Light Enrichment Research Articles

Origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England

Chalks are generally believed to have formed in shallow marine environments. However, the consensus on sources of Fe and origin of hematite in red chalks remains elusive. To better understand the origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England, the evidence has been gathered through diffuse reflection, trace element and iron isotope analysis and combining with the lithologic and stratigraphic characteristics. Rare earth element (REE) patterns exhibit upper-continental-crust-like signatures such as enrichment in light REEs and depletion in heavy REEs. The Fe isotopic composition of the red chalk is typically heavier than that of igneous rocks (except for one sample) and shows negative Eu anomalies (Eu/Eu* = 0.58–0.73), which indicates that the hematite in the red chalk is not of microbial or hydrothermal, but of sedimentary origin. At the same time, the analysis of trace elements suggests that the felsic rocks and sedimentary derivatives are the source of non-carbonate components in the red chalks. The pedogenic features such as mudcracks, secondary white chalks, and calcareous nodules appear in the red chalks, as documented through the field stratigraphic observations. Based on the above evidence and the sedimentary environment of the overlying and underlying layers, it is inferred that the red chalk was deposited in the supratidal environment that had undergone some degree of desiccation and pedogenic modification.

The Transitional Gabbroic Rocks in Bayah Geological Complex, Western part of Java, Indonesia, Inferred from XRF, ICP-MS, and Microprobe Analysis

Gabbro, is a fossil remnant of oceanic crust in western part of Java, found at Bayah Geological Complex (BGC) and Ciletuh Melange Complex (CMC), Indonesia. It has been studied by using petrographic, X-Ray Fluorescence (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) and mineralogical (microprobe) analyses. Mineral and geochemical composition of these rocks provide important clues to their origins since the rocks have been deformed and gone through auto metamorphism, beside they contain the economic mineral and or rare earth elements (REE). Gabbroic rocks in these two areas generally shows phaneritic to porphyritic texture, granular texture. These rocks in CMC are dominated by plagioclase (oligoclase to albite), hornblende, pyroxene, partly altered to tremolite, actinolite, chlorite, epidote, and sericite; meanwhile those of BGC dominantly consist of plagioclase, pyroxene, hornblende, some present of chlorite, actinolite, epidote and biotite as secondary minerals. In multi-element diagrams, gabbroic rocks in CMC show strong negative Sr and Zr, but positive Nb anomaly, while those of BGC show strong negative anomaly of Nb and Zr. In addition, based on rare earth elements (REE) diagrams, gabbroic rocks in CMC show depleted of light rare earth elements (LREE) with negative Eu anomaly, while gabbro’s in BGC show enrichment of LREE. These characteristics indicate that GBC’s and CMC’s gabbroic rocks came from different magma sources, one was formed by partial melting of depleted upper mantle reservoir while the other one was formed by partial melting of mantle wedge with active participation of subducted slab in an arc tectonic setting, suprasubduction zone which were formed at started Upper Cretaceous to Paleogene, and they had retrograde metamorphism to epidote amphibolite facies.

Elemental and isotopic evidence for the presence of a microbial thriving environment during the deposition of Early Cambrian Xiaoerbulake Formation in the Tarim Basin, NW China

Microbial communities regained prominence in the Cambrian after a prolonged downturn in the Neoproterozoic, but the reasons for their revival are unclear. The earliest microbialites formed during this period within the Tarim Basin are well displayed in the Xiaoerbulake Formation in the Keping area. Major, trace, rare earth element (REE) analyses were carried out using samples collected from two sections of the Xiaoerbulake Formation which present two different palaeogeographical settings, namely, the intraplatform and platform margin. The samples were divided into seven groups based on their REE + Y (yttrium) distributions. Among these, the YG3, YG4, and PG3 groups correspond to microbial boom phases. The high contents of Al and other nutrient elements (e.g., P, Ni, U, Cu, Fe, Mn) in these three groups indicate that the microbial booms are controlled by nutrient concentration of different sources. In relatively deep‐water settings, aeolian dust and suspension particles modified the seawater geochemical characteristics to form shale‐like (REE + Y)N patterns. The nutrient‐rich fine‐grained deposits were blown up by storms and released nutrients. This eutrophication ultimately promoted microbial development and is demonstrated by light rare earth element enrichment patterns. In shallow‐water environments, microbes seemed to prefer quiet conditions, with sufficient nutrients provided by fluvial influx. In contrast, excess clastic materials inputs, be they carbonate or terrigenous debris, are unfavourable to the development of microbialites. Hence, it is expected that seawater REE preserved in ancient carbonates are capable of providing information on the impacts of seawater geochemical conditions on the evolution of microbialites.

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.

Juvenile continental crust evolution in a modern oceanic arc setting: Petrogenesis of Cenozoic felsic plutons in Fiji, SW Pacific

Viti Levu, Fiji, provides one of the best exposed Phanerozoic analogues for the formation of juvenile continental crust in an intra-oceanic setting. Tonalites and trondhjemites are present in several large (75–150 km2) adjacent, mid-Cenozoic plutons. We report major and trace element data including rare earth element (REE) and high-precision high field strength element (HFSE) compositions, new Hf-Nd-Sr-Pb isotope data, and zircon U/Pb-ages, O-Hf isotopes, and trace elements, from five different plutons. The Eocene Yavuna pluton and the Miocene Colo plutons are mainly composed of tonalites and trondhjemites and represent the exposed middle crust of the former Vitiaz island arc. The plutons can be divided into three suites. One suite is light REE (LREE) depleted with some trace element ratios lower than average normal mid-ocean ridge basalts (N-MORB). A second suite has flat REE patterns similar to local island arc basalts. Both suites occur near the coast of Viti Levu, include a wide compositional spectrum from gabbro to tonalite, and can be produced mostly by fractional crystallization of mafic precursor melts. The third suite is characterized by LREE enrichments with higher LaN/YbN (2.3–4.9), higher Zr/Y (4.3–7.1), and lower Nb/Ta (9.6–12.4). They occur closer to the center of the island and are bimodal trondhjemite-gabbro intrusions. These characteristics are consistent with formation mostly by partial melting of mafic crust. Trace element modeling shows that the trace element ratios of the third suite can be produced by 10–20 % melting of the mafic crust in the presence of residual amphibole, resulting in the retention of the medium REE (MREE) and diagnostic trace element ratios including low Nb/Ta and high Zr/Y. Geochemical similarities of the LREE enriched suite to typical “low”-pressure Archean tonalites-trondhjemites-granodiorites (TTGs) imply a common petrogenetic origin and similar mechanisms for the generation of juvenile Archean and modern differentiated crust by partial melting of mafic crust with residual amphibole. In modern oceanic arcs, genetically unrelated felsic plutonic as well as volcanic rocks co-exist, and in this regard, the Fijian plutons accompany major tectonic disruptions to arc processes.

Petrogenesis and tectonic implications of the Paleoproterozoic Gaoliduntai plagiogranites in the Jiao–Liao–Ji Belt, North China Craton

Paleoproterozoic plagiogranites were identified in the Gaoliduntai area of the Jiao–Liao–Ji Belt (JLJB), North China Craton (NCC). We conducted a geochronological, geochemical, isotopic, and mineral chemical investigation of the plagiogranites to constrain their petrogenesis and tectonic implications. The Gaoliduntai plagiogranites yielded zircon U–Pb ages of 1802 ± 10, 1828 ± 18, and 1853 ± 11 Ma, indicating emplacement at the late Paleoproterozoic. Geochemically, They are characterized by high SiO2 and Na2O, and low K2O and TiO2 contents, with low total rare earth element (REE) concentrations. They also exhibit enrichment in light REEs and large-ion lithophile elements (e.g., Cs, Rb and Sr), and depletion in heavy REEs and high-field-strength elements (e.g., Nb and Ta). The plagiogranites show varying initial 87Sr/86Sr ratios (0.7012–0.7053), whole-rock εNd(t) values (–8.23–+1.15) and apatite εNd(t) values (–4.78–+3.30), with whole-rock 206Pb/204Pb(t) (14.223–16.733), 207Pb/204Pb(t) (15.310–15.644) and 208Pb/204Pb(t) (35.310–35.998) values, indicating a depleted mantle source modified with an enrichment component. Zircons in the plagiogranites have positive εHf(t) values (+1.0 to +6.4) and TDM2(Hf) ages of 2408–2078 Ma. Zircons δ18O compositions (2.1‰ to 4.5‰) are lower than the zircons from normal mantle but show similar features to typical plagiogranites. Furthermore, the plagioclases in these plagiogranites are albite (Ab = 95–99). These features lead us to propose that the Gaoliduntai plagiogranites formed by partial melting of high-temperature mafic oceanic crust, with contributions from delaminated crustal components. We suggest that the Paleoproterozoic Gaoliduntai plagiogranites provide evidence of the involvement of ancient oceanic crust in the tectonic evolution of the JLJB. Based on regional geological features and previous studies, we propose that the JLJB was an arc–continent collision belt during the Paleoproterozoic, and that the Gaoliduntai plagiogranites formed in an extensional tectonic setting.

Paleoenvironment reconstruction of the upper Paleozoic in the Linxing area, northeastern Ordos Basin, China

The interpretation of the depositional environment, provenance, and paleoclimate provides important information for understanding the formation and distribution petroleum systems. The trace element and rare earth element (REE) concentrations and the heavy mineral distribution in upper Paleozoic sedimentary rocks in the Linxing area, northeastern Ordos Basin, were determined to reconstruct the paleoenvironment. The samples show clear light REE enrichment, even though the degree of enrichment is generally poor. Redox proxies indicate that most of the samples were deposited in relatively dysoxic-oxic conditions. A warm and humid climate prevailed during deposition of the Benxi to Xiashihezi Formations, while extremely dry climate also occurred during Benxi Formation and Taiyuan Formation deposition. Shiqianfeng Formation deposition occurred in a completely dry climate. The increase in ΣREE concentrations and the decrease in the Sr/Ba ratios above the Benxi Formation indicate a rapid increase in terrigenous clastic input and sea-level fall, which was related to Permian tectonic movement. Bivariate plot diagrams show mixed felsic-basic source influences for all the formations, with the Shanxi Formation displaying certain influences from acidic sources. From late Carboniferous to early Permian, the Daqingshan uplift was the primary provenance. Subsequently, the Yinshan paleocontinent played a more important role as the main provenance as it was being continuously uplifted.

Importance of the Spatial Distribution of Rare Earth Elements in the Bottom Sediments of Reservoirs as a Potential Proxy for Tracing Sediments Sources. A Case Study in the Dominican Republic

The geochemical composition of rare earth elements (REE) in the bottom sediments of two Dominican reservoirs and in soils from their catchments was studied to identify possible sources of the deposited materials. Knowledge of the origin of the sediments will serve to control the excessive rates of erosion and sedimentation that occur annually due to periodic extreme climatic events that promote excessive silting of the lakes, followed by loss of storage capacity and degradation of water quality. The REE contents of sediments and soils were normalized to the North American Shale Composite (NASC) and the ratio of light/heavy rare earths (LREE/HREE ratio), Ce and Eu anomalies, and some fractionation parameters were determined. The REE patterns are more homogeneous in the sediments, indicating uniform sedimentation in both deposits. The sediment data reflect depletion of REE from the sources, enrichment of light REE (LREE) and some middle REE (MREE), and positive Eu and Ce anomalies. All data were plotted in correlation diagrams between some fractionation parameters of light–middle–heavy REE and anomalies of Ce and Eu. The similarity of the ratios between these parameters in all samples and the overlap of data from soils and rocks on the sediment projection in the diagrams allowed a good discrimination of the main sources of the materials.

Petrography and Geochemistry of Gabbroic Rock from the Penjwin Ophiolite, Kurdistan Region, Northeastern Iraq

The gabbroic rocks as a part of Zagros ophiolite are exposed in northeastern Iraq, Penjwin area. These rocks with granular to ophitic textures are widely distributed without metamorphic halos. The main minerals are plagioclase (An90-99), olivine, clinopyroxene (Wo27-47 En 45-64 Fs8-14) and orthopyroxene (Wo2 En78 Fs20) respectively based on the abundances. The major elements show a broad range of compositional variations, with SiO2 (46.2–50.9 wt. %), and low concentrations Na2O (0.15–0.62 wt. %), K2O (0.01–0.03 wt. %) and TiO2 (0.06–0.2) and high concentrations, Al2O3 (6.4–19.75 wt. %), total Fe2O3 (6.29–11.6 wt. %), MgO (9.63–24.5 wt. %), CaO (8.02–18 wt. %) and low alkali contents (Na2O + K2O = 0.16–0.65 wt. %). On Ti-V diagram, all of the gabbroic samples have Ti/V less than 10 and consequently fall in the low Ti- Island arc tholeiitic. Whole rocks chemistry shows a depletion of High field strength elements in comparison with the primitive mantle with an arched upward rare earth elements pattern, characterized by light rare earth elements depletion (La N/Sm N = 0.05–0.8) and enrichment in the High field strength elements. Whole rocks chemistry, mineral paragenesis and chemistry of these rocks are more consistent with tholeiitic magma series. Based on our findings in this research, the primary magma has been produced from the depleted mantle with a high degree of partial melting.

Neoproterozoic protracted arc basaltic magmatism in the southern margin of the Yangtze Block, south China: New constraints from mafic-ultramafic intrusive rocks

Mantle plume activity and subduction are two popular geodynamic models that have been used to explain the widespread Neoproterozoic basaltic magmatism in the Jiangnan Orogenic Belt along the southern margin of the Yangtze Block, South China. We have evaluated the competing hypotheses using new geochronological, petrological and geochemical data of two mafic–ultramafic intrusions in the Yuanbaoshan (YBS) area of this orogenic belt, namely the YBS south and west intrusions. Zircons from the YBS south intrusion yielded a U-Pb age of 835 ± 3 Ma. This new age and the previously reported age for another mafic–ultramafic intrusion situated < 10 km to the northwest (the YBS west intrusion) reveal a protracted (12–20 Myr) episode of mafic magmatism in a very small area. Such temporal-spatial relationship is more consistent with the style of subduction-related magmatism than that of mantle plume-related magmatism. The YBS south and west intrusions are characterized by arc-like geochemical features such as light REE enrichments and pronounced negative Nb-Ta anomalies in whole rocks. Olivine Mn/Fe and Ca contents, clinopyroxene AlZ/TiO2 and Cr-spinel TiO2/Al2O3 of these intrusions are similar to those of some undisputed subduction-related mafic–ultramafic intrusions elsewhere in the world. Zircons from the YBS south intrusion have mantle-likeHf-O isotope compositions, with εHf(t)varying from + 3.3 to + 8.7 and δ18Oof5.32 ± 0.29‰. The whole rock samples yielded εNd(t) from + 5.2 to + 8.8 and (87Sr/86Sr)i from 0.7034 to 0.7051 for the YBS south intrusion, and εNd(t) from + 1.7 to + 5.2 and (87Sr/86Sr)i from 0.7036 to 0.7051 for the YBS west intrusion. The Sr-Nd isotope compositions of these intrusions are similar to those of global Cenozoic arc basalts, but are also consistent with mantle plume-derived magma with < 5% of crustal contamination. However, the lack of correlation between Th/Nb and εNd(t) or (87Sr/86Sr)i for the intrusive rocks is inconsistent with the contamination model. The results from this study support for a popular view that northward oceanic subduction beneath the southern margin of the Yangtze Block took place in the Neoproterozoic.

Geochemical and REE mineralogical characteristics of the Zhaibei Granite in Jiangxi Province, southern China, and a model for the genesis of ion-adsorption REE deposits

The Zhaibei Granite in Jiangxi Province, southern China, hosts an ion-adsorption light rare earth element (LREE) deposit. Recently, heavy REE (HREE) ores have also been reported from weathered crusts of the granitic rocks. In this study, petrological, geochemical, and mineralogical characteristics of the Zhaibei pluton were analysed to establish the genesis of this REE deposit. The pluton contains coarse-grained biotite syenogranite and minor hornblende–biotite–quartz monzonite in its central zone, medium-grained biotite syenogranite in its transitional zone, and fine-grained muscovitic alkali-feldspar granite and porphyritic muscovitic biotite–alkali-feldspar granite together with monzogranite porphyry intrusions in its marginal zone. The REE minerals include titanite, allanite, monazite, bastnasite, and thorite-(Y) in the central zone, allanite, monazite, xenotime, bastnasite, and thorite-(Y) in the transitional zone, and synchysite-(Y), thorite-(Y), and xenotime in the marginal zone. The SiO2 content increases from 62 to 77 wt% from the centre to the margin, whereas the Al2O3, TiO2, MgO, FeO, and CaO contents decrease. In addition, the size of the negative Eu anomaly and HREE content increase, accompanied by decreases in Co, Zr, Hf, Sr, and Ba and increases in Rb, Cs, Nb, and Sn. These observations indicate that the Zhaibei Granite might have been formed by crystallisation differentiation of magma. The granitic pluton was also influenced by reactions with late-magmatic F-, CO2–, and REE-rich fluids that altered magmatic minerals and crystallised as REE-fluorocarbonates and thorite. The hydrothermal REE minerals constitute ∼ 40% of the total REEs in the bedrock and are the major supplier of ion-exchangeable REEs in the ores of the Zhaibei deposit. LREE-rich ores may have been derived from the LREE-rich granitic rocks, which contain bastnasite, titanite, and allanite, whereas HREE-rich ores were sourced mainly from the HREE-rich granites, which contain synchysite-(Y) and thorite-(Y). Furthermore, REE ores with enrichment in both HREEs and LREEs were probably sourced from the monzogranite porphyry bedrock, which contains bastnasite-(Y), bastnasite, allanite, and thorite-(Y).

Coeval calc-alkaline and alkaline Cadomian magmatism in the Bafq, central Iran: Insights into their petrogenesis

The Chah-Gaz and Mishdovan areas in the Bafq magmatic complex, central Iran, contain thick series of terrigenous sediments (the Rizu-Dezu complex), and arc-related calc-alkaline and alkaline igneous rocks. Geochemical analyses of igneous rocks from both areas indicate two distinct rock clans: (1) high-K, calc-alkaline-shoshonitic rocks with strong depletions in Nb, P, and Ti, and (2) an alkaline quartz gabbro-diorite, with trace element patterns resembling oceanic island basalts (OIB). New geochronological data reveal that magmatic rocks of both clans crystallized almost simultaneously, with zircon UPb ages of 534 Ma and 537 Ma, respectively. The whole-rock NdSr isotopic data ((87Sr/86Sr)(I) = 0.7052 to 0.7064 and ɛNd(t) = +1.3 to +2.7) of alkaline quartz gabbro-diorite indicate an enriched OIB-like mantle source, while the high-K, calc-alkaline-shoshonitic rocks have εNd(t) = −5.5 to −7.6, clearly reflecting significant contributions from pre-existing Proterozoic basement. Apatite in both the Chah-Gaz and Mishdovan magmatic rocks is of magmatic origin, with light rare earth element (LREE) enrichment patterns. The low Sr/Y and Eu/Eu⁎ values in apatite demonstrate the non-adakitic character of the investigated rocks, while the moderately negative Eu anomaly and inverse correlation between δCe and δEu in the analyzed apatites may reflect reduced parental magmas. The geochemical and isotopic results presented here indicate that slab rollback and opening of an extensional basin could have initiated concurrent Cadomian arc-related calc-alkaline and rift-associated alkaline magmatism in the Chah-Gaz and Mishdovan areas.

Identification of ca. 520 Ma mid-ocean-ridge–type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite

AbstractAn ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)–like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621–0.12008 and very low 187Re/188Os values of 0.031–0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2–4.1) and relatively high (87Sr/86Sr)i ratios (0.7046–0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09–0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6–1.65 wt%), Cr (0.22–6.26 ppm), and Ni (0.77–4.74 ppm) compositions and low Mg# (22.4–31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰–5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy–type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic.

Redox state and water content changes of magma during amphibole accumulation process: Tongling example

The mechanism by which amphibole-dominated differentiation controls the redox state and water content of derivative melt was explored through petrographic studies and in situ major- and trace-element and Sr–Nd isotope analyses of apatites from amphibole-rich cumulate xenoliths and their porphyry hosts in the Tongling ore cluster, China. Apatite is typically dominated by F-rich components, with increasing OH and SO3 contents from primitive cumulates to evolved porphyry hosts. Chondrite-normalised rare-earth-element (REE) patterns of apatites are right-inclined with strong light REE enrichment; the negative Eu anomaly is more pronounced in apatites from porphyry hosts than in those from cumulates. Plagioclase crystallisation prior to apatite is a predominant factor controlling the contrasting differences in the REE contents and patterns and the Sr contents of apatites. We conclude that amphibole-dominated magma differentiation has only weak effects on the redox state of the melt but can efficiently increase the water content of derivative melt; the oxidised nature of the porphyries is inherited from their source. In porphyry Cu mineralisation systems, deep accumulation processes may offset metal loss incurred through sulphide saturation as its function in increasing the water content of evolved melts.

Controls on the formation of porphyry Mo deposits: Insights from porphyry (-skarn) Mo deposits in northeastern China

AbstractPorphyry Mo deposits have traditionally been classified into two major classes, arc-related and Climax-type, based on the tectonic setting and chemistry of associated intrusions. Although there is a consensus that porphyry Mo systems were formed by the optimal coincidence of geological processes operating at different scales, it is unclear what key parameter(s) render systems productive and whether the two classes of porphyry Mo deposits are unique in their mode of formation, or if they share fundamentally similar geological processes. These questions are important as a clearer understanding of the optimum conditions for the formation of porphyry Mo deposits is a prerequisite for more efficient exploration.This contribution presents a detailed assessment of the factors affecting the formation of porphyry Mo deposits through the investigation of barren and mineralized intrusions from the arc-related Songbei-Yangjiazhangzi-Lanjiagou (SYL) ore zone and the Climax-type Hashitu deposit of northeastern China. Our results show that the syn-mineralization intrusions from the SYL ore zone are quite evolved (SiO2 ~ 75 wt%; Na2O+K2O ~ 8.7 wt%) and are characterized by apparent light rare earth element (LREE) enrichments (LaN/YbN = 2.7–33.1) and moderate negative Eu anomalies (Eu/Eu* = 0.4–0.7). They show enriched zircon Hf isotopic compositions [εHf(t) = –11.9 to –4.8], indicating their parental magmas were likely derived from an ancient crustal source. Melt inclusions from the SYL syn-mineralization intrusions contain negligible F and Cl. In contrast, Hashitu syn-mineralization intrusions are characterized by weak LREE enrichments (LaN/YbN = 2.2–6.9) and strong negative Eu anomalies (Eu/Eu* = 0.02–0.10), with SiO2 and Na2O+K2O contents similar to the SYL syn-mineralization intrusions. They show depleted zircon Hf isotopic compositions [εHf(t) = 3.1–5.0], indicating their parental magmas were likely derived from a juvenile crustal source. Melt inclusions from the Hashitu syn-mineralization intrusions contain up to 0.4 wt% F and 0.03 to 0.09 wt% Cl. However, in both cases, the syn-mineralization intrusions are Mo-poor (1–7 ppm Mo), oxidized (above the quartz-fayalite-magnetite buffer), water-saturated (4.4–7.8 wt% H2O), and were emplaced at palaeodepths of 3.3 to 8.3 km. These data imply that magma source composition is not a key factor in the formation of porphyry Mo deposits. In contrast, magma oxygen fugacity, water content, and emplacement depth appear to play fundamental roles in the formation of porphyry Mo deposits of both arc-related and Climax-type.Within individual deposits, no systematic differences between pre- and syn-mineralization intrusions are observed in terms of magma source, fractionation degree, oxygen fugacity, emplacement depth, and volatile and Mo contents. Instead, a crucial apparent difference lies in the geometry of the intrusions, i.e., pre-mineralization intrusions generally occur as flat, ponded bodies, whereas syn-mineralization intrusions commonly develop as small stocks or dikes. Our results, in combination with an examination of other porphyry Mo systems, suggest that the sudden depressurization of magma chambers and subsequent venting of voluminous fluids along focusing structures (such as small stocks or dikes) most likely plays a critical role in the formation of porphyry Mo deposits of both arc-related and Climax-type. The findings of this study indicate that fluid processes in the shallow crust are pivotal for the formation of porphyry Mo deposits and that settings with ideal magmatic-hydrothermal architectures are most likely to develop into productive porphyry Mo systems.

Petrogenesis of Botorubuh igneous rocks at Gunungkidul: A preliminary study for paleomagmatism phenomenon in the southern beach of Yogyakarta - Indonesia

Botorubuh Beach is located at Southern Beach of Gunungkidul, in the Southern Mountain of Yogyakarta - Indonesia and is a promontory composed of Middle - Late Miocene igneous rock with a columnar jointing structure. This isolated igneous rock area is surrounded by limestone. Therefore, the regional geological map of Surakarta-Giritontro classifies this area as the Punung-Wonosari Formation which is dominated by limestone. Because of the geological phenomenon of the isolated igneous rock area, it is necessary to study the petrogenesis of the igneous rocks at the area. The petrogenesis research is based on a preliminary study of the petrographical and geochemical characteristics of this igneous rock samples. The petrographic identification of andesite samples shows porphyritic, trachytic, oscillatory zoning, and sieve textures. The results of geochemical analysis (major and trace elements) show that the rock samples are are classified to andesite rocks and calc-alkaline suites. These rocks are enriched in LILEs (Rb, Ba, K) and depleted in HFSEs (Nb, Ti, Ce). Additionally, REE shows a slight enrichment of light-REE and a slight negative anomaly of Eu. The patterns of the trace elements including REE show a typical pattern of calc alkaline arc. Petrographical and geochemical characteristics suggest evidence of magma differentiation process, that is by a mechanism of crystallization fractionation. The andesite was formed in relation to a Middle – Late Miocene paleomagmatism and the Late Miocene-Pliocene subduction zone.

Geochemical constraints of hydrothermal alteration of dolostones: An example of Lower Cretaceous Qamchuqa Formation, Kurdistan Region, northern Iraq

This paper integrates major, minor, trace and rare-earth element (REE) data into C-, O-, Sr-isotopes, fluid-inclusions microthermometry of replacive matrix dolomite and vug/fracture-filling saddle dolomite and calcite of the Lower Cretaceous Qamchuqe Formation, the Kurditan Region of Iraq, in order to get further insights into the origin and geochemical evolution of their parent diagenetic fluids. Multiple generations of dolomite have been observed. These dolomites constitute replacive matrix dolomite and saddle dolomite cement, dominantly poorly ordered, non-ferroan, non-stoichiometric, with an average of 59.4 and 60 mol% CaCO3, respectively, and have low Sr, Fe and Mn concentrations. Replacive matrix dolomites display fairly comparable REE patterns to saddle dolomite, but with light REE (LREE) enrichment, and moderate low (∑REEs) contents (18.9–1111.5 and 16.6–593.4 ppb, respectively). Saddle dolomite has moderately positive Eu anomalies while replacive matrix dolomite has slightly less positive Eu anomalies and negative Dy, Ho and Yb anomalies. Both dolomites are characterized by slightly radiogenic 87Sr/86Sr ratios ranging from 0.707702 to 0.707839, overlapping, wide-ranging and negative δ18O values from −10.43 to −4.47 and −12.87 to −7.79 ‰VPDB, and δ13C values ranging from 0.38 to 3.57 and −3.45 to 3.11‰VPDB, respectively. The primary fluid inclusions microthermometry of replacive matrix and saddle dolomites yield homogenization temperatures (Th) ranging from 71.3 to 228.3 °C and 82–188 °C, respectively, and salinity values ranging between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These data suggest multiple hydrothermal fluid fluxes during and post Zagros orogeny. The geochemical data also show that these fluids derived from similar source of mixed hydrothermal and marine fluids, which caused the dolomitization of host rock and precipitation of dolomite cements.

Cumulate Mush Hybridization by Melt Invasion: Evidence from Compositionally Diverse Amphiboles in Ultramafic–Mafic Arc Cumulates within the Eastern Gangdese Batholith, Southern Tibet

Abstract Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic–mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates that they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE–HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, which is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest that the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25 to ~44 %. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate that the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart to residual melts a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt–rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution.

From subduction initiation to hot subduction: Life of a Neoarchean subduction zone from the Dengfeng Greenstone Belt, North China Craton

Abstract We report a spatially and temporally linked arc magmatic association of 2.55–2.50 Ga mid-ocean ridge basalt (MORB)-affinity gabbros, arc-affinity basalts, high-Mg basalts/basaltic andesites (HMBA), Nb-enriched basalts (NEB), and a tonalite-trondhjemite-granodiorite (TTG)-like granodiorite from the Dengfeng greenstone belt (DFGB), North China Craton. These are identical to arc magmatic rock suites from hot subduction in the present plate mosaic, and resemble suites formed during subduction initiation. Laser ablation–inductively coupled plasma–mass spectrometry zircon U-Pb dating indicates that gabbros and granodiorites intruded at ca. 2.55 Ga and the HMBAs formed at ca. 2.50 Ga, indicating a long duration of subduction zone magmatic activity of at least 50 m.y. in the DFGB. The gabbros are characterized by flat light-rare earth element (LREE) patterns without negative Nb and Zr anomalies, showing a MORB-like geochemical affinity. Combined with previously reported normal-MORB-type basalts, we suggest that they may represent magmatic products during intraoceanic subduction initiation where their mantle source was slightly metasomatized by subduction-derived fluids during the initiation stage. The arc-affinity basalts are characterized by slightly enriched LREE patterns with negative Nb and Zr anomalies. The HMBAs have higher contents of MgO, Ni, and Cr and display more fractionated rare earth element (REE) patterns and large negative Nb and Zr anomalies than the arc-affinity basalts. The NEBs are characterized by high absolute contents of Nb (9.9–14 ppm) and high ratios of Nb/Laprimitive mantle (pm) (0.40–0.46) and Nb/Thpm (0.44–0.54), and moderately fractionated REE and minor negative Nb anomalies with no Zr anomalies. One granodiorite sample is characterized by a highly fractionated REE pattern and negative Nb and Zr anomalies. Geochemical modeling suggests that the association of the normal arc-affinity basalt (NAB)-HMBA-NEB can be interpreted to be generated by hybridization of mantle wedge peridotites by slab-derived TTG-like melts. Positive values of whole-rock εNd (t) and zircon εHf (t) of the 2.55–2.50 Ga magmatic associations are consistent with a long-term depleted mantle source for the mafic assemblage. The 2.55–2.50 Ga magmatic association from MORB-affinity gabbros and basalts to arc-affinity NAB-HMBA-NEB may record a continuous Neoarchean geodynamic process from intraoceanic subduction initiation to mature arc magmatism. The NAB-HMBA-NEB association with regional sanukitoids and high-Al TTGs indicates that hot subduction may have played an important role in the production of arc-related magmatism during the Archean.

Petrogenesis of amphibole-rich ultramafic rocks in the Hida metamorphic complex, Japan: Its role in arc crust differentiation

Amphibole controls major and trace element chemistry of arc magma via fractionation, therefore its formation process is the key to understand arc crustal evolution. Amphibole-rich cumulates, occasionally found associated with felsic plutons or metamorphic rocks, provide complementary insights into the arc magma differentiation. In this study, we conducted a petrological examination of amphibole-rich ultramafic rocks from the Hida metamorphic complex in Japan to better understand its role in arc crust evolution. The studied samples are characterized by large poikilitic amphibole and orthopyroxene enclosing olivine grains with corroded margins. Amphibole showed high Cr2O3 and TiO2 contents (up to 0.9 and 1.2 wt%) and light rare earth element enrichment relative to heavy rare earth elements. Microscopic and geochemical features of amphibole and olivine suggest that the amphibole formed at the expense of early crystalized olivine (± clinopyroxene) from hydrous juvenile magma at a deep crustal level, which is consistent with the estimated crystallization pressure of 0.7–1.0 GPa based on an empirical Al-in-hornblende geobarometer. Considering the arc affinity of parental magma and the amphibole-fractionation trend in bulk chemistry of the Mesozoic plutonic rocks from the Hida Belt, it is implied the formation of amphibole-rich plutonic rocks contributed to arc crustal evolution as amphibole-rich residue complementary to differentiated magma.