Negative Sr Anomalies Research Articles

Geochronology and Geochemistry of Paleoproterozoic Mafic Rocks in Northern Liaoning and Their Geological Significance

Petrological, geochronological, and geochemical analyses of mafic rocks in northern Liaoning were conducted to constrain the formation age of the Proterozoic strata, and to further study the source characteristics, genesis, and tectonic setting. The mafic rocks in northern Liaoning primarily consist of basalt, diabase, gabbro, and amphibolite. Results of zircon U-Pb chronology reveal four stages of mafic magma activities in northern Liaoning: the first stage of basalt (2209 ± 12 Ma), the second stage of diabase (2154 ± 15 Ma), the third stage of gabbro (2063 ± 7 Ma), and the fourth stage of magmatic protolith of amphibolite (2018 ± 13 Ma). Combined with the unconformity overlying Neoproterozoic granite, the formation age of the Proterozoic strata in northern Liaoning was found to be Paleoproterozoic rather than Middle Neoproterozoic by the geochronology of these mafic rocks. A chronological framework of mafic magmatic activities in the eastern segment of the North China Craton (NCC) is proposed. The mafic rocks in northern Liaoning exhibit compositional ranges of 46.39–50.33 wt% for SiO2, 2.95–5.08 wt% for total alkalis (K2O + Na2O), 6.17–7.50 wt% for MgO, and 43.32–52.02 for the Mg number. TiO2 contents lie between 1.61 and 2.39 wt%, and those of MnO between 0.17 and 0.21 wt%. The first basalt and the fourth amphibolite show low total rare earth element contents. Normalized against primitive mantle, they are enriched in large ion lithophile elements (Rb, Ba, K), depleted in high field strength elements (Th, U, Nb, Ta, Zr, Ti), and exhibit negative anomalies in Sr and P, as well as slight positive anomalies in Zr and Hf. The second diabase and the third gabbro have similar average total rare earth element contents. The diabase shows slight negative Eu anomalies (Eu/Eu* = 0.72–0.88), enrichment in large ion lithophile elements (Ba), depletion in Rb, and slight positive anomalies in high field strength elements (Th, U, Nb, Ta, Zr, Hf, Ti), with negative anomalies in K, Sr, and P. The gabbro is enriched in large ion lithophile elements (Rb, Ba, K), depleted in high field strength elements (Th, U, Nb, Ta, Zr, Hf), and exhibits positive anomalies in Eu (Eu/Eu* = 1.31–1.37). The contents of Cr, Co, and Ni of these four stages of mafic rocks are higher than those of N-MORB. The characteristics of trace element ratios indicate that the mafic rocks belong to the calc-alkaline series and originate from the transitional mantle. During the process of magma ascent and emplacement, it is contaminated by continental crustal materials. There are residual hornblende and spinel in the magma source of the first basalt. The other three magma sources contain residual garnet and spinel. The third gabbro was formed in an island arc environment, and the other three stages of mafic rocks originated from the Dupal OIB and were formed in an oceanic island environment. The discovery of mafic rocks in northern Liaoning suggests that the Longgang Block underwent oceanic subduction and extinction in both the north and south in the Paleoproterozoic, indicating the possibility of being in two different tectonic domains.

Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high εNd(t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable εNd(t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded UPb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young UPb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma.

APATITE GEOCHEMISTRY IN ROCKS OF KORSUN-NOVOMYRHOROD ANORTOSITE-RAPAKIVI GRANITE PLUTON

Trace element concentrations (REEs, Y, Sr, Th, U) in apatites from the most typical rocks of the Korsun-Novomyrhorod anorthosite-rapakivi pluton (KNP) were measured. Apatite was the last liquidous phase to crystallize in all the studied rocks because of the low P2O5 content in the primary melts. In the basic rocks, apatite is the main source of REEs, especially LREEs, except for Eu and Y. In intermediate and acidic rocks, apatites have minor influence on REEs concentrations, but they are more enriched in Th and U. All chondrite-normalized REE patterns of apatite show large negative Eu-anomalies (Eu/Eu* 0.03-0.21). This anomaly as well as increased Sr contents is less prominent (0.39) only in apatite from the norite of the external part of the pluton. With increased magmatic differentiation, apatite becomes more enriched in REEs, but Sr is depleted in the basic and intermediate rocks. In addition, all apatites from these rocks are LREE enriched and depleted in HREE and Sc which is consistent with earlier clino- or orthopyroxene fractionation, considering the minor amounts of in REE in plagioclase. REE patterns normalized to the primitive mantle indicate LREE enrichment ((La/Lu)N = 10.8-36) of apatites from all types of rocks with pronounced negative anomalies of Sr and Eu. This is the result of their low compatibility with apatite compared to other REEs. Inasmuch as the distribution of Eu is related to oxygen fugacity, we think that such unusually low Eu-anomalies reflect the initial melt differentiation under abyssal conditions. Thus, ubiquitous strong negative Eu-anomalies in apatites of all types of rocks are a result of two factors occurring during parent melt fractionation, namely early intensive plagioclase fractionation and highly reduced conditions during melt differentiation. By similar trace elements concentration and similarity of REE distribution in apatites it is probably that basic rocks and monzonitesare are derived from the same primary melt. At the same time, apatites from granites and syenites show maximum enrichment of REEs, especially HREE and Y. This indicates crystallization from residual differentiated melts (for syenites) or partial interaction of melts of various composition (basic and acidic) with migration of enriched LILE and REE solutions into acidic melts. The low content of P2O5 in ore-bearing rocks and ores of KNP, as well as the absence of Ti-magnetite and high iron enrichment of mafgic minerals and the unusually strong and negative Eu-anomalies in apatites, clearly distinguishes them from the Fe-Ti-P ores of the Korosten pluton. This suggests differences in the chemical composition of primary melts as well as the physical conditions of their differentiation.

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Geochemistry of volcanic flows of Nakora area of Malani igneous suite, Northwestern India: Constraints on magmatic evolution and petrogenesis

The geochemical characteristics of volcanic flows of Nakora area of Malani Igneous Suite have been determined to understand their magmatic evolution and petro-genetic aspects. Geochemically, they are high in silica, total alkalis, high field strength elements (HFSE), low ion lithophile elements (LILE), rare metals and rare earth elements; represent A-type affinity with potential mineralization associations. Here, we carried out average geochemical data bank of representative samples of 44 individual lava flows of isolated hill-locks. The relative enrichment of trace elements and negative anomalies of Sr, Eu, P and Ti in the multi-element spider diagrams suggests that the emplacement of the lava flows was controlled by complex magmatic processes i.e. fractional crystallization, partial melting, magma mixing, crustal contamination and assimilation. Moreover, NRCmagma provides new geochemical approaches to understand geodynamic evolution of MIS and emplaced in plume related extensional geodynamic settings in NW Indian shield.
 Keywords: Geochemistry; Volcanic flows; Nakora; Malani Igneous Suite; Rajasthan; Rodina

Paleoproterozoic A-type granite from the southwestern margin of the North China block: high temperature melting of tonalitic crust in extensional setting

ABSTRACTProterozoic igneous rocks in the southern margin of the North China Craton can place constraints on its crustal evolution. This paper reports geochemistry, zircon U-Pb ages, Sr-Nd isotopes and Lu-Hf isotopes for the Proterozoic Angou granite from the southwestern margin of the North China Craton (NCC). The Angou granite has zircon LA-ICP-MS U-Pb age of 1819 ± 14Ma (MSWD = 1.3, n = 37), which is synchronous with the Proterozoic Xiong’er Group in the southeastern margin of the NCC. The granite has A-type granite affinity: (1) high SiO2 (67.12% to 73.16%) and alkali (Na2O+K2O = 7.18% to 8.75%) contents, with K2O/Na2O ratios of 1.04 to 4.78; (2) high FeOT/MgO values (4 to 36); (3) significantly negative Eu anomalies (Eu*/Eu = 0.33 to 0.53) and negative Sr anomalies (38.1 to 102ppm); (4) high 104× Ga/Al (2.55 to 4.75, with an average of 3.43) values and Zr+Ce+Nb+Y (634 to 1022ppm) contents; (5) high zircon saturation temperatures (871 to 957°C). In combination with its slightly peraluminous (A/CNK = 1.02 to 1.26) nature, the Angou granite can be classified as slightly peraluminous A-type granite. It displays negative ɛNd(t) values of −2.0 to −3.9, variable εHf(t) values (−8.81 to +1.98) with Hf model ages of 2194 to 2721Ma, combined with its high Rb/Sr (1.05 to 3.26) and low Rb/Ba (0.08 to 0.18) ratios, suggesting that the Angou granite was derived from high temperature melting of tonalite in middle-lower crust. The formation of Angou A-type granite is consistent with the mafic dyke swarms and Xiong’er bimodal volcanic rocks in NCC, indicating a Paleoprotozoic extensional setting in the southwestern margin of the NCC.

Petrogenesis and Tectonic Implications of the Paiku Leucogranites, Northern Himalaya

The Himalayan leucogranites provide insights into the partial melting behavior of relatively deeper crustal rocks and tectono-magmatic history of the Himalayan Orogen. The Paiku leucogranites of northern Himalaya can be subdivided into two-mica leucogranite (TML), garnet-bearing leucogranite (GL), cordierite-bearing leucogranite (CL), and tourmaline-bearing leucogranite (TL). All of them are high-K, peraluminous, calc-alkalic to alkali-calcic rocks. They are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and show pronounced negative anomalies of Sr, Ba, K and Ti, but positive anomalies of Nb and Rb. LA-ICP-MS U-Pb zircon dating of one TML, one GL, and two CL samples yielded variable 206Pb/238U ages ranging from 23.6 to 16.1 Ma, indicating the Paiku leucogranites underwent a low degree of partial melting process. Combining with previous studies, we suggest the Paiku leucogranites were derived from partial melting of metasedimentary rocks of the Higher Himalayan Sequence (HHS). The GL and TL mainly resulted from the muscovite-dehydration melting, whereas the TML and CL were mainly derived from the biotite-dehydration melting. Finally, it is concluded that the Paiku leucogranites were probably formed during the subduction of the Indian crust.

Origin of Late Permian amphibole syenite from the Panxi area, SW China: high degree fractional crystallization of basaltic magma in the inner zone of the Emeishan mantle plume

ABSTRACTThe c.a. 259–260 Ma Emeishan mantle plume caused extensive flood basalts and associated peralkaline rocks; petrogenesis of these peralkaline rocks can provide detailed information about the melting process in the mantle. In this paper, we present detailed petrology, chronology, major and trace element geochemistry, Sr-Nd isotope and in situ zircon Hf isotope for an amphibole syenite from Panxi area, inner zone of the Emeishan large Igneous province (ELIP). The amphibole syenite has a zircon U-Pb ages of 260 ± 2 Ma (MSWD = 1.2, n = 22), which is synchronous with Emeishan mantle plume. It displays moderate SiO2 (64.89–65.67 wt.%), high Na2O (7.35–7.71 wt.%) and K2O (4.78–4.96 wt.%) contents, with A/CNK values of 0.86 to 0.90, with high Rittman index of 6.6 to 7.1, showing peralkaline affinity. It displays significantly negative Sr anomalies (133 to 200 ppm) and slightly negative Eu anomalies (Eu/Eu* = 0.78–1.04). The rock displays high εNd(t) values of +2.4 to +2.5. Zircons from the amphibole syenite display positive εHf(t) values from +8.9 to +11.0, with single-stage Hf model ages ranging from 0.47 to 0.55 Ga, combined with the well-developed correlation of TFe2O3, CaO/Al2O3, TiO2, Ni and Cr with MgO, suggesting the amphibole syenite was derived from the mantle source, and the fractional crystallization played a significant role in the generation of the magma. The MELTS model results show that Houzishan amphibole syenite can be formed by the high degree (~80%) fractional crystallization of ~24% clinopyroxene, ~4% orthopyroxene, ~35% plagioclase and ~5% oxide from parental magma resembling high-Ti basalts. These results indicate that Houzishan amphibole syenite represents the juvenile mantle-derived peralkaline melts addition to the continental lower crust in the inner zone of the ELIP and assimilating with minor crust material.

Tectonic setting and mineralization potential of the Zefreh porphyry Cu-Mo deposit, central Iran: Constraints from petrographic and geochemical data

The Zefreh porphyry Cu-Mo deposit is located in the central part of the Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA), 75 km NE of Isfahan city, central Iran. The Zefreh rocks, ranging from diorite to granodiorite in composition, formed in a subduction-related arc setting, and likely in a pre-plate collisional environment. They are enriched in Th, U, Rb, Pb, light rare earth elements (LREE), but depleted in Ti, Nb, Ta, Ba, Sr, and P. Negative to slightly positive Eu anomalies (0.5–1.1), low to moderate Sr contents, (189–567 ppm), low to moderate Sr/Y ratios (5–23), and low La/Yb ratios (3–8) in the Zefreh magmatic suite rule out considerable amounts of fractionated amphibole from a hydrous magma in the lower crust. Arc crustal thickness in the Zefreh area was probably <40 km at the time of emplacement of the granitoids, under which amphibole did not fractionate considerably from the primitive magma. Melting of garnet-free lower-crustal amphibolites, together with fractional crystallization and crustal assimilation processes may be the main source of the Zefreh granitoids. On the basis of geochemical characteristics of the granitoids (e.g., low to moderate Sr contents, moderate to high contents of Y and Yb, low to moderate ratios of Sr/Y and Dy/Yb, low La/Yb ratios, negative to slightly positive Eu anomalies, and negative anomalies of Sr) and immature arc with thin crust, the Zefreh deposit is likely to be a small, sub-economic porphyry Cu-Mo deposit.

Geochemical constraints on the link between volcanism and plutonism at the Yunshan caldera complex, SE China

The Yunshan caldera complex is part of a larger scale, ca. 2000-km-long volcanic–plutonic complex belt in the coastal region of SE China. The volcanic rocks in the caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton. In this study, we present zircon U–Pb, Hf and stable O isotopes along with geochemical data of both volcanic and plutonic rocks to evaluate the potential petrogenetic link between volcanism and plutonism in the Yunshan caldera complex. SHRIMP zircon U–Pb geochronology of both volcanic and plutonic rocks yields almost identical ages ranging from 95.6 to 93.1 Ma. The peraluminous and peralkaline rhyolites show negative anomalies of Sr, P, Ti and Ba and to a lesser extent negative Nb and Ta anomalies, along with positive Rb anomalies and ‘seagull-like’ rare earth element (REE) patterns with negative Eu anomalies and low (La/Yb)N ratios. The intracaldera porphyritic quartz monzonite displays minor negative Rb, Nb, Ta, Sr, P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high (La/Yb)N ratios and lack significant Eu anomalies. The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent e Nd(t) of − 3.7 to − 2.2 and display zircon e Hf(t) values of − 2.1 to 3.7. They further have similar, mantle-like, zircon oxygen isotopic compositions (δ18OVSMOW mainly = 4.63 to 5.76‰). We interpret these observations to be in agreement with a crystal mush model in which the parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge. The peralkaline rhyolites are interpreted to represent the most differentiated magma that has subsequently experienced significant fluid–melt interactions, whereas the porphyritic quartz monzonite may be representative of the residual crystal mush. The Yunshan rhyolites typically match the geochemical characteristics of ‘hot-dry-reduced’ rhyolites indicating that, during the late Cretaceous, the tectonic setting of SE China changed from a compressional environment to an extensional environment, i.e., from an arc into a back-arc setting. Our results imply that volcanic and plutonic rocks in caldera systems may provide unique constraints on the evolution of the magmatic system in which both the erupting melt and the residual crystalline material are being preserved.

Late Triassic post-collisional slab break-off along the Ailaoshan suture: insights from OIB-like amphibolites and associated felsic rocks

What dynamic processes the South China-Indochina collision had gone through is still a pending problem. Our recent investigations identified a Late Triassic gabbroic intrusion in Mengdong village and several granitic plutons near Wana village, Yunnan province (SW China). Both have underwent strong metamorphism and been altered to amphibolite and granitic gneiss, respectively. We carried out SIMS and LA-ICPMS zircon U–Pb dating for the amphibolites and granitic gneisses, respectively. Dating results yield weighted mean 206Pb/238U ages of 221.5 ± 5.3 Ma for the amphibolites and 224.0 ± 1.8 and 235.4 ± 0.6 Ma for the granitic gneisses. The amphibolite samples have low Mg# (41.7–42.8), high TiO2 (~3.7 wt%) and Na2O (Na2O/K2O = 1.89–2.68) contents and depleted Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707647 and e Nd(t) = +1.17]. They exhibit OIB-like REE patterns and spidergrams, with strongly enriched LREE contents, insignificant Eu anomalies (Eu* = 0.99–1.03) and moderate negative Sr anomalies (Sr* = 0.46–0.56). The protolith gabbro of the Mengdong amphibolites is derived from low-degree partial melting of a homogeneous OIB-type mantle source in the garnet stability field and experienced significant fractionation of olivine, clinopyroxene and plagioclase during magma evolution. The Wana samples are strongly peraluminous (A/CNK > 1.1) and K-enriched (K2O/Na2O = 2.93–3.63). They show enriched Sr–Nd isotopic compositions with (87Sr/86Sr)i = 0.718589–0.719754 and e Nd(t) = −11.34 to −10.92. The Wana plutons are typical S-type granite and product of the dehydration melting of meta-sedimentary rocks. We summarized Late Triassic age data in the literature along the South China-Indochina suture zone and recognized a significant Late Triassic magmatic flare-up. In combination with previous studies on the Late Triassic HP–UHP metamorphic rocks, we proposed that during the Late Triassic the South China-Indochina suture zone had transformed into post-collisional setting, and accompanied slab break-off triggered the OIB-type asthenospheric mantle upwelling and provided the heat source for the Late Triassic magmatic flare-up.

Apatite geochemistry of the Taihe layered intrusion, SW China: Implications for the magmatic differentiation and the origin of apatite-rich Fe-Ti oxide ores

The Taihe intrusion is one of the layered intrusions situated in the central zone of the Emeishan Large Igneous Province (ELIP), SW China. The cyclic units in the Middle Zone of the intrusion are composed of apatite-magnetite clinopyroxenite at the base and gabbro at the top. The apatite-rich oxide ores contain 6–12modal% apatite and 20–50modal% Fe-Ti oxides evidently distinguished from the coeval intrusions in which apatite-rich rocks are poor in Fe-Ti oxides. Most of apatites of the Taihe Middle and Upper Zones are fluorapatite, although four samples show slightly high Cl content in apatite suggesting that they crystallize from a hydrous parental magma. Compared to the apatite from the gabbro of the Panzhihua intrusion, situated 100km to the south of the Taihe intrusion, the apatite of the Taihe rocks is richer in Sr and depleted in HREE relative to LREE. The calculated magma in equilibrium with apatite of the Taihe Middle and Upper Zones also shows weakly negative Sr anomalies in primitive mantle normalized trace element diagrams. These features indicate that the apatite of the Taihe Middle and Upper Zones crystallizes after clinopyroxene and before plagioclase. The apatite of the Taihe Middle and Upper Zones shows weakly negative Eu anomalies suggesting a high oxygen fugacity condition. The high iron and titanium contents in the oxidizing magma result in crystallization of Fe-Ti oxides. Crystallization of abundant Fe-Ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates SiO2 concentration in the magma triggering the saturation of apatite. The positive correlations of Sr, V, total REE contents and Ce/Yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma. Early crystallization and accumulation of Fe-Ti oxide together with apatite produced the apatite-rich oxide ores at the base of the cyclic units of the Taihe Middle Zone.

Geochemical and Geochronological Constraints on the Origin of the Meta‐basic Volcanic Rocks in the Tengtiaohe Zone, Southeast Yunnan

AbstractThe meta‐basic volcanic rocks in the Tengtiaohe Zone yield zircon U‐Pb ages of 258.8±2.5 Ma and 259.2±1.8 Ma, respectively which agree with the ages of flood basalts of ELIP and are similar to the basaltic rocks and komatiites from the Song Da Zone in northern Vietnam. The results suggest that the age of meta‐basic volcanic rocks is Late Permian, rather than the Early Permian or Early Carboniferous ages as previously inferred. Most meta‐basic volcanic rocks are strongly enriched in LREEs relative to HREEs and display trace element patterns similar to the ELIP high‐Ti basalts, and are enriched in LILEs with negative Sr anomalies. Their initial 87Sr/86Sr ratios range from 0.705974 to 0.706188 and εNd(t) from –0.82 to –2.11. Their magmas were derived from an enriched and deep mantle source without significant crustal contamination. These meta‐basic volcanic rocks formed in ELIP. Therefore, the Tengtiaohe Zone is not an ophiolite zone and can link to the Song Da Zone in northern Vietnam.

Provenance and Tectonic Setting of Late Lower Cretaceous (Albian) Kazhdumi Formation Sandstones (SW Iran)

Provenance and tectonic setting of the Late Lower Cretaceous (Albian) sandstones in SW Iran have been interpreted on the basis of geochemistry (major and trace elements) of 35 samples from Iranian offshore oil fields (Soroosh, Nowrooz, Foroozan and Hendijan). Geochemically, trace-element concentrations, such as La, Th, Sc and Zr, and ratios, such as La/Sc, Th/Sc, La/Co, and Th/Co, of sandstones from Kazhdumi Formation at four localities indicate that they were derived from felsic source rocks and deposited in a passive continental margin. Most major and trace element contents of Kazhdumi sandstones are generally similar to upper continental crust (UCC) values. However, some samples are depleted in major and trace elements (such as K2O, Na2O, Sr and Ba) relative to UCC, which is mainly due to absence or smaller amount of Na-rich plagioclase and K-feldspar and also paucity of clay minerals, which is possibly due to intense weathering and recycling. Enrichments in Zr, negative Sr anomalies and Th/U ratios higher than 4 for Kazhdumi sandstones are further evidence for recycled sources. The study of paleoweathering conditions based on chemical index of weathering (CIW) indicates a moderate to intense weathering of first cycle sediments, or it may alternatively reflect recycling under semi-arid to semi-humid climate conditions in Kazhdumi sandstones. The results of this study suggest that the main source for the Kazhdumi sandstones is likely located in uplifted shoulders of a rifted basin (Neo-Tethys) in its post-rift stage (Arabian basement).

Geochemistry and petrogenesis of the K-rich ‘Bongo-type’ granitoids in the Paleoproterozoic Bole–Nangodi greenstone belt of Ghana

The ‘Bongo-type’ granitoids intrude the northern portion (where it cross cuts the Tarkwaian), central and south-central portions of the Bole–Nangodi greenstone belt in Ghana. The non-foliated, porphyritic granitoids are characterised by microcline (5–18 mm), often set in a medium to fine grained matrix of quartz and plagioclase, contains granular quartz and show incipient development of mymerkites under thin section. The major mafic phases are a brownish-green variety of hornblende ± biotite, with sphene, epidote and apatite occurring as accessory minerals. The pink looking ‘Bongo-type’ granitoids are classified as granites with subordinate granodiorites and quartz monzonites. Whole-rock geochemical analyses show that the granitoids are magnesian and metaluminous to weakly peraluminous. On the basis of their Th–Co–K2O and SiO2 contents, the granitoids classified as high-K calc-alkaline and shoshonite series rocks. They show enriched LILE trends, decoupled LILE–HFSE patterns, negative Ta–Nb anomaly, pronounced negative Ti anomaly and variable positive and negative Sr anomalies. The K-rich granitoids from the middle and south-central portions of the Bole–Nangodi belt show the lowest concentrations of LILE and some HFSE with the most prominent negative Ti and Si anomalies. The ‘Bongo-type’ granitoids show I-type characteristics. Based on Ba–U–Th concentrations, they are interpreted to have had some interaction with crustal materials or some influence of subducted sediments. The granitoids unequivocally show subduction-related signatures and mimic high silica adakites (HSA). They show evidence of being emplaced in a volcanic arc geotectonic environment. Their geochemical similarities to A-type granitoids, however, suggest an arc–back-arc tectonic setting.

Petrography and Geochemistry of the Mbip Granitic Massif, SW Tchollir&amp;#233; (Central North Cameroon): Petrogenetic and Geodynamic Implication

In this article, we discussed about the petrography and geochemistry of magmatic rocks of the Mbip massif located SW of Tcholliré, in Central North Cameroon (Central African Fold Belt). Petrographic study shows that this massif is made of granodiorite, amphibole-biotite granite, and biotite granite which often contain enclave of mafic rocks (gabbro). Granodiorites and granites show porphyritic texture and consist dominantly of plagioclase and alkaline feldspar phenocrysts, quartz, biotite and often few amphibole. The gabbro enclaves are characterized by a granular porphyroid texture constituted of amphibole, plagioclase and olivine phenocrysts, all in a matrix of small crystals of plagioclase, olivine, amphibole, pyroxene and opaque minerals. All the analyzed rocks provide geochemical features of sublkaline serie and have nothing to do with the alkaline nature previously signaled in the former works. Granodiorites and granites are shoshonitic to calc-alkaline strongly potassic affinity consistent with emplacement in the continental collision setting. They present positive anomalies in Rb, Th, U; negative anomalies in Sr, P, Ti, Nb, Ta and some facies contain hydrated minerals such as amphibole. These characters are consistent with crustal and mantle contribution in their genesis. The mafic rocks (gabbro) have geochemical characteristics conferring a mantle origin, as confirmed by the high Mg# value (49.9). On the whole, the fractional crystallization of the magma formed by melting of crustal material and the magmatic mixing are the main petrogenetic process in the Mbip massif. Petrographic studies and geochemical data shows that the Mbip massif was emplaced in an active continental margin, into transitional regime from the end of maximum compression to the beginning of relaxation.

Geochronology, geochemistry, and mineralization of the granodiorite porphyry hosting the Matou Cu–Mo (±W) deposit, Lower Yangtze River metallogenic belt, eastern China

Porphyry and skarn Cu–Fe–Au–Mo deposits are widespread in the Middle and Lower Yangtze River metallogenic belt (MLYMB), eastern China. The Matou deposit has long been regarded as a typical Cu–Mo porphyry deposit within Lower Yangtze part of the belt. Recently, we identified scheelite and wolframite in quartz veins in the Matou deposit, which is uncommon in other porphyry and skarn deposits in the MLYMB. We carried out detailed zircon U–Pb dating and geochemical and Sr–Nd–Hf isotopic studies of the granodiorite porphyry at Matou to define any differences from other ore-related granitoids. The porphyry shows a SiO2 content ranging from 61.85wt.% to 65.74wt.%, K2O from 1.99wt.% to 3.74wt.%, and MgO from 1.74wt.% to 2.19wt.% (Mg# value ranging from 45 to 55). It is enriched in light rare earth elements and large ion lithophile elements, but relatively depleted in Nb, Ta, Y, Yb and compatible trace elements (such as Cr, Ni, and V), with slight negative Eu anomalies (Eu/Eu*=0.88–0.98) and almost no negative Sr anomalies. Results of electron microprobe analysis of rock-forming silicate minerals indicate that the Matou porphyry has been altered by an oxidized fluid that is rich in Mg, Cl, and K. The samples show relatively low εNd(t) values from −7.4 to −7.1, slightly high initial 87Sr/86Sr values from 0.708223 to 0.709088, and low εHf(t) values of zircon from −9.0 to −6.5, when compared with the other Cu–Mo porphyry deposits in the MLYMB. Zircon U–Pb dating suggests the Matou granodiorite porphyry was emplaced at 139.5±1.5Ma (MSWD=1.8, n=15), which is within the age range of the other porphyries in the MLYMB. Although geochemical characteristics of the Matou and other porphyries in the MLYMB are similar and all adakitic, the detrital zircons in the samples from Matou suggest that Archean lower crust (2543±29Ma, MSWD=0.25, n=5) was involved with the generation of Matou magma, which is different from the other porphyries in the belt. Our study suggests that the Matou granodiorite porphyry originated from partial melting of thickened lower crust that was delaminated into the mantle, similar to the other porphyries in the MLYMB, but it has a higher proportion of lower crustal material, including Archean rocks, which contributed to the formation of the porphyry and related W-rich magmatic-hydrothermal system.

Petrogenesis of adamellites from eastern Shandong Province: geochronological, geochemical, and Sr–Nd–Pb isotopic evidence

Geochronology, geochemistry, and whole-rock Sr–Nd–Pb isotopes were studied on a suite of Mesozoic adamellites from eastern China to characterize their ages and petrogenesis. Sensitive high-resolution ion microprobe U–Pb zircon analyses were done, yielding consistent ages of 123.2 ± 1.8 to 122.1 ± 2.1 Ma for the samples. These rocks belong to the alkaline magma series in terms of K2O + Na2O contents (8.45–9.58 wt.%) and to the shoshonitic series based on their high K2O contents (5.23–5.79 wt.%). The adamellites are further characterized by high light rare earth element contents [(La/Yb)N = 14.96–45.99]; negative Eu anomalies (δEu = 0.46–0.75); positive anomalies in Rb, Th, Pb, and U; and negative anomalies in Sr, Ba, and high field-strength elements (i.e. Nb, Ta, P, and Ti). In addition, all of the adamellites in this study display relatively low radiogenic Sr [(87Sr/86Sr)i = 0.7081–0.7089] and negative ϵNd(t) values from –16.70 to –17.80. These results suggest that the adamellites were derived from low-degree partial melting of an enriched lithospheric mantle below the North China Craton (NCC). The parent magmas likely experienced fractional crystallization of potassium feldspar, plagioclase and Fe–Ti oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during the ascent of alkaline rocks without crustal contamination.

MAGNETIC SUSCEPTIBILITY AND GEOCHEMISTRY OF GRANITIC ROCKS IN CAMBODIA

Granitic rocks in Cambodia are divided into two groups, i.e. ilmenite-series in southern Cambodia and magnetite-series in northern Cambodia. Both groups belong to high-K calc-alkaline series, metaluminous to slightly peraluminous and display typical features of I-type granitic rocks. The ilmenite-series granitic rocks are characterized by high SiO2 contents (70-75 wt. %) with abundance of quartz and K-feldspar, enrichment of LILEs, and strong negative anomalies of Ba, Sr, Eu (Eu/Eu*=0.1-0.5), Nb, and Ti. Chondrite-normalized REE patterns exhibit enrichment of LREE ([La/Yb]N=1.4-17.6) with flat HREE patterns and flat to concave-upward MREE patterns. These granitic rocks have high CaO+FeO+MgO+TiO2 and intermediate Al2O3/(FeO+ MgO+TiO2) with intermediate magnesium number (Mg# = 22-38) (Mg# = 100 x MgO/[MgO+Total FeO]). Geochemical features of the granitic rocks suggest partial melting of crustal igneous rocks of intermediate composition where plagioclase was a major fractionating and/or residual phase. On the other hand, the magnetite-series granitic rocks show wide range of SiO2 contents (59-70 wt. %), higher TiO2, Al2O3, CaO, and MgO contents than ilmenite-series granitic rocks, and smallnegligible negative anomalies of Sr and Eu. The high CaO+FeO+MgO+TiO2 and low Al2O3/(FeO+ MgO+TiO2) coupled with high Mg# (32-48) suggest partial melting of amphibolite-type source with presence of plagioclase. The granites of Cambodia were formed in subduction-related tectonic setting. The ilmenite-series granites range from volcanic-arc granites to syn- and postcollision granites while the magnetite-series granitic rocks belong to volcanic arc granites.

Trace-element patterns of fibrous and monocrystalline diamonds: Insights into mantle fluids

During their growth diamonds may trap micron-scale inclusions of the fluids from which they grew, and these “time capsules” provide insights into the metasomatic processes that have modified the subcontinental lithospheric mantle. LAM-ICPMS analysis of trace elements in > 500 fibrous and monocrystalline diamonds worldwide has been used to understand the nature of these fluids. Analyses of fibrous diamonds define two general types of pattern, a “fibrous-high” (FH) one with high contents of LREE, Ba and K, and a “fibrous-low” (FL) pattern characterized by depletion in LREE/MREE, Ba and K, negative anomalies in Sr and Y, and subchondritic Zr/Hf and Nb/Ta. Both types may be found in fibrous diamonds from single deposits, and in three Yakutian pipes some diamonds show abrupt transitions from inclusion-rich cores with FH patterns to clearer rims with FL patterns. Most monocrystalline diamonds show FL-type patterns, but some have patterns that resemble those of FH fibrous diamonds. Peridotitic and eclogitic monocrystalline diamonds may show either patterns with relatively flat REE, or patterns with more strongly depleted LREE. Kimberlites that contain peridotitic diamonds with “high” patterns also contain eclogitic diamonds with “high” patterns. Strong similarities in the patterns of these two groups of diamonds may suggest high fluid/rock ratios. Many diamonds of the “superdeep” paragenesis have trace-element patterns similar to those of other monocrystalline diamonds. This may be evidence that the trace-element compositions of deep-seated fluids are generally similar to those that form diamonds in the subcontinental lithospheric mantle. The element fractionations observed between the FH and FL patterns are consistent with the immiscible separation of a silicic fluid from a carbonatite–silicate fluid, leaving a residual carbonatitic fluid strongly enriched in LREE, Ba and alkalies. This model would suggest that most monocrystalline diamonds crystallized from the more silicic fraction. Comparison with studies of single fluid inclusions in fibrous diamonds suggests that the FH patterns reflect trapped inclusions of high-Mg and low-Mg carbonatitic high-density fluids. In terms of the rock-forming elements, the fluids that precipitated the rims of some fibrous diamonds (FL pattern) and most monocrystalline diamonds are broadly similar to some hydro-silicic high-density fluids found in fibrous diamonds. However, there are still significant differences between the trace-element patterns of most monocrystalline diamonds and known high-density fluids, and further research is required to understand the formation and growth of these diamonds.