Late-stage Rocks Research Articles

Different characteristics of barren and W–Mo–productive Cretaceous granites in the Hwanggangri mineralized district, South Korea

The Hwanggangri mineralized district (HMD) in South Korea contains a variety of ore deposits such as porphyry or skarn (Mo, Cu–Pb–Zn), carbonate replacement, and fissure–filling vein types (W–Mo, Au–Ag, fluorite). The HMD is located on the boundary of the Okcheon metamorphic belt and the Taebaeksan Basin. Most of the deposits are hosted in limestones/dolomitic limestones of the Joseon Supergroup, and the age of mineralization is Late Cretaceous (96.5–76.2 Ma) including Geumseong, Jungbo, Kwangsin, and Geumsil deposits. These granites are mainly magnetite-series biotite granites, and they can be categorized into early- and late-stage on the basis of mineral assemblage, texture, grain size, and magnetic susceptibility. Magnetite is present in some of the granites, and some of the early-stage granites retain the high magnetic susceptibility values typical of the magnetite series. However, the late-stage granites show a wide range of magnetic susceptibility values, including low ones due to fractionation. The emplacement ages of the Sokrisan, Weolaksan, Muamsa, and Susan granite bodies are 94–88 Ma, similar to the ages of mineralization in the study area. The Cretaceous granites reported to be related to W–Mo mineralization in the HMD show miarolitic, porphyritic, and micrographic textures indicating considerable hydrothermal fluid activity in a shallow environment. The rocks of the Sokrisan granite are mostly early stage, those of the Weolaksan and Muamsa granites include both early and late stages, and the studied samples of the Susan granite are entirely late stage. These four Cretaceous granites have more than 70 wt% SiO2, high Rb/Sr ratios, depletions in compatible elements, enrichments in incompatible elements, and tetrad patterns with depleted LREEs, enriched HREEs, and strong negative Eu anomalies. They all belong to the high–K calc–alkaline series, are dominantly peraluminous, and can be classified as strongly fractionated I–type granites. The Sokrisan, Weolaksan, Muamsa, and Susan granites have similar emplacement ages and geochemical characteristics that indicate the same magma source. However, the Sokrisan granite, with its mainly early-stage rocks, consistently higher magnetic susceptibility values, and relatively high Nb/Ta values, differs from the other Cretaceous granites in the HMD. This suggests that the widely exposed Sokrisan granite of batholith type has been eroded substantially, removing the late-stage rocks and ore deposits. In contrast, the stock–type Susan granite has undergone relatively little erosion, and it represents well the late-stage parts of the Cretaceous granites in the HMD. The Cretaceous granites in the HMD have mostly highly-fractionated geochemical characteristics, and the late-stage rocks that are more associated with W–Mo mineralization are more fractionated than the early-stage rocks. The respective productivities of the Susan, Muamsa, Weolaksan, and Sokrisan granites range from high to low, and the more-productive late-stage granites are still present, as they have undergone less erosion.

Geochronological and geochemical comparison of Precambrian meta-mafic volcanics in Zhongtiaoshan and Lüliangshan Regions in Shanxi, North China Craton

To unravel the geochemical heterogeneity and its origin in different terranes of North China, we conducted geochronological and geochemical analyses of the meta-mafic rocks from the Luliang–Zhongtiao rift zone (Shanxi Province). LA-ICP-MS zircon U–Pb dating yielded mostly End-Neoarchean to Proterozoic ages for the basement rocks (Sushui Complex: 2516 ± 26 Ma; Meta-mafic rocks: 2494 ± 31 Ma), Jiangxian Group (~ 2213 Ma), Zhongtiao Group (2077 ± 29 Ma), Jiehekou Group (1998 ± 23 Ma), and Luliang Group (2152 ± 52 Ma). Petrographic characteristics show that the meta-mafic rocks from the Neoarchean–Paleoproterozoic Zhongtiaoshan (Sushui Complex) have similar geochemical characteristics to the overlying Jiangxian and Zhongtiao Groups. The Paleoproterozoic Luliang and Yejishan Group meta-mafic rocks from Luliangshan also have similar geochemical characteristics but are geochemically different from similar-age rocks from Zhongtiaoshan. This shows that the late-stage rocks have a geochemical inheritance from the early-stage rocks in the same region and that the geochemical heterogeneity of rocks from different areas was originated from the inherited heterogeneity of the magma source.

Magmatism in the Shapinggou district of the Dabie orogen, China: Implications for the formation of porphyry Mo deposits in a collisional orogenic belt

The Shapinggou molybdenum deposit is located in the Qinling-Dabie Orogen, which hosts the world's largest molybdenum belt. The igneous rocks at Shapinggou can be divided into two stages (136–127 Ma and 118–114 Ma), the early suite of felsic (136–127 Ma, SiO2 = 58.0 to 72.9 wt%) and mafic rocks (133–128 Ma, SiO2 = 45.2 to 57.0 wt%), and a later suite comprising syenite (117 Ma, SiO2 = 64.2 to 65.0 wt%), quartz syenite porphyry (116 Ma, 62.5 to 70.0 wt%), granite porphyry (112 Ma, SiO2 = 75.5 to 77.6 wt%) and diorite porphyry (111 Ma, SiO2 = 56.6 to 59.7 wt%). The early-stage felsic rocks display high SiO2, Al2O3, Na2O, K2O, Sr, LREE contents, and Sr/Y, (La/Yb)N ratios, initial Sr isotope ratios of 0.7076 to 0.7089, but low MgO, FeOT, Y, Yb contents and negative εNd(t) values, consistent with partial melting of the lower continental crust. The early-stage mafic rocks exhibit low SiO2, high MgO, Ni and Cr contents, consistent with an upper mantle source, but trace element and isotope data suggest a role for crustal contamination. The late-stage syenite and quartz syenite porphyry show high abundances of Na2O, K2O, Al2O3, HFSEs (e.g., Th, U, Zr, Hf) and significant negative Eu anomalies. The late-stage granite porphyry displays high SiO2 contents, and depletions in Ba, Sr, Eu and Ti. The geochemical features of the late-stage intrusions are similar to A-type granites. Crystal fractionation of plagioclase, K-feldspar, biotite/ muscovite, amphibole/ garnet and Fe-Ti oxides controlled the evolution of the magma. The geochemical and isotopic data suggest that the rocks at Shapinggou were likely derived from a mixed source of lithospheric mantle, subducted continental crust of the Yangtze Block (Kongling Group) and partial melts of the Dabie Complex. Early stage rocks represent melts of the source with a lower proportion of Dabie Complex materials, whereas late stage rocks were derived from a source with a higher proportion Dabie Complex component. The geochemical and isotopic variations of the intrusions at Shapinggou were controlled by both source characteristics and fractional crystallization. Although the Shapinggou deposit is located in a continental collision orogen, the magmas formed in an intraplate extension setting, with an increase in the amount of extension from the early to late stages. As well, both stages intrusions at Shapinggou were generated by the addition of heat, due to lithospheric delamination, mantle upwelling and rapid mantle convection, related to the far-field effects of the westward subduction of the paleo-Pacific Plate beneath the Asian continent. The geochemistry and setting suggest that the formation of a giant Mo deposit does not require a Mo-rich magma source, but rather an efficient convection mechanism for the transport of volatiles and Mo in a granitic magma system. The fluids derived from the granite porphyry at Shapinggou were more oxidised than that from the barren intrusions.

Hydrocarbon potential of the Lower Cretaceous mudstones in coastal southeastern China

The Cretaceous hosts the highest hydrocarbon potential among all geological stratigraphic sequences worldwide, and the Lower Cretaceous in coastal southeastern China, with the development of black mudstones and shales, has potential for hydrocarbon generation. This potential, therefore, is of significance to regional hydrocarbon exploration both onshore and offshore; however, it has not been investigated in detail. Here, we address this issue based on the results of integrated organic geochemistry and petrology of four representative outcrop sections in the region. Results show that there are two possible source rock sequences in the Lower Cretaceous of coastal southeastern China, that is, a lower sequence represented by the Bantou Formation black mudstones in the Fujian Province and an upper interval represented by the Guantou Formation black mudstones or calcareous mudstones in the Zhejiang Province. The organic matter abundance of these two formations reaches levels of poor to good source rocks. The kerogen is dominated by type III, whereas type II can also be found locally in the northwestern Fujian Province. Most samples are highly mature to overmature, except for the samples with less influence of volcanism in the northern Guangdong Province. Comparatively, the organic matter abundance of the early-stage source rocks is higher than for the late-stage rocks, and the kerogen type is less favorable; however, the maturity shows little correlation with age and is largely related to volcanism. Therefore, the Lower Cretaceous in coastal southeastern China seems to have hydrocarbon potential (gas in particular) in local areas, including the northern Guangdong Province and its analog in offshore basins for the early stage and the northwestern Fujian Province and its analog in offshore basins for the late stage. These results and understanding provide insights for expanding new hydrocarbon exploration targets in southeastern China both onshore and offshore and may also have implications for understanding the regional Cretaceous geology.

Insights into the late-stage differentiation processes of the Catalão I carbonatite complex in Brazil: New Sr–Nd and C–O isotopic data in minerals from niobium ores

The Late Cretaceous Catalão I carbonatite complex consists of ultramafic silicate rocks, phoscorites, nelsonites and carbonatites. The latest stages of the evolution of the complex are characterized by several nelsonite (magnetite–apatite rock) and carbonatite dykes, plugs and veins crosscutting earlier alkaline rocks. The interaction between the latter and late-stage carbonatites and/or carbo-hydrothermal fluids, converted the original dunites and bebedourites to metasomatic phlogopitites. Late-stage nelsonites (N1), pseudonelsonites (N2) and various types of dolomite carbonatites (DC) including norsethite-, magnesite- and/or monazite-bearing varieties show significant whole-rock Nd and Sr isotopic variations. To elucidate whether magmatic or metasomatic processes, or both, were responsible for these isotope variations we characterized the Nd and Sr isotope compositions of major mineral phases (i.e. apatite, dolomite, norsethite, pyrochlore and tetraferriphlogopite) in these late-stage rocks. Mineral isotope data recorded the same differences observed between N1 and N2 whole-rocks with N2 minerals showing more enriched isotopic signatures than minerals from N1. Sr isotopic disequilibrium among minerals from N2 pseudonelsonites and spatially related dolomite carbonatite pockets implies formation from batches of carbonate melts with distinct isotopic compositions. A detailed investigation of Nd and Sr isotopes from whole-rocks and minerals suggests that the most evolved rocks of the Catalão I complex probably derive from two different evolution paths. We propose that an earlier magmatic trend (path A) could be explained by several batches of immiscible and/or residual melts derived from carbonated-silicate parental magma (e.g. phlogopite picrite) contaminated with continental crust to a variable extent, in an AFCLI-like process. A second trend (path B) comprises highly variable 143Nd/144Ndi at nearly constant 87Sr/86Sri coupled with high δ18O in carbonates. This is interpreted here as the result of the interaction of previously-formed dolomite carbonatites with carbo-hydrothermal fluids.

Origin of rhythmic anorthositic–pyroxenitic layering in the Damiao anorthosite complex, China: Implications for late-stage fractional crystallization and genesis of Fe–Ti oxide ores

The ∼1.7Ga Damiao anorthosite complex (DAC) in the North China Craton contains abundant Ti–magnetite-dominated ore deposits. Both the Fe–Ti–P-rich silicate rocks and massive Fe–Ti–(P) ores occur as discordant late-stage dikes cross-cutting early-stage anorthosites with irregular but sharp boundaries. Field and petrographic observations indicate that some late-stage dikes are composed of unique oxide–apatite gabbronorites (OAGNs), whereas others comprise well-developed alternating late-stage anorthosites and Fe–Ti–P-rich pyroxenites defining rhythmic layers. Massive Fe–Ti–(P) ores are closely related to the Fe–Ti–P-rich pyroxenites. Plagioclase and whole-rock compositions of different rock types were analyzed to constrain the late-stage magma evolution and genesis of the Fe–Ti oxide ores. The similar mineralogical assemblages, REE and HFSE patterns suggest that the different rock types formed by differentiation from a common parental magma. Early-stage anorthosites are characterized by positive Eu anomalies and low REE contents, whereas the late-stage dike-like rocks display no significant Eu anomalies and high REE contents. Plagioclase compositions in the late-stage rocks show a decrease of An contents when compared to that of the early-stage rocks. Based on field relations, petrography and well-defined linear compositional trends, the sequence of crystallization is inferred as: early-stage anorthosites+leuconorites+norites, OAGNs, late-stage anorthosites+Fe–Ti–P-rich pyroxenites+massive Fe–Ti–(P) ores, and massive Fe–Ti–(P) ores. The OAGNs which underwent relatively rapid crystallization represent an early phase during the residual magma evolution after anorthosite separation, whereas the rhythmic layers formed by slow but extensive fractional crystallization of interstitial melt. High solubility of phosphorous played an important role in the formation of rhythmic layering. Massive Fe–Ti–(P) ores crystallized and segregated directly from the magma of Fe–Ti–P-rich pyroxenites, thus representing the most evolved products of the residual magma. Progressive and extensive fractional crystallization of residual ferrobasaltic magma is probably responsible for the formation of large-scale massive Fe–Ti–(P) ores.

Deep mantle storage of the Earth’s missing niobium in late-stage residual melts from a magma ocean

The origin of the observed niobium deficit in the bulk silicate Earth (BSE) compared to chondritic meteorites constitutes a long-standing problem in geochemistry. The deficit requires a large-scale process fractionating niobium from tantalum, and a super-chondritic Nb/Ta reservoir hidden in the deep silicate Earth and/or in the metallic core. The only voluminous super-chondritic Nb/Ta silicate reservoir analysed to date is found in lunar basalts that assimilated highly evolved Fe-rich rocks associated with anorthosites in the lunar crust. These Fe-rich rocks, enriched in incompatible elements, are thought to represent the last fractions of melt remaining at the end of lunar magma ocean crystallization. Here we report high-precision Nb–Ta data for a Fe-rich, late-stage rock suite associated with a terrestrial anorthosite from the Proterozoic Bolangir complex in India. The geochemical characteristics of this rock suite resemble those expected for late-stage residual melts from a terrestrial magma ocean. Samples show extreme, super-chondritic Nb/Ta up to 31.1 and highly elevated Nb concentrations up to 338 ppm. We argue that formation of an early enriched crustal reservoir (EECR) with these characteristics (high Fe, high Nb, superchondritic Nb/Ta) is likely in the course of Hadean late-stage terrestrial magma ocean solidification. Subduction and subsequent permanent deep mantle storage in the D′′ layer of a minor amount (∼0.5% of the BSE mass) of this EECR can readily explain the terrestrial Nb deficit, without the need to invoke core Nb storage. Our model is consistent with short-lived 142Nd and long-lived 176Hf– 143Nd isotope models for early differentiation of the Earth’s crust. In addition, the inferred Lu/Hf of this EECR implies that this reservoir can also balance the offset of terrestrial Hf isotope ratios compared to the chondritic reservoir. As such, late-stage magma ocean residual melts may constitute the enigmatic parental reservoir of Hadean zircons with low time-integrated Hf isotope compositions.

Geochronology on Cenozoic volcanic rocks of a profile Linzhou Basin, Tibet, China and their geological implications

Geochronology on Cenozoic volcanic rocks of a profile Linzhou Basin, Tibet, China and their geological implications

Geochemical evolution of the metaluminous and peraluminous granites of Ganawuri Younger Granite Complex, northern Nigeria

The Ganawuri Complex is one of the numerous high level composite granitoid bodies occurring in the central plateau sector of the Nigerian Younger Granite province.Lithologically the Complex is composed of a hastingsite-fayalite granite, hastingsite-biotite granite and biotite granite.Although the major element chemistry of the constituent rock types shows only subtle variation typical of granites with minimum melt composition, the trace elements data and variation indicate that the granites in the Complex formed essentially by fractional crystallization. Geochemically the late-stage rock types are characterized by anomalously high contents of F, Li, Y, Th, Ga, Rb, Sn and Nb and by unusually low contents of Ba, Sr and Zr. These geochemical peculiarities are interpreted to be due to extreme fractionation. The ultimate product of fractional crystallization was a water-saturated melt, enriched in incompatible elements whose crystallization resulted in tin mineralization.The chemistry of the rocks can be compared with A-type granites which are considered typical of anorogenic extensional environments and/or Continental rifts.

The origins of late-stage rocks in the St Austell granite—a re-interpretation

The origin of the granite varieties present in the St Austell granite body (biotite granite, megacrystic and non-megacrystic lithium-mica granite, fluorite granite) is discussed in the light of new petrographic, chemical and experimental evidence. There were essentially two stages of magmatic activity—the first represented by the intrusion of several phases of biotite granite, the second by a later intrusion of non-megacrystic lithium-mica granite which crystallized from a volatile-rich residual differentiate of the biotite granite magma. Both intrusions suffered hydrothermal alteration which primarily involved at least two stages of exchange of components via a single aqueous fluid phase. Within the non-megacrystic lithium-mica granite, topaz and lithium-mica were among the first phases to break down, releasing F and Li to this aqueous phase. F was locally fixed as fluorite, removing Ca (derived from the host biotite granite) from the fluid and giving rise to pockets of fluorite granite. Apart from losing Ca from plagioclase, the biotite granite gained Li, expressed as lithium-rich mica, resulting in an aureole of megacrystic lithium-mica granite surrounding the second intrusion. At least one pervasive hydrothermal system, using joints and grain boundaries as fluid pathways, may have been in existence. Water-rock interaction during fluid circulation is believed to have modified considerably the composition of predominantly meteoric water to give rise to the hydrothermal fluid responsible for greisening and mineralization within its sphere of influence. The alteration phenomena initiated by the proposed hydrothermal system may have culminated in the formation of the major kaolinite deposits currently worked in the area.

Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon pluton, Scotland

The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles. Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions. It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.

Petrography of the Manu'a Islands, Samoa

The rocks of the Manu'a Islands are predominantly olivine basalt with lesser amounts of picrite basalt (both ankaramite and oceanite), basalt, hawaiite, olivine gabbro, basaltic lapilli tuff, and ash. Alkali ∶ silica ratios obtained from chemical analyses of twenty widely selected samples place the rocks in the alkali basalt suite. All of the samples are also high in titanium. Plots of the chemical analyses, including samples from nearby Tutuila Island, show a progressive enrichment in alkalis. The occurrence of hawaiite and picrite basalt indicates that a primitive alkalic olivine basalt magma was undergoing differentiation. The most important factor in this process was crystal settling, especially of olivine. The dunite xenoliths in the late-stage rocks of Ta'u Island probably came from a residual olivine layer near the bottom of the magma chamber. The magma did not become sufficiently silicic to produce the trachytic end-member of the series, which probably would have contained normative and possibly some modal quartz, as on Tutuila Island. With increasing silica content, iron and titanium generally decrease slightly, whereas alkalis increase.

Differentiation of quartz-bearing syenite (nordmarkite) and riebeckitic-arfvedsonite granite (ekerite) of the Oslo Series

The quartz-bearing syenite (nordmarkite) and the riebeckitic-arfvedsonite granite (ekerite) members of the Oslo Series subvolcanic comagmatic suite exhibit both inter- and intra-member differentiation relationships. The differentiation seems to involve: late stage relative enrichment in Be, Si, Li, Nb, Zr, Pb, Tl, Rb and Cs; late stage relative impoverishment in Al, Mg, Ti, V, Ca, Nd, Sr, La, K + Na, Ba and Fe; and general decreases in the K/Rb and Ba/Rb ratios with fractionation. The rocks exhibiting the late stage chemical characteristics are typically massive whereas the rocks with earlier stage chemical characteristics are miarolitic. In addition, the seemingly late stage rocks occur in loci where there probably was relatively more rapid cooling. These features appear to support the suggestion that the above alluded to enrichment of some of the elements, at least of the highly volatile ones, may be apparent rather than real—that is, the concentration of at least the more volatile of the apparently enriched elements may have been essentially the same in the magma(s) that consolidated to form each of the investigated rocks but the massive, apparently late stage rocks retained these elements whereas the miarolitic, apparently earlier stage rocks lost large percentages of them.

K/Rb ratios in the evolution of the Younger Granites of Northern Nigeria

K/Rb ratios have been determined in about 70 granites from Northern Nigeria. It is shown that late stage rocks such as albite-riebeckite-granites have ratios below 30 while early-stage rocks such as Ca-amphibole-granites can have ratios up to 330; biotite-granites have intermediate ratios. In the Liruei complex the ratio decreases progressively with time of emplacement but in the Amo complex the ratio varies irregularly.