Mafic Source Research Articles

Indium in cassiterite and ores of tin deposits

The results obtained with LA-ICP-MS by less abundant lighter 113In isotope and EPMA show that in cassiterite of cassiterite–quartz veins the indium contents do not exceed 160ppm, while cassiterite from Sn–sulfide veins is characterized by higher indium contents from 40 to 485ppm; sulfides of Sn–sulfide veins unlike sulfides of cassiterite–quartz veins also have the highest indium contents: Fe-sphalerite (100–25,000ppm), chalcopyrite (up to 1000ppm), and stannite (up to 60,000ppm). Indium contents in the Sn–sulfide ore of the Tigrinoe and Pravourmiiskoe deposits obtained using SR-XRF, ICP-MS and atomic absorption methods range from 10 to 433ppm with average values of 56–65ppm. Indium-rich Sn–sulfide mineralization in five large Sn–Ag ore districts of the Far East Russia (Khingansky, Badzhalsky, Komsomolsky, Arminsky, Kavalerovsky) provides the impetus for further exploration of deposits with Sn–sulfide mineralization as the most promising indium resources in Russia. Empirical observations from geology and geochronology of cassiterite–quartz and Sn–sulfide mineralization show that the combined contribution from granite and alkaline–subalkaline mafic sources and multistage ore-forming processes doubled indium resources of deposits being the main factors in the formation of high grade indium mineralization.

Composition, origin and industrial suitability of the Aswan ball clays, Egypt

Although Aswan ball clays occur in relatively large reserves, information on their geochemistry and source is still lacking. This paper presents detailed petrographic, mineralogical, and geochemical investigations on these clays to examine their source as well as their possible use as raw materials for ceramic and refractory industries.Aswan ball clays occur as gray, yellowish gray, reddish to brownish gray, massive to faint laminated, and moderately hard clays. Grain-size distributions indicate the dominance of clay fractions (45–57wt.%). Low-ordered kaolinite is the main constituent (39–60wt.%) along with quartz (24–46wt.%) and low crystalline illite (10–19wt.%). Fine-grained anatase is reported as a minor constituent (~2wt.%). Aswan clays are typically ball clays (SiO2/Al2O3=2.3–4.1) with relatively high Fe2O3 and TiO2 contents. The trace elements occur in two assemblages. Elements associated with the Fe-bearing phases include Cu, Ni, Co, Zn, V, and Pb as indicated from the positive correlations with the Fe2O3 contents. Elements occur as silicate minerals but not in clay minerals such as Nb, Zr, Y, Hf, Ta, and U as revealed from the positive correlations of these elements with the SiO2 and the negative correlations with the Al2O3 contents. The sum of rare earth elements (REEs) ranges from 291 to 335ppm with negative correlations with the Al2O3 and positive correlations with the SiO2 suggesting the occurrence of REE as silicates but not in clay minerals. Chondrite-normalized REE patterns exhibit light rare earth element enrichment relative to heavy rare earth elements ((La/Yb)N=9.2–11.7) and slightly negative Eu anomalies (Eu/Eu*=0.74–0.83) without Ce anomalies (Ce/Ce*=0.96–1.03).Major, trace, and rare earth elements geochemistry of the clay fractions indicates a mixture of more than rock types as a source of the Aswan ball clays. The high Zr and Y contents and La/Yb ratios suggest a contribution of granitic rock, while the relatively high contents of TiO2 and Ti are suggestive for a contribution of mafic source rock. Plot of the study clays in the Rb-K2O and Hf–La/Th binary plots supports mixed felsic and mafic source rocks of the study clays. In addition, high Nb contents indicate a contribution of alkaline source rock. Plot of the study clays in the phyllite and schist field in the Co–Th binary diagram also indicates contribution of metamorphic source rock to the source of the clays.The abundance of clay size fractions (<2μm), low-order kaolinite and illite, absence of I/S minerals, low fluxing agents such as alkali oxides (Na2O and K2O) and alkaline earth oxides (CaO and MgO), low S and Cl contents, and low contents of toxic elements (As, Cd, Hg, and Pb) reveal the suitability of the Aswan ball clays as a good quality and environment-friendly raw material for ceramic and refractory industries.

Age, composition, and source of continental arc- and syn-collision granites of the Neoproterozoic Sergipano Belt, Southern Borborema Province, Brazil

The Sergipano belt is the outcome of collision between the Pernambuco-Alagoas Domain (Massif) and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Although the understanding of the Sergipano belt evolution has improved significantly, the timing of emplacement, geochemistry and tectonic setting of granitic bodies in the belt is poorly known. We recognized two granite age groups: 630–618 Ma granites in the Canindé, Poço Redondo and Macururé domains, and 590–570 Ma granites in the Macururé metasedimentary domain. U–Pb SHRIMP zircon ages for granites of first age group indicated ages of 631 ± 4 Ma for the Sítios Novos granite, 623 ± 7 Ma for the Poço Redondo granite, 619 ± 3.3 Ma for the Lajedinho monzodiorite, and 618 ± 3 Ma for the Queimada Grande granodiorite. These granitoids are dominantly high-K calc-alkaline, magnesian, metaluminous, mafic enclave-rich (Queimada Grande and Lajedinho), or with abundant inherited zircon grains (Poço Redondo and Sitios Novos). Geochemical and isotope data allow us to propose that Sítios Novos and Poço Redondo granites are product of partial melting of Poço Redondo migmatites. Sr-Nd isotopes of the Queimada Grande granodiorite and Lajedinho monzodiorite suggest that their parental magma may have originated by mixing between a juvenile mafic source and a crustal component that could be the Poço Redondo migmatites or the Macururé metasediments. Other 630–618 Ma granites in the belt are the mafic enclave-rich Coronel João Sá granodiorite and the Camará tonalite in the Macururé sedimentary domain. These granites have similar geochemical and isotopic characteristics as the Lajedinho and Queimada Grande granitoids. We infer for the Camará tonalite and Coronel João Sá granodiorite that their parental magmas have had contributions from mafic lower crust and felsic upper crust, most probably from underthrust São Francisco Craton, or Pernambuco-Alagoas Domain. The younger 590–570 Ma granite group is confined to the Macururé metasedimentary domain. Although these granites do not show typical features of S-type granites, their U–Pb age, field relationships, geochemical and Sr-Nd data suggest that their parental magmas have originated from high degree melting of the Macururé micaschists. Field observations support a model in which the Macururé domain, limited by the Belo Monte-Jeremoabo and São Miguel do Aleixo shear zones, behaved as a ductile channel flow for magma migration and emplacement during the Neoproterozoic, very much like the channel flow model proposed for emplacement of leucogranites in the Himalayas.

Timing and significance of gabbro emplacement within two distinct plutonic domains of the Peninsular Ranges batholith, southern and Baja California

Evaluating the crucial role postulated for mantle-derived mafic magmas in the formation of silicic batholiths requires well-determined igneous crystallization ages from mafic rocks as well as petrologic data reflecting possible mantle source heterogeneities and/or variations in subcrustal processes. The abundant mafic plutons that crop out within the mid-Cretaceous Peninsular Ranges batholith (PRB, southern and Baja California) provide an important opportunity to investigate the relationship of mafic and silicic magmatism in Cordilleran batholiths. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry ages are reported for 24 samples of PRB gabbro collected throughout its 1400-km-long extent. The samples span the entire compositional spectrum of PRB gabbro from cumulate olivine gabbro with <10 ppm whole rock Zr content, to gabbronorite, to more fractionated hornblende gabbro. Eighteen of the samples are from the western zone “gabbro belt” of the batholith including the type locality for San Marcos Gabbro, while six are from the eastern zone where gabbro is rare, composing <1% of the outcrop area. The morphology of zircon in PRB gabbro ranges from anhedral fragments to euhedral grains, and this zircon has distinctly lower uranium concentrations and higher Th/U ratios than zircon from PRB granitoids. Most samples exhibit simple U-Pb systematics, but several contain inherited or entrained zircons, and two eastern gabbro samples contain distinctly bimodal age populations. Weighted mean 206Pb/238U zircon ages are interpreted as crystallization ages and range from 130.5 ± 1.5 Ma to 100.2 ± 3.0 Ma for western zone samples. The ages demonstrate that gabbros were emplaced throughout the ∼30 m.y. interval during which the bulk of the western zone was constructed, and conflict with the earlier interpretation of western gabbros as mafic forerunners that preceded emplacement of granitoid plutons. Eastern zone gabbros, in contrast, yield U-Pb ages that range mostly from 109.4 ± 2.9 Ma to 97.8 ± 2.3 Ma, narrowly predating the ca. 92–98 Ma eastern zone tonalite-trondhjemite-granodiorite batholith flareup, and thus may have played a role in crustal thickening and triggering of the flareup. Despite their partially cumulate character, whole rock major and trace element compositions of PRB gabbro closely match high-alumina olivine tholeiite and arc basalt and basaltic andesite from modern arcs. Gabbros from the west and east sides of the batholith, transverse to regional strike, have similar nearly flat rare earth element patterns that reflect derivation from a common partially contaminated depleted mantle source. The apparent across-strike petrologic uniformity of the gabbros contrasts strongly with PRB granitoids, which are well known for across-strike asymmetries in trace elements, initial 87Sr/86Sr, and δ18O, reflecting progressively deeper partial melting of mafic source rocks from west to east with additional input from a high-18O, high-87Sr end member on the east side of the batholith. The western PRB gabbros represent a plausible deep crustal source composition for derivation of western granitoids by partial melting or fractional crystallization. For the eastern batholith, the mafic source region requires modification by input of accretionary complex supracrustal rocks delivered via forearc subduction erosion underthrusting.

PROVENANCE OF THE NEOPROTEROZOIC ROCKS OF THE GIFBERG GROUP (WESTERN SOUTH AFRICA)

New petrographic and lithogeochemical data are presented for siliciclastic units of the Gifberg Group (western South Africa), a succession rarely studied in the past. The main detrital source for the oldest succession of the Gifberg Group, the Karoetjes Kop Formation, has been identified as sourced by local metamorphic rocks with a felsic geochemical composition. Nearly unweathered detritus (Chemical index of alteration: 50 to 60; K/Cs >>5000) occurs in the metadiamictite of this formation and is interpreted as palaeovalley infill, deposited only a short distance from its sources. The provenance signature of the overlying Aties Formation indicates significant mafic and/or intermediate sources. Possible sources are rocks of the Bushmanland Group and an unknown unmetamorphosed mafic to intermediate rock succession, which might have been exposed during the deposition of the Aties Formation. The overlying Bloupoort Formation is characterised by a geochemically homogenous metadiamictite (Swartleikrans Bed) with only slightly weathered detritus (K/Cs >10000) and overlying mature silica-rich metarenites and metaconglomerates. It is proposed that the Gifberg basin formed as a small-scale rift basin and then widened through time with subsequent longer sedimentary transport of the detritus which have caused the higher compositional maturity. The predominance of detrital input from local sources in the Karoetjes Kop Formation hinders correlation based on its mineralogical and geochemical content alone. A glacial origin for the Swartleikrans Bed was proposed previously based on the occurrence of possible dropstones. Certain criteria for a glacial depositional environment as outlined by Arnaud and Etienne (2011) were met in our study and include poor sorting, rapid lateral facies changes, crude stratification, variable thicknesses, compositional immaturity, and a gradational contact plus a chaotic texture with large boulders. While these observations point to a ‘rainout’ deposit, there is still a lack of evidence for glacial transport on grain surfaces or in the form of faceted clasts and pebbles.

GEOCHEMISTRY AND HEAVY MINERALS ANALYSES ON "BLACK SANDS" OF THE LOWER CAMBRIAN FISH RIVER SUBGROUP (NAMA GROUP, NAMIBIA)

Heavy mineral rich-sandstones (“black sands”) of the Lower Cambrian upper Fish River Subgroup in Namibia were used to reveal the provenance of the detrital material. The host rocks of the black sands from the Haribes and Rosenhof Members are mainly composed of partially rounded quartz, feldspar (K-feldspar > plagioclase), polycrystalline quartz, few siltstone fragments and labile altered or metamorphosed volcanic lithoclasts. Geochemical proxies are similar to those from typical foreland deposits and point to slightly recycled upper continental crustal composition, e.g.: Th/Sc ratios between 1.23 and 1.95, Zr/Sc ranging from 24 to 38 and La/Sc from 4.8 to 10.8. Heavy mineral addition of up to 30% does not significantly change Th/Sc, La/Sc and La/Th values, but slightly changed the Zr/Sc ratios, which range from 38 to 53. The heavy mineral-enriched beds (50 to 75% of the whole rock), display nearly similar La/Sc ratios compared with other lithotypes. They are 9x enriched in rare earth elements and show a significantly stronger negative Eu-anomaly, and an enrichment of heavy over light rare earth elements, as well as higher Zr/Sc ratios (82 to 299), typical for reworked passive margin or foreland deposits. The chemical composition of separated heavy minerals point to a relatively strong influence of metapelitic and metamorphosed mafic sources, which were not identified using geochemical data. Mineral chemical analyses of chromian spinels point to island arc and mid-ocean ridge basalts as protoliths, whereas rutiles were derived from metamorphosed mafic and pelitic rocks, and abundant garnet (mainly almandine) derived from high-grade and low-grade metapelitic sources. Titanite shows dominance of felsic and mafic plutonic rocks as sources, while epidote point to low-grade metamorphic protoliths. Magnetite was eroded from felsic plutonic and/or mafic metamorphosed rocks. Previous works using U-Pb detrital zircon dating show typical Namaquan and Pan-African age populations as expected for a collisional tectonic setting. Palaeocurrents indicate that the main source region of the heavy and the host rock minerals is within the Damara Belt which resulted from the convergence of Congo and Kalahari Cratons. Although the exact age of thrusting is debated, the data presented suggest the exhumation of metamorphosed crustal fragments and plutonism related to the Damara orogeny during the deposition of the upper Nama Group (ca. 520 Ma).

Zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical constraints on the provenance and age of the Shuangshanzi and Qinglonghe Groups in Eastern Hebei: Implications for the tectonic evolution of the Eastern Block

Eastern Hebei is located in the northern part of the Eastern Block, which is considered to have collided with the Western Block along the Trans-North China Orogen to form the basement of the North China Craton. It consists mainly of Eoarchean to Mesoarchean rocks, Neoarchean granitoid plutons and supracrustal rocks. The minor amounts of supracrustal rocks can be further divided into the high-grade Zunhua, Qianxi, Luanxian and Dantazi Groups and the low-grade Shuangshanzi and Qinglonghe Groups. The Shuangshanzi Group is composed of amphibolites, leptynites, schists and quartzites, whereas the overlying Qinglonghe Group consists predominantly of meta-conglomerates and leptynites. Geochemistry of the metasedimentary samples from the two low-grade groups indicates weak to moderate source weathering, dominantly chemically immature and major felsic sources with few mafic source rocks. On the tectonic discrimination plots, most of the metasedimentary samples display continental island arc feature. U-Pb ages of detrital zircons from the Shuangshanzi and Qinglonghe Groups yielded three age populations of 2600–2400Ma, 2400–2161Ma and 3662–2609Ma. The primary 2600–2400Ma detrital zircons were most probably sourced from the late Neoarchean to early Paleoproterozoic granitoids and amphibolites in Eastern Hebei. The subordinate 2400–2161Ma detrital zircons may have mostly derived from the meta-volcanic rocks in the Shuangshanzi Group, whereas minor amounts of 3662–2609Ma detrital zircons were most likely directly derived from the Paleoarchean to Neoarchean granitics or recycled from the Caozhuang quartzites in Eastern Hebei. The crystallization age of 2352±4Ma of a meta-porphyritic dacite and the youngest detrital zircon age peaks of 2337–2329Ma from the Shuangshanzi Group suggest that the sedimentation of the group began at ∼2.35Ga. The nearly contemporary youngest age peaks of 2331–2329Ma have also been identified from the overlying Qinglonghe Group. In combination with the ∼1.85Ga metamorphic event in the Trans-North China Orogen, the formation ages of the two groups can be constrained in the period between ∼2.35Ga and ∼1.85Ga. Taking into account the lithostratigraphic features, provenances and formation ages, the Shuangshanzi and Qinglonghe Groups are interpreted as having formed possibly in a continental intra-arc basin.

Detrital rutile geochemistry and thermometry from the Dabie orogen: Implications for source–sediment links in a UHPM terrane

This study explores the potential of detrital rutile geochemistry and thermometry as a provenance tracer in rocks from the Central Dabie ultrahigh-pressure metamorphic (UHPM) zone in east-central China that formed during Triassic continental collision. Trace element data of 176 detrital rutile grains selected from local river sediments and 91 rutile grains from distinct bedrocks in the Shuanghe and Bixiling areas, obtained by both electron microprobe (EMP) and in situ LA-ICP-MS analyses, suggest that geochemical compositions and thermometry of detrital rutiles are comparable to those from their potential source rocks. After certification of the Cr–Nb discrimination method for the Central Dabie UHPM zone, we show that 29% of the detrital rutiles in the Shuanghe area were derived from metamafic sources whereas in the Bixiling area that it is up to 76%. Furthermore, the proportion of distinct types of detrital rutiles combined with modal abundances of rutile in metapelites and metamafic bedrocks can be used to estimate the proportion of different source lithologies. Based on this method the proportion of mafic source rocks was estimated to ∼10% at Shuanghe and >60% at Bixiling, respectively, which is consistent with the proportions of eclogite (the major rutile-bearing metamafic rock) distribution in the field. Therefore, the investigation of detrital rutiles is a potential way to evaluate the proportion of metamafic rocks and even to prospect for metamafic bodies in UHPM terranes. Zr-in-rutile temperatures were calculated at different pressures and compared with temperatures derived from rock-in rutiles and garnet-clinopyroxene Fe–Mg thermometers. Temperatures calculated for detrital rutiles range from 606°C to 707°C and 566°C to 752°C in Shuanghe and Bixiling, respectively, at P=3GPa with an average temperatures of ca. 630°C for both areas. These temperature averages and ranges are similar to those calculated for rutiles from surrounding source rocks. Combined with comparable Zr distribution characteristics between detrital and source rock rutiles, demonstrating a close source–sediment link for rutiles from clastic and rock in UHPM terranes. Thus rutiles can be accurate tracers of source rock lithologies in sedimentary provenance studies even at a small regional scale. In Bixiling, Nb/Ta ratios of metamafic and metapelitic detrital rutiles fall between 11.0 to 27.3 and 7.7 to 20.5, respectively. In contrast, in Shuanghe, these ratios are highly variable, ranging from 10.9 to 71.0 and 7.6 to 87.1, respectively. When ignoring four outlier compositions with extremely high Nb/Ta in Shuanghe, a distinct clustering of Nb/Ta ratios in rutiles is shown: metapelitic detrital rutiles have Nb/Ta of 7–40 vs. metamafic detrital rutiles with Nb/Ta=11–25. The Nb/Ta characteristics in detrital rutiles from both areas may reflect the degree of fluid–rock interaction during metamorphism and/or different source lithologies. Therefore, the trace element compositions in detrital rutiles can accurately trace the lithology, proportion and fluid–rock interaction of different source rocks.

Structural, petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region, Norway

AbstractEvidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P(ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in theWGRdefines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. TheREEpatterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in theWGRbefore or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H2O‐rich fluid phase at great depth to facilitate H2O‐present partial melting.

Provenance, tectonic setting and age of the sediments of the Upper Disang Formation in the Phek District, Nagaland

Integrated petrographic and geochemical studies of sandstones, and geochemical studies of shales of turbidites from the Upper Disang Formation, Phek district, Nagaland have been carried out to determine their provenance, weathering conditions and tectonic setting. Paleomagnetic studies were carried out for magnetostratigraphic purposes. Studies indicate that most of these sediments were derived from felsic and mafic sources with minor contribution from low to medium grade metamorphic rocks. Most of the felsic components have been transported from distant sources as evidenced from extensive reworking of grains. The most likely source rocks are the granite/granite gneiss of the Karbi Anglong crystalline complex to the west of the study area. However, the bulk of the sediments have been contributed from nearby basic and ultrabasic sources. This would correspond to the fast rising Naga Ophiolite, which probably emerged above sea level during the Mid-Eocene. Prevailing high temperatures and humid climate caused intense chemical weathering of the source rocks. The sediments from the west were transported great distances by turbidity currents into an easterly deepening basin. Sediments from the nearby east were rapidly dumped on the seafloor causing rapid mixing, leading to textural and chemical immaturity. Paleomagnetic studies endorse published paleontological evidence to indicate that most of the sediments of the Upper Disang Formation were deposited during the Late Eocene. Deposition took place in a westward-migrating accretionary-prism complex in an active-margin setting at the convergence of the Indian and Burma plates. This was a rapidly-closing basin where anoxic conditions prevailed. Towards the end of the Eocene this basin closed completely with the destruction of the Tethyan Ocean.

A lower crustal mafic source for the ca. 2550 Ma Qôrqut Granite Complex in southern West Greenland

The late Neoarchaean Qôrqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite–trondhjemite–granodiorite (TTG) composition. The Qôrqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547±4Ma (MSWD=0.63). Initial εHf values range from −12 to −18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon δ18O values range from 6.1±0.2‰ to 6.5±0.3/0.7‰ best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a 176Lu/176Hf around 0.015–0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to >ca. 13–18kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750–850°C. The obtained P–T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting.

Provenance of Cored Sediments from Active Margin off Southwestern Taiwan Deduced from Geochemical Constraints

AbstractThe cored sediments sampled by R/V Marion Dufrense in the active margin off southwestern Taiwan on the east side of the Manila Trench are mostly muds consisting mainly of quartz, feldspar, illite, chlorite + kaolinite with a trace amount of montmorillonite. The cored sediments have higher Fe, Ti, Cr, Cs, Li, Ni, and V but lower Ca, Na, Mn, Hf, Nb, Sr, Ta, U and Zr contents when compared with the upper continental crust (UCC, Taylor and McLennan, 1985). The Eu/Eu* ratios of the sediments averaging 0.69 indicate that the recycling effect on the Eu anomaly may be minor and the contribution of mafic source rock was unimportant, which can also be depicted in the La‐Th‐Sc plot (Cullers, 1994) showing a “mixed origin” close to the felsic end member. The chemistry of the cored sediment can be explained by a mixing model designed by Ho and Chen (1996) with four end members i.e., greywacke, shale, quartzite and limestone, which are widely distributed in the Cenozoic formations of southern Taiwan with greywacke as the dominant contributor followed by shale.

Geochemistry and geochronology of Mesoproterozoic basement rocks from the Eastern Amery Ice Shelf and southwestern Prydz Bay, East Antarctica: Implications for a long-lived magmatic accretion in a continental arc

The high-grade metamorphic rocks from the eastern Amery Ice Shelf and southwestern Prydz Bay of East Antarctica represent reworked Rayner Complex during the late Neoproterozoic/Cambrian metamorphism. These Mesoproterozoic basement rocks can provide important information for the earlier tectonic evolution of the Rayner orogen. Chemical compositions of the mafic granulites from the Søstrene Island and Munro Kerr Mountains area suggest that their protoliths resemble Nb-enriched arc basalts, whereas those from the McKaskle-Mistichelli Hills, Reinbolt Hills and Manning Nunataks show typical characteristics of island arc basalts. Nd isotopic analyses yielded initial Nd isotope compositions, expressed as ε~Nd~(T) values, ranging from +4.1 to −0.4 for the former, and mostly from −3.2 to −4.7 for the latter. The felsic orthogneisses have the characteristics of volcanic arc granites, and one fifth of the samples belong to high Sr/Y types. The felsic orthogneisses yielded ε~Nd~(T) values of −2.4 to −7.6 and Nd depleted-mantle model ages of 2.2 to 1.9 Ga, implying an important episode of crustal formation in the Paleoproterozoic. The high Sr/Y orthogneisses have high K~2~O/Na~2~O ratios (\>1), positive Eu anomalies, significant HREE depletion and negative ε~Nd~(T) values, suggesting their derivation from partial melting of garnet-bearing K-rich mafic sources at the lower crust of the continental arc. Geochronological data give protolith ages ranging from 1380 to 1020 Ma for these arc-related rocks, indicating a long-lived magmatic accretion in the Rayner continental arc for ∼360 Ma. In addition, a younger island arc (\<1080 Ma) was probably formed in the south of the Fisher oceanic arc. Coupled with the available data for the adjacent areas, we propose that the tectonic process of the Rayner orogen involved the collision of several island arcs with East Antarctica (the Lambert Terrane or the Ruker craton), followed by the closure of ocean and final collision of the Indian craton with the newly accreted Antarctic margin.

Transport-Limited Denudation Regime Inferred from Sand Petrography and Chemical Composition: Cenozoic Sediments from the Guadiana Basin (SW Spain)

The Guadiana Basin is an E-W intraplate Cenozoic basin located in SW of the Iberian Peninsula, covering an area close to 6000km^2. NE-SW Alpine faults divide the basin into two subbasins: the Vegas Altas (VA) to the East, and the Vegas Bajas (VB) to the West. Sedimentary record consists of ＜200m thick of sediments, arranged in two main depositional sequences, DS1 and DS2. Petrographic analysis of sediment sandy-fraction combined with geochemical analysis of ＜2mm fraction permit to infer provenance and factors controlling sediment composition. Petrographic analysis of sandy-fraction reveals provenance from basin margins lithologies. In VA subbasin, quartzolithic petrofacies characterized eastern subbasin infill related to the erosion of Neoproterozoic and Palaeozoic metasediments. To the West quartzofeldspathic petrofacies predominates, associated with the erosion of coarse-grained plutonites and short distance transport of their products. Intermediate petrofacies characterizes clastic deposits from the VB. Chemical analysis of clastic deposits corroborates a main felsic origin of sediments with a composition that fits well with the upper crust compositional norm and with the Neoproterozoic Iberian Average Shale (NIBAS). In addition, minor contributions from mafic sources can be identified in DS1 from VB, manifested by anomalies in the content of Co, Ni, Cr, V and Sc. Different stages of weathering (from intermediate to intense) can be deduced by the analysis of both, sandy-fraction petrography and bulk sediment geochemistry. This fact is related to the great hiatus at the base of the Cenozoicbasin infill. Transport-limited denudation regimes can be inferred by the presence of high and intermediate weathered and texturalyinmature sediments. In addition, trace element ratios suggest local enrichment in dense minerals, attributable to recycling processes from metasediments. Finally, the sedimentary record of the Guadiana Basin is constituted by immature sediments formed by limited transport processes of very variable weathered products from recycled metasedimentary and first-order sediments from granitoids.

GEOCHEMISTRY OF PALEOGENE RED BEDS IN THE NORTHERN IRAQ FORELAND BASIN: EVIDENCE FOR PROVENANCE

The Paleogene Red Bed deposits in northern Iraq crop out as a narrow northwest-southeast trending belt within the thrust zone in an active foreland basin developed adjacent to the Zagraos orogenic belt. The Red Beds are divided vertically into four units. The lower part (unit 1) is mainly composed of red mudstone and siltstone. The middle part (unit 2) is mainly composed of sandstone with thin interbeds of red siltstone. The upper part is composed of conglomerate (unit 3) covered by 100-120 m of red mudstone, siltstone and sandstone (unit 4). The geochemical stratigraphy shows an increase in transition elements and REEs while LILE and HFSE elements decrease in the lower part of the sequence. These features indicate that a mafic source supplied detritus during the deposition of the lower Red Beds and decreased in importance during the deposition of the middle and upper parts as a result of erosion or tectonic activity.

Provenance versus weathering control on the composition of tropical river mud (southern Africa)

This study presents an integrated mineralogical–geochemical database on fine-grained sediments transported by all major rivers of southern Africa, including the Zambezi, Okavango, Limpopo, Olifants, Orange and Kunene. Clay mineralogy, bulk geochemistry, Sr and Nd isotopic signatures of river mud, considered a proxy of suspended load, are used to investigate the influence of source-rock lithology and weathering intensity on the composition of clay and silt produced in subequatorial to subtropical latitudes.Depletion in mobile alkali and alkaline-earth metals, minor in arid Namibia, is strong in the Okavango, Kwando and Upper Zambezi catchments, where recycling is also extensive. Element removal is most significant for Na, and to a lesser extent for Sr. Depletion in K, Ca and other elements, negligible in Namibia, is moderate elsewhere. The most widespread clay minerals are smectite, dominant in muds derived from Karoo or Etendeka flood basalts, or illite and chlorite, dominant in muds derived from metasedimentary rocks of the Damara Orogen or Zimbabwe Craton. Kaolinite represents 30–40% of clay minerals only in Okavango and Upper Zambezi sediments sourced in humid subequatorial Angola and Zambia. After subtracting the effects of recycling and of local accumulation of authigenic carbonates in soils, the regional distribution of clay minerals and chemical indices consistently reflect weathering intensity primarily controlled by climate.Bulk geochemistry identifies most clearly volcaniclastic sediments and mafic sources in general, but cannot discriminate the other sources of detritus in detail. Instead, Sr and Nd isotopic fingerprints are insensitive to weathering, and thus mirror faithfully the tectonic structure of the southern African continent. Isotopic tools thus represent a much firmer basis than bulk geochemistry or clay mineralogy in the provenance study of mudrocks.

UNDERSTANDING ARC-CONTINENT COLLISION AND CRUSTAL GROWTH: GEOCHEMISTRY OF PHILIPPINE SEDIMENTARY ROCK SEQUENCES

This work synthesizes the results of studies utilizing whole rock sedimentary geochemistry of samples collected from different tectonic settings. Major, trace and rare earth element diagrams effectively discriminate the nature of the source rocks and tectonic setting of the sedimentary rocks. The continental character of Palawan, Buruanga and Mindoro are affirmed by their geochemical components which point to felsic and quartzose recycled source rocks in a continental setting. The Antique samples, when plotted on the various diagrams, are shown to have contributions from both mafic and felsic sources. This indicates their deposition in an overlap basin which marks province linking resulting from the collision of the two blocks – Palawan Microcontinental Block and Philippine Mobile Belt. In stark contrast, the sedimentary rocks from Baguio (Klondyke, Amlang and Cataguintingan) have geochemical signatures consistent with their derivation from mafic igneous rocks in an oceanic island arc setting. The variety of diagrams used in this study illustrates their effectiveness in characterizing the provenance and tectonic setting of sedimentary rocks. These results underscore the value of using whole rock sedimentary geochemistry in deciphering the evolution of the Philippine island arc system.

The Itsaq Gneiss Complex of Greenland: Episodic 3900 to 3660 Ma juvenile crust formation and recycling in the 3660 to 3600 Ma Isukasian orogeny

From the 3000 km2 Eoarchean Itsaq Gneiss Complex (IGC) of Greenland, zircon U-Pb dating of numerous meta-granitoid and orthogneiss samples is integrated with geologic observations, whole rock geochemistry and a strategic subset of zircon Hf and whole rock Nd isotopic measurements. This shows that there are multiple episodes of TTG suite formation from ∼3890 to 3660 Ma, characterized by zircon initial εHf≈0 and whole rock initial εNd of > +2. These rocks mostly have geochemical signatures of partial melting of eclogitized mafic sources, with a subset of high magnesian, low silica rocks indicating fusion by fluid fluxing of upper mantle sources. The TTG suites are accompanied by slightly older gabbros, basalts and andesites, which have geochemical signatures pointing to magmas originating from fluid fluxing of upper mantle sources. The data show the formation of juvenile crust domains in several discrete events from ∼3900 to 3660 Ma, probably at convergent plate boundaries in an environment analogous, but not identical to, modern island arcs. In the Isua area, a northern ∼3700 Ma terrane formed distal from a predominantly ∼3800 Ma terrane. These terranes were juxtaposed between 3680 and 3660 Ma—respectively the age of the youngest rocks unique to the northern terrane and the lithologically distinctive ultramafic-granitic Inaluk dykes common to both terranes. This shows the assembly of different domains of juvenile rocks to form a more expansive domain of “continental” crust. A rare occurrence of high-pressure granulite is dated at ∼3660 Ma, demonstrating that assembly involved tectonic crustal thickening. This continental crust was then reworked in the 3660 to 3600 Ma Isukasian orogeny . In the northern part of the Isua area, 3660 to 3600 Ma granites were emplaced into ∼3700 Ma tonalites. The earliest granites are nebulous, and sigmoidal schlieric inclusions within them demonstrate ductile extension. Younger granite sheets were emplaced into extensional ductile-brittle fractures. These granite-tonalite relationships are overprinted by widespread development of late Eoarchean (pre-3500 Ma Ameralik dyke) brittle-ductile extensional cataclastic textures, together demonstrating that extension was polybaric. The southern part of the Isua area largely escaped 3660 to 3600 Ma high temperature processes and has sparse granite sheets commonly focused into coeval shear zones. In the rest of the complex, deeper crustal levels during the Isukasian orogeny are widely preserved. These experienced upper amphibolite to granulite facies moderate- to low-pressure syn-kinematic metamorphism, forming complex migmatites rich in granitic-trondhjemitic neosome. The migmatites were intruded by composite ferrogabbro and granite bodies, in which syn-magmatic extensional features are locally preserved. Thus 3660 to 3600 Ma crustal recycling involved elevated crustal thermal gradients in an extensional regime. Crustal melts formed in the Isukasian orogeny have zircon initial εHf<0 and whole rock initial εNd of ≤0, showing incorporation of slightly older Eoarchean juvenile crust. A Phanerozoic example of collisional orogeny followed by crustal thinning is explored as an analog for the Isukasian orogeny.

Assessing the influence of lithology on weathering indices of Changjiang river sediments

We present major and trace element data in suspended particulate matter (SPM) collected in the Changjiang main channel and its tributaries. Suspended sediments are derived from a mafic source related to the Emeishan Large Igneous Province (ELIP) in the Upper Reach and from a more felsic source in the Middle/Lower Reaches. The difference in chemical composition between the two sources has a strong influence on the apparent weathering intensity. Although the apparent loss of soluble elements follows a climatic gradient from the Upper Reach to the Lower Reach, the co-variation of weathering indices with different proxies for igneous differentiation suggests that a lithologic control cannot be ruled out. By taking into account the variability in chemical composition of the parent rocks, we show that river suspended sediments from the upper reach may have not experienced less intense chemical weathering than those transported by rivers from the Lower/Middle Reaches characterized by higher runoff and surface temperature. We argue that the relationships observed for different indices of weathering with climate are probably an artifact and are mainly driven by the change in chemical composition of the sediment's parent-rocks.

Intracontinental Eocene-Oligocene Porphyry Cu Mineral Systems of Yunnan, Western Yangtze Craton, China: Compositional Characteristics, Sources, and Implications for Continental Collision Metallogeny

The Yao’an porphyry Au system, Machangqing porphyry Cu-Mo system, and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oligocene epochs at 37 to 33 Ma in a postcollisional intracontinental setting in western Yunnan, western Yangtze craton, China. The Yao’an monzonite and quartz monzonite porphyry intrusions are alkaline and potassic with high K 2 O/Na 2 O ratios (1.1–1.5). They have Sr-Nd-Pb isotopes similar to coeval lamprophyres and are characterized by uniform zircon ɛ Hf (−6.4 to −8.7) and δ 18 O values (6.6–7.0‰). They are interpreted as products of fractional crystallization of lamprophyre-like potassic mafic magma derived from ancient metasomatized lithospheric mantle, a scenario similar to the mid-Cretaceous postcollisonal Scheelite Dome gold system in Yukon, Canada. The Machangqing granitic intrusions are high K calc-alkaline and show high Sr, Sr/Y, and La/Yb, but low Y and Yb geochemical signatures. They have Sr-Nd-Pb isotope compositions similar to amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The zircon ɛ Hf values of the Machangqing granitic intrusions are positive (0.3–4.7), and the zircon-depleted Hf mantle model ages are 1.1 to 0.8 Ga. They also have mantle-like zircon δ 18 O values (5.5–6.4‰). The Machangqing granites were most likely derived from partial melting of Neoproterozoic lower crust. The Beiya granitic intrusions are alkaline, with high K 2 O/Na 2 O (1.9–2.7), Sr/Y and La/Yb ratios, high Sr contents, and low Y and Yb contents. They contain abundant zircon inheritance and have variable magmatic zircon ɛ Hf (−4 to +4) and the highest magmatic zircon δ 18 O values (6.6–7.8‰). The Beiya felsic intrusions are interpreted to be derived from partial melting of a K-rich mafic source mixed with a metasedimentary component. The Eocene-Oligocene intracontinental potassic intrusions and associated mineralization in western Yunnan are located proximal to the Mesozoic Jinsha suture, suggesting that this Mesozoic lithospheric boundary may have provided a first-order control on localization of Cenozoic mineral systems. These potassic felsic intrusions are coeval with regional potassic mafic magmatism in western Yunnan and were emplaced between 37 to 33 Ma, after the collision between India and Asia at ca. 60 to 55 Ma. It is therefore postulated that continental collision may have preferentially thickened the continental lithospheric mantle (CLM) adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed during 37 to 33 Ma, inducing melting of residual metasomatized lithospheric mantle as well as lower crust. The gold-rich Yao’an and Beiya intrusions are alkaline and potassic, characterized by high zircon δ 18 O values (>6.5‰), which is consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high K calc-alkaline with mantle-like zircon δ 18 O values ( ɛ Hf signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan. In western Yunnan, gold tends to be associated with alkaline and potassic melts with a supracrustal contribution, whereas Cu-Mo mineralization seems to be more related with juvenile crustal sources with little supracrustal influence.