by a significant fractionation between light REE (LREE) and heavy REE (HREE) with slightly positive Eu anomalies (Eu/Eu* = 1.08–1.12). Large ion lithophile elements (e.g., Rb, U, K, and Pb) are enriched, whereas high field strength elements (e.g., Nb, Ta, Ti, HREEs, and Yb) are strongly depleted. The granodiorite porphyry is also characterized by initial strontium isotope ratios (87Sr/86Sr)i of 0.70460–0.70482 and magmatic zircon δ18O values of 5.2–6.5 ‰ that are similar to those of the mantle. Zircon ɛHf(t) and whole‐rock εNd(t) values range from 5.6 to 9.9 and 0.8 to 1.1, respectively. The two‐stage Nd model ages (TDM2) are in the range of 868–894 Ma, similar to Hf model ages, indicating that the parent magma has a uniform source and primarily originated from a juvenile crustal source. Combined with the regional geological history, geochemistry of the Daheishan granodiorite porphyry, and new isotopic age data, we propose that the formation of the Daheishan porphyry Mo deposit is likely related to the subduction of the Paleo‐Pacific Plate.

Mesoarchean gabbroanorthosite magmatism of the Kola region: Petrochemical, geochronological, and isotope-geochemical data

The paper presents results of petrochemical, geochemical, and isotope-geochemical study of the Patchemvarek and Severnyi gabbroanorthosite massifs of the Kola Peninsula. It was shown that the rocks of these massifs differ from the gabbroanorthosite massifs of the Neoarchean Keivy-Kolmozero Complex in the more calcic composition (70–85% An) of normative plagioclase, and low contents of TiO2, FeO, and Fe2O3. In terms of chemical composition, the gabbroanorthosites of the studied massifs are close to the rocks of the Fiskenasset Complex (Southwestern Greenland) and to the anorthosites of the Vermillion Lake Complex (Canada). U-Pb zircon dating established Mesoarchean ages of 2925 ± 7 and 2935 ± 8 Ma for the gabbroan-orthosites of the Patchemvarek and Severnyi massifs, respectively. It was shown that the gabbroanorthosites of the studied massifs have fairly low REE contents (Ce n = 2.2−4.2, Yb n = 1.6−2.6) and distinct positive Eu anomaly. Comagmatic ultrabasic differentiates have practically unfractionated REE pattern, low total REE contents (Ce n = 1.2, Yb n = 1.1, La/Yb n = 1.3), and no Eu anomaly. The studied samples of the Archean gabbroanorthosites are characterized by positive eNd = +2.68 for the gabbroanorthosites of the Severnyi Massif and from + 2.77 to + 1.66 for the Patchemvarek Massif. Initial strontium isotope ratios are 87Sr/86Sr i = 0.70204 ± 8 and 87Sr/86Sr i = 0.70258 ± 8 for the rocks of the Severnyi and Patchemvarek massifs, respectively. Our study showed that the obtained U-Pb zircon ages for the gabbroanorthosites of the Patchemvarek and Severnyi massifs represent the oldest date for the Kola peninsula, thus marking the oldest, Mesoarchean stage in the evolution of region. The differences in the initial 143Nd/144Nd ratios between the Neoarchean gabbroanorthosites of the Keivy-Kolmozero Complex and the Mesoarchean gabbroanorthosites of the studied massifs suggest the existence of two mantle sources. One of them produced intrusions with an age of 2.67–2.66 Ga, while other was responsible for the formation of massifs with an age of 2.93–2.92 Ga. The composition and temperature of “parental” melt of the gabbroanorthosites were simulated using COMAGMAT-3.5 program. According to the calculations, the parental melt represented aluminous basalt, whose differentiation at T = 1280°C and P = 7 kbar at the crust-mantle boundary was accompanied by plagioclase floatation and formation of “crystal mesh” that produced anorthosite complexes. The gabbroanorthosies of the Patchemvarek and Severnyi massifs were presumably derived from MORB-type basalts of oceanic settings, while the Tsaga, Achinskii, and other anorthosite massifs of the Neoarchean age were generated from subalkaline magma formed in within-plate anorogenic setting. Sm-Nd isotope data suggest the existence of several mantle sources in the Kola region, which produced melts for different-age gabbroanorthosite massifs since Mesoarchean to the middle Paleoproterozoic. The Archean-Early Proterozoic anorthosite magmatism of the Kola region records a complete cycle (∼ 800 Ma) of the formation and consolidation of continental block.

Mantle sources of the Cenozoic volcanic rocks of the Lake Kizi region in the eastern Sikhote Alin

The Middle Cenozoic lava sequence of the Lake Kizi region was studied. It characterizes the activity of sources in the Northern zone of the eastern Sikhote Alin: a Middle Eocene pulse of slab-related magmatism and prolonged injection of magmas from the sublithospheric convecting mantle in the Late Oligocene. Low contents of high field strength elements (Nb and Ta) with low Nb/Ta, Ce/Pb, and Nb/La and high K/Nb ratios and a low (87Sr/86Sr)0 of 0.703399 were determined in a Middle Eocene dacite with an age of ∼43.5 Ma. Three phases of Late Oligocene volcanic eruptions were distinguished: (1) basaltic andesites (29–27 Ma), (2) basaltic trachyandesites and trachyandesites (27–24 Ma), and (3) andesites (∼23 Ma). The lavas of the first and third phases showed low Ce/Pb, Nb/La, and Ba/La and high K/Nb ratios, which are also characteristic of supraslab processes. The lavas of the second phase are shifted with respect to these ratios toward ocean island basalt compositions. The entire Late Oligocene volcanic sequence falls within a narrow range of the initial strontium isotope ratios, (87Sr/86Sr)0, from 0.703661 to 0.703853. Such ratios are characteristic of volcanic and subvolcanic rocks with ages of ∼37, 31–23, and ∼16 Ma over the whole region of the Tatar Strait coast.

From andesitic volcanism to the formation of a porphyry Cu-Au mineralizing magma chamber: the Farallón Negro Volcanic Complex, northwestern Argentina

The geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the Upper Miocene Farallón Negro Volcanic Complex (NW Argentina) were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3.5 my. This paper is part of an integrated study of the processes that generate magmatic-hydrothermal fluids responsible for the formation of porphyry-Cu-Au deposits. Here we present new data on bulk rock chemical and Nd, Sr and Pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks, for which geological relations and ages are constrained by field-based stratigraphy and extensive Ar–Ar geochronology. The combined results indicate that voluminous volcanism started at ca. 9.7 Ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits. This basic to intermediate volcanism persisted until 7.5 Ma and was followed at 7.35 Ma by the eruption of dacites as the last recorded major extrusive event. The oldest andesitic intrusions, emplaced at 9.0 Ma, were followed by hypabyssal intrusions of basaltic andesite, andesite and dacite composition, which intruded the volcanic sequence between 8.3 and 6.1 Ma. Intrusions at 6.8±0.2 Ma led to the formation of over 800 Mt of porphyry-Cu-Au ore at Bajo de la Alumbrera. It was formed by a large flux of magmatic-hydrothermal fluids channeled through several phases of fractured dacitic porphyry, which together with immediately following barren andesitic intrusions make up a composite stock. After this magmatic-hydrothermal event, only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6.1 Ma. Geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material, as indicated by linear compositional correlations between compatible and incompatible elements. Initial strontium isotopic ratios vary between 0.7047 and 0.7118 while 143Nd/ 144Nd range from 0.5127 to 0.5124, and both isotopic variations correlate with each other and with SiO 2 content of the rocks. After 8.5 Ma, a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber. A substantial magma reservoir (>7 km 3) must have been established by the time of porphyry-Cu-Au mineralization, to allow rapid release of a large volume of ore-forming magmatic brine as a result of wholesale crystallization and de-volatilization of the magma chamber.

Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the origin and evolution of the magmas postdating the Ross Orogen. The δ 18O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts ( δ 18O WR=6.8–7.4‰). The O-isotope character of the mantle source inferred from the δ 18O values of cumulous olivine in gabbros (5.7–6.8‰) is enriched in 18O compared to modern arc-related magmas. Geochemical data and concurrent high δ 18O values, and initial strontium ( 87Sr/ 86Sr=0.7060) and neodymium ( 143Nd/ 144Nd=0.5122) isotope ratios indicate that the olivine gabbros formed by crustal contamination of a primary calc-alkaline basaltic melt. The diorites have high δ 18O values, among the highest ever measured for dioritic rocks (8.7–10.3‰), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708–0.710). The diorites have pyroxene with high, nearly constant δ 18O values (8.2–8.6‰) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calc-alkaline basalt experiencing different amounts of crustal contamination, and underwent different degrees of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The δ 18O, 87Sr/ 86Sr and 143Nd/ 144Nd compositions of the granites and granodiorites overlap (10.8–12.1‰, 0.7096–0.7108, 0.5119–0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile meta-igneous protoliths.

Trace element and isotope (Sr, Nd) geochemistry of porphyry- and skarn-mineralising Late Cretaceous intrusions from Banat, western South Carpathians, Romania

Geochemical and isotopic (strontium, neodymium) data have been obtained from 24 samples of seven Late Cretaceous intrusions associated with skarn (Tincova, Bocsa, Ocna de Fier – Dognecea) and porphyry copper-molybdenum/skarn deposits (Oraviţa, Ciclova, Sasca and Moldova Nouă) in the South Carpathians of Romania. The intrusions cut the Supragetic-Getic nappes and form a 100-km-long, NNE-SSW-trending lineament in the Banat metallogenic province. The samples range in composition from gabbro to granodiorite and define, in major and trace element variation diagrams, a calc-alkaline to high-potassium calc-alkaline trend ranging from 50 to 67% SiO2. They are magnetite-bearing, I-type granitoids enriched in LILE/LREE and depleted in Nb, Ta and Zr, common features for magmas associated with subduction zones. The lack of cumulate textures and of positive europium anomalies seems to indicate that the samples represent liquid compositions. The observed trend thus approximates a liquid line of descent resulting from fractional crystallisation of parental magmas with similar major and trace element compositions. Initial strontium isotopic ratios (Sri) vary from 0.7042 to 0.7058 and eNd(t) values range from +3.9 to –0.2. Variations of Sri and eNd(t) values with SiO2 show minor crustal assimilation. The generally positive eNd(t) values, together with moderate Sri, indicate that the parental magmas result from partial melting of a rubidium-enriched and LREE-depleted source, such as a heterogeneous lithospheric mantle or a young mafic lower crust derived from it. Selective rubidium enrichment probably occurred during a previous subduction event by metasomatism of the overlying mantle wedge, either in the Early Cretaceous or earlier, as suggested by TDM model ages (ca. 600 Ma). The Cu-Mo(-Pb-Zn-Fe) deposits are intimately associated with the intrusions, and the Banat (Romania) region can be subdivided in two metallogenic zones. In northern Banat, Fe-Cu-Pb-Zn skarns occur whereas in southern Banat, porphyry-style copper and molybdenum deposits predominate. These differences result from a combination of several parameters: (1) magma composition, whereby copper- and molybdenum-rich deposits tend to be associated with calc-alkaline compositions; (2) an increase of the present-day erosion level, from south to north, as indicated by the presence of large equigranular plutons in northern Banat, and of porphyritic cupolas and apophyses associated with porphyry-style mineralisation in southern Banat; (3) the nature of the host rocks, with skarns preferentially developed in calcareous host rocks; and (4) local variations of conditions controlling the infiltration of fluids and the precipitation of ore minerals.

Petrology of early Tertiary nephelinites off mid-Norway. Additional evidence for an enriched endmember of the ancestral Iceland plume

Plugs of alkaline volcanic rocks (≈56 Ma) penetrating Mesozoic sediments crop out at 63°25′N on the continental shelf off Norway. The rocks are highly silica-undersaturated, comprising both nephelinites and basanite. The nephelinitic rocks are porphyritic, with abundant phenocrysts of Mg-rich olivine and some clinopyroxene. Many olivine crystals are altered along grain margins and fractures. The fine-grained matrix consists of fresh augite, biotite, rhönite, chlorite, oxides, and altered nepheline. Apatite and calcite are accessories. The basanite has much less olivine and abundant matrix plagioclase. The occurrence of rhönite indicates crystallization in the presence of a fluid phase at P<60 MPa and temperatures between 840 and 1200°C. The rocks are characterized by relatively high contents of MgO, CaO, P 2O 5, Ni, Cr, Sr, and incompatible elements. The REE are highly fractionated with LREE up to 500× chondrite. Initial strontium isotope ratios ( 87 Sr/ 86 Sr ) i range from 0.703685 to 0.704025 and ϵ Nd from +3.13 to +3.31 in the nephelinites. In the basanite ( 87 Sr/ 86 Sr ) i is 0.704447 and ϵ Nd+0.01. The nephelinites are enriched in 208 Pb/ 204 Pb . We suggest that the basanite formed from enriched lithospheric mantle and that the nephelinites were low melt fractions of a sublithospheric (asthenospheric) source. This source probably represents an enriched, low-temperature melting end member component of the North Atlantic (Icelandic) plume.

The Advent of the Proterozoic - a Trigger for Extensive Intracrustal Processes in the South Indian Shield?

Geochemical, Sr-isotopic and geochronological data on the major granitic activity in the south Indian shield suggest a significant event of intracrustal melting and differentiation processes, particularly at middle- and lower crustal levels, resulting in irreversible geochemical differentiation of the crustal column into a large ion lithophile (LIL) element - depleted anhydrous lower crust and an upper crust characterised by copious granitic activity, marking the Archean-Proterozoic boundary. A perceptible link between the formation of granulites in the lower crust and the granitic activity in the upper crust is recognised in this region. Major element geochemistry of the granites suggests an anatectic origin for these granites. The geochemical signatures of the granites in the Closepet, Hampi (Karnataka), Lepakshi, Kadiri, Tanakallu and Hyderabad regions (Andhra Pradesh) suggest that the source regions for the granitic melts were mainly the juvenile crustal additions of the tonalilte-trondhjemite-granodiorite (TTG) types at lower crustal levels. Sources which had little crustal residence time are indicated by relatively low initial strontium isotopic ratios. This granitic activity was mostly of the granodiorite, quartz-monzonite and granite types which exhibit I-type character. These granites usually have highly fractionated REE patterns, mostly with negative Eu anomalies, implying significant fractionation of palgioclase into the residue and the strong effects of intracrustal differentiation processes. The heat required for melting might have been provided by crustal thickening and radioactive decay processes, with possible involvement of minor amounts of mantle input at times. CO 2- streaming, which reduces considerably the melting temperatures and helps in the removal of LILE into the melt, may also have played an important role. Most of the granites have geochemical characteristics of origins in volcanic arc or syn-collisional settings, suggesting the operation of plate tectonic processes and crustal accretion mechanisms during the Archaean-Proterozoic transition in the south Indian shield. Proterozoic crustal accretion and the granitic crustal differentiation contributed to relative stability of the continental crust and resulted in cratonisation process.

Oxygen isotope evidence for extensive crustal contamination in the Okenyenya igneous complex, Namibia

Oxygen isotope data are presented for silicate minerals separated from a variety of rock types from the 130 Ma Okenyenya igneous complex of northwestern Namibia. These rock types include a tholeiitic suite ranging from olivine gabbro to quartz monzodiorite and syenite, and an alkaline suite including gabbro, syenite, nepheline syenite, and essexite. In general, the difference in δ18O values between coexisting feldspar, pyroxene, and biotite are consistent with equilibrium at high temperatures and preclude significant interaction with magmatic or external fluids.The δ18O values of pyroxene can be used to model magmatic processes and show a systematic increase from 7.1–9.0‰ along a transect through a tholeiitic olivine gabbro-quartz monzodioritic body, corresponding to a systematic decrease in Mg#. Pyroxene δ18O values also show a strong correlation with initial strontium and neodymium isotope ratios, with the alkaline suite having lower δ18O values, lower initial strontium-isotope ratios, and higher initial neodymium isotope ratios than the tholeiitic suite. Simple oxygen mass balance calculations suggest that the extremes of isotope composition resulted from at least 60% contamination by material of similar isotope composition to the nearby southern Etendeka quartz latites. Assimilation/crystal fractionation models of oxygen, strontium, and neodymium isotope data suggest that the rate of assimilation was high and may have approached simple mixing. The contaminated tholeiitic olivine gabbro-quartz monzodioritic body was the earliest to crystallize in the complex and this was followed by relatively uncontaminated silica-undersaturated magmas.

A Rb-Sr whole-rock isochron of 1.5 Ga in early Proterozoic pillow lavas in SW Finland

Abstract Svecofennian pillowed metalavas from SW Finland give Rb-Sr whole-rock ages close to 1.5 Ga and an initial strontium isotope ratio around 0.705. The low age is synchronous with the ages of two large rapakivi massives north and west of the pillow lavas, and it is suggested that it dates the thermal halo around the massives. The primitive and uniform initial strontium isotope ratios for the chemically slightly different tholeiitic pillow lavas show that they are partial melts of similar source rocks and that their different trace element composition is not due to contamination with older crustal material.

Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, Puno, southeastern Peru: Geochronology and petrochemistry

The Inner Arc domain, the easternmost magmatic manifestation of the post-Paleozoic Central Andean orogeny in southeastern Peru and western Bolivia, comprises a remarkably diverse assemblage of plutonic and volcanic rocks, many of which would be more characteristic of ensialic rifts or collisional mountain belts than of Andean-type convergent plate boundaries. Marked petrologic contrasts with the more homogeneous Main Arc domain, which underlies the westerly provinces of the orogen, have been maintained since the initiation of Andean orogeny in the Late Triassic. Constraints on the chronology and petrogenesis of the early stages in the protracted evolution of the Inner Arc and its Permian antecedents are provided herein by, respectively, KAr and RbSr geochronologic data and major and minor element analyses of representative pre-Cretaceous igneous rocks of the Cordillera de Carabaya, southeastern Peru. Our studies confirm the following sequence of magmatic events, which temporally overlapped with the initial stages of Andean orogeny: 1. i) Intrusion of the gabbroic-to-granitic San Gabán (Corani) complex, a calc-alkaline, but crustally contaminated, suite that cores an extensive area of high-grade, low-pressure metamorphism in lower Paleozoic strata. The complex has been assigned to the mid-Paleozoic, but its age remains poorly defined. The foliated, markedly peraluminous, two-mica granites of the smaller Limacpampa pluton may also have been emplaced during the Paleozoic, but a Triassic age is favored on the basis of our RbSr data. 2. ii) Eruption of alkali basaltic lavas of the Lower Permian Mitu Group along the northeastern margin of a longitudinal ensialic rift that developed in response to extensional tectonism in the interval between the pre-Andean (“late Hercynian”) and Andean orogenies. 3. iii) Emplacement of large granitoid plutons (Coasa, Limbani, and Aricoma centers), with I-Caledonian affinities, along the northeastern boundary of the Mitu rift during the Late Triassic ( ca. 225 Ma). The metaluminous to weakly peraluminous monozogranites and granodiorites comprising the greater part of the Carabaya Batholith (new term) were closely associated with mafic dikes of alkaline composition, similar in many respects to the preceding alkali basalts. 4. iv) Development of the Allinccápac Group or peralkaline complex (new term), an assemblage of Middle (and Lower(?) Jurassic lavas, pyroclastics, and plutons exhibiting alkaline to peralkaline affinities. Whereas each of the above suites may be assigned to either a predominant mantle or crustal source, it is evident from the chemical and isotopic data that varied mantle and crustal environments have been involved. Thus, the distinctive chemistries of the coeval granitoid intrusions — as expressed, for example, in the trace element contents of whole-rocks and biotites, the oxidation states of both rocks and biotites, and the initial strontium isotope ratios — demonstrate the contributions of several distinct protoliths. The close spatial and temporal association of mantle and crustal suites during the Permian-to-Jurassic interval strongly implies a cause-and-effect relationship. In particular, the role of basaltic injection in generating large volumes of peraluminous granitoid magmas is amply supported.

Meteoric depositions on a forest in southern Sardinia (Italy)

A strontium and neodymium isotopic investigation of plagioclase-rich cumulate rocks in the 1.43 Ga Laramie anorthosite complex (LAC) provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of Proterozoic anorthosites. The 725 km2 LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island arc terranes in the south, the Cheyenne belt, at pressures of 3–4 kilobars. Initial strontium and εND isotopic ratios from the three large composite anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034–0.7055 and +0.9 to −4.9, respectively. The isotopic ratios in the cumulates vary as a result of (1) the isotopic composition of the parental mantle-derived high-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement.Strontium isotopic and normative An variations across the 5–7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km2 Poe Mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The earliest and stratigraphically lowest intrusion was emplaced as a plagioclase-rich magma that crystallized large volumes of compositionally homogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt shifts in ISr and/or normative An in the stratigraphically higher levels of the Poe Mountain anorthosite. Strontium and neodymium isotopic disequilibrium between a high-Al clinopyroxene megacryst and the layered cumulates is consistent with a high-pressure origin for Al-rich pyroxene megacrysts in Proterozoic anorthosites. The megacryst crystallized in a magma chamber at pressures of 10–12 kilobars from relatively uncontaminated basaltic parent magma (high-Al gabbro) and was transported through the crust in progressively contaminated anorthositic magma.The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAC, supporting the proposition that high-Al gabbros are parental to anorthosite (Mitchell et al., 1995). The isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing ISr and increasing εNd with a relative decrease in age of the intrusions (Snow Creek → Poe Mountain/Chugwater → troctolites) indicates that the magma conduit became increasingly insulated from crustal contamination over time.This study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.

The geology of the Tangse porphyry copper-molybdenum prospect, Aceh, Indonesia

The porphyry mineral deposit at Tangse is hosted by a multiphase intrusion of predominantly quartz diorite composition termed the Tangse stock. Emplacement along a segment of the transcurrent Sumatera fault system was related to the northward subduction of the Indian plate. The intrusive rocks belong to a normal K calc-alkaline suite and are characterized by low abundances of K (1.09-1.35 wt %), Rb (26-40 ppm), Nb (<5 ppm), Th (<4 ppm), and U (<1.5 ppm), high Sr (314-488 ppm) abundance, moderately high K/Rb and K/Ba ratios, and low Rb/Sr and Th/U ratios. Initial strontium isotope ratios (0.70423-0.70453) are uniformly low and prohibit any significant involvement of a sialic crustal component in magma genesis. K-Ar analysis yields middle to late Miocene ages for the intrusion and cooling of the Tangse stock (13.1 m.y.) and the development of the associated hydrothermal alteration-mineralization (9.0 m.y.). The porphyritic intrusive phases are divisible into three groups (older, intermediate, and younger porphyries). The older porphyries form the bulk of the Tangse stock; the other phases are volumetrically unimportant, occurring typically as dikes. The distribution of the hypogene sulfide minerals, chalcopyrite, molybdenite, and pyrite, within the older porphyry mass is related to the development of a complex series of aluminosilicate alteration assemblages. A core of early chalcopyrite-bearing biotite alteration (Cu = 0.1-0.2%, Mo: 60-90 ppm, total sulfide = 1 vol %) is enclosed within a halo of barren chlorite-epidote alteration. Both have been overprinted by feldspar-destructive, sericitic alteration with quite variable metal contents. Copper and molybdenum grades in the chalcopyrite-molybdenite-bearing sericite-chlorite-quartz alteration (total sulfide = 2-4 vol %) typically exceed 0.2 percent and 100 ppm, respectively, and locally may reach values of 0.8 percent and 300 ppm, reflecting an enrichment relative to the early biotite alteration. By contrast the quartz-sericite and quartz-sericite-andalusite alteration assembldges (total sulfide content = 2-10 vol %) with grades of less than 0.1 percent Cu and 20 ppm Mo are depleted in both metals.

Field relations and strontium isotope systematics of the marginal rocks of the eastern Bushveld Complex

The well-exposed basal contact of the eastern Bushveld Complex was mapped at scales of 1:10,000 and 1:20,000 along 130 km of strike and through a 9-km vertical section. Field relations, petrography, and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups. A broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone. A gabbroic group occurs above this level. Each group consists of a number of closely related rock types, most of which have sill analogs. Rocks of the first group (B1) include pyroxenites, norites, and peridotites, all of which may have quench textures. All contain orthopyroxene (>En 78 ), plagioclase (>An 65 ), and chromite, with subordinate clinopyroxene and biotite. Rare earth element patterns are mostly strongly fractionated (Ce N /Yb N approximately 10) and have 40X chondritic abundances at Ce. Their initial strontium isotope ratios at 2.05 b.y. fall in the range 0.7032 to 0.7057 and Rb/Sr ratios approximate 0.2. The gabbroic group consists of microcrystalline, two-pyroxene gabbros (B2N and B3N) and their coarser grained equivalents (B2 and B3) that border the upper critical and main zones, respectively. Mineral assemblages are dominated by plagioclase ( approximately An 55 ), clinopyroxene, and orthopyroxene ( approximately En 63 ), with abundant magnetite and traces of ilmenite and biotite. Their rare earth element patterns are less fractionated than B1 rocks. The B2N and B2 family have Ce N /Yb N approximately 6 and 30X chondritic abundances at Ce; the B3N and B3 rocks have Ce N /Yb N approximately 3, 6X chondritic abundances at Ce, and pronounced positive Eu anomalies. Strontium isotope initial ratios at 2.05 b.y. fall in the range 0.706 to 0.708 and the Rb/Sr ratios are approximately 0.02. Whole-rock and Sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial Sr isotope ratio (R 0 ) similar to that of the Transition Zone (0.7056). A fraction of such liquid, enriched in cumulus olivine crystals, when mixed with up to 30 percent of a pyroxenitic liquid with low R 0 (0.7034), yields a product having the isotopic and compositional characteristics of the peridotite sills.A continuum exists in the initial Sr ratios through the pyroxenitic and gabbroic marginal rocks, suggesting that these represent mixtures of at least two sources with distinct initial Sr isotope ratio characteristics. It is argued that neither magma aging nor crustal contamination adequately accounts for the Sr isotope characteristics of the marginal rocks or their derived cumulates. The unusual chemistry of the pyroxenitic early liquids (SiO 2 approximately 55%, MgO approximately 13%, coupled with high concentrations of incompatible elements) should be considered primary.

Strontium isotope evidence for preserved density stratification in the main zone of the Bushveld Complex, South Africa

The Bushveld Complex formed from two isotopically distinct magmas. Magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates. Rocks with ratios of 0.7065 lie below the lower 2.2 km of the main zone (ratio = 0.7085), whereas those with ratios of 0.7073 lie above it. This unit represents a liquid layer intruded into the density-stratified magma pile near the base of the chamber. The Merensky Reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals.

The petrology and geochemistry of alkaline granites from Nigeria

The Nigerian younger granites, as well as the petrologically similar but chronologically older northern centres in Aïr and south Niger, constitute the best example of epidosically migrating mid-plate magmatism. The Nigerian anorogenic ring complexes provide a classic example of alkaline granitoid evolution. Each anorogenic centre represents the exposed roots of an eroded volcano. Volcanic assemblages preserved by caldera collapse at some centres confirm that early erupted liquids evolved from olivine basalt through hawaiite, mugearite and trachyte. Although there are volumetrically abundant rhyolitic ignimbrites, there are no volcanic equivalents of biotite granite. However, at the subvolcanic level biotite granites are extensively mineralized with rich deposits of columbite, sphalerite, cassiterite and wolframite. Related peralkaline granites contain enriched concentrations of ore elements but apart from pyrochlore, thorium-rich monazite, and topaz, most potential ores are contained in alkali amphiboles or alkali pyroxene. Fluid interaction in the granite cupolas has caused subsolidus mineral growth and modified original element abundances. In some instances fluid reaction has been so intense that original granite has been locally converted to albite, microclinite, and greisen. These reactions can be monitored by the new mineral assemblages, the major element chemistry, trace elements (e.g., the rare earths) and isotopically by the initial strontium isotopic ratios.

Cretaceous Sinistral Strike Slip Along Nacimiento Fault in Coastal California

The San Andreas and Nacimiento faults of coastal California both separate granitic and metamorphic basement rocks of the Salinian block from partly coeval but contrasting Mesozoic terranes underlain by the Franciscan subduction complex. By analogy with Neogene dextral strike slip along the San Andreas fault, Cretaceous sinistral strike slip can be inferred along the Nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount. Following restoration of known San Andreas and inferred proto-San Andreas dextral displacements, reversal of about 560 km (350 mi) of postulated sinistral slip on the Nacimiento fault brings four major Mesozoic lithotectonic belts of California and Baja California into simple al gnment as subparallel terranes related to Mesozoic subduction along the continental margin. Neogene deformation within the San Andreas transform system involved (a) elongation of the Salinian block by dextral slip along subsidiary faults that branch from the San Andreas fault, and (b) dextral rotation of crustal panels within the Transverse Ranges. Latest Cretaceous and/or earliest Paleogene dextral slip along a proto-San Andreas fault followed the San Andreas course in central California, but diverged westward in southern California. Nacimiento sinistral displacements occurred in mid-Cretaceous to early or medial Late Cretaceous time, after Cretaceous emplacement of plutons now within the Salinian block but prior to deposition of uppermost Cretaceous sedimentary sequences in central California. Available data on Mesozoic relative and absolute plate motions in the Mesoameric n region support the likelihood of Cretaceous sinistral strike slip subparallel to the California continental margin. Paleotectonic reconstruction of crustal blocks in California and Baja California to their inferred mid-Cretaceous relative positions shows the Salinian block inserted on a bias between the flanking Mojave and Peninsular Ranges blocks. Salinian granitic rocks thus formed an interior part of the Mesozoic batholith belt, and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks. The similar Mesozoic terranes that now lie east and west of the Salinian block were then adjacent to one another west of the Sierra Nevada block. Available paleomagnetic data neither support nor preclude the reconstruction, but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents. A correct interpretation of the Nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal California.

Differentiation of calc-alkaline magmas: Evidence from the Coquihalla Volcanic Complex, British Columbia

The Coquihalla Volcanic Complex is part of the Miocene, calc-alkaline Pemberton Volcanic Belt, whose formation in southwestern British Columbia is associated with subduction of the Juan de Fuca plate. The complex consists of voluminous rhyolitic ash-flow tuffs cross-cut by hypabyssal andesitic to dacitic intrusive bodies and a diorite stock. Whole-rock compositions of members of the Coquihalla Volcanic Complex show a range in silica from 54 to 76 wt.% (volatile-free). Uniform initial strontium isotopic ratios (0.7037 ± 0.00008) argue against derivation of magmas by crustal melting or mixing of basaltic magmas with crustal material. Trace element concentrations of whole-rocks cannot be modelled by varying degrees of partial melting of mantle material. Compositional variations among whole-rocks and constituent phenocrysts form the basis for major element mass-balance calculations which indicate that: (1) hornblende dacites can be derived from basaltic andesites by 50% crystallization of a mixture of plagioclase, hornblende, clinopyroxene, titanomagnetite, and apatite, and (2) rhyolites can be derived from dacites by roughly 45% crystallization of a mixture of plagioclase, hornblende, biotite, titanomagnetite, and apatite. Mineral/liquid distribution coefficients for eight trace elements and the observed mineral assemblages (plagioclase, clinopyroxene, hornblende, titanomagnetite, biotite, apatite, and zircon) have been compiled in an appendix. Large uncertainties in these values have led to a reformulation of the Rayleigh fractionation model so that an internally consistent set of distribution coefficients has been calculated for the proposed differentiation sequence.

Stable isotope systematics in mesozoic granites of Central and Northern California and Southwestern Oregon

18O, D, and H2O+ contents were measured for whole-rock specimens of granitoid rocks from 131 localitics in California and southwestern Oregon. With 41 new determinations in the Klamath Mountains and Sierra Nevada, initial strontium isotope ratios are known for 104 of these samples. Large variations in δ 18O (5.5 to 12.4), δD (−130 to −31), water contents (0.14 to 2.23 weight percent) and initial strontium isotope ratios (0.7028 to 0.7095) suggest a variety of source materials and identify rocks modified by secondary processes. Regular patterns of variation in each isotopic ratio exist over large geographical regions, but correlations between the ratios are generally absent except in restricted areas. For example, the regular decrease in δD values from west to east in the Sierra Nevada batholith is not correlative with a quite complex pattern of δ 18O values, implying that different processes were responsible for the isotopic variations in these two elements. In marked contrast to a good correlation between (87Sr/86Sr)o and δ 18O observed in the Peninsular Ranges batholith to the south, such correlations are lacking except in a few areas. δD values, on the other hand, correlate well with rock types, chemistry, and (87Sr/86Sr)o except in the Coast Ranges where few of the isotopic signatures are primary. The uniformly low δD values of samples from the Mojave Desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area. Even so, the δ 18O values and 18O fractionations between quartz and feldspar are normal in these same rocks. This reconnaissance study has identified regularities in geochemical parameters over enormous geographical regions. These patterns are not well understood but merit more detailed examination because they contain information critical to our understanding of the development of granitoid batholiths.

Development of heterogeneities in the subcontinental Mantle: Evidence from the Ferrar Group, Antarctica

Tholeiitic lava flows (Kirkpatrick Basalts) and dolerite sills and dikes (Ferrar Dolerites) of the Jurassic Ferrar Group from Antarctica and dolerite sills from Tasmania, Australia are characterised by initial strontium isotope ratios ranging from 0.7089 to 0.7153. The mean and standard deviation of 85 analyses is 0.7115±0.0012. Some of the scatter in the initial 87Sr/86Sr ratios can be attributed to sample inhomogeneity, analytical uncertainties and sample alteration. The published major element data show well-defined trends that are consistent with an evolution by fractional crystallization. Recognition of a parental magma is difficult due to the fractionated nature of the rocks. Trace element analyses, particularly the rare earth elements (REE) support a differentiation model. Compared to mid-ocean ridge basalts, Ferrar Group rocks are enriched in light REE. Kirkpatrick Basalts from the central Transantarctic Mountains show significant correlations between initial 87Sr/86Sr ratios and major elements only for SiO2 and CaO. The general lack of strong correlation is the basis for rejecting the possibility of wholesale contamination by sialic material as a possible cause of the high 87Sr/86Sr ratios. Selective contamination of the basaltic magmas is a possibility and cannot be completely discounted. It would probably involve a fluid phase in order to transport and mix the light REE, Rb, 87Sr, and other elements. By analogy with selective contamination of ocean ridge basalts by sea water it is difficult to envisage a similar process acting on magma emplaced in a non-marine environment. Because of the elevated values of the initial 87Sr/86Sr ratios, their similar average value over 2,500 km and the large volume of magma involved (4× 105 km3) a mantle origin for the high Sr ratios is preferred. Models to account for the enrichment of Rb and light REE in the Antarctic mantle during or prior to the Jurassic include: 1. addition of continental material from a Palezoic Mesozoic subduction zone; 2. metasomatism of volatile elements from the lower mantle; and 3. evolution of a mantle with a high Rb/Sr ratio.