Mafic Magmatic Rocks Research Articles

Continental crust recycling in collisional zones: insights from Li isotope compositions of the syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatism in continental collision orogenic belts may provide insights into the nature of orogenic lithospheric mantle and recycled continental components in continental subduction zones. Lithium and its isotopes have emerged as potentially valuable tools for shedding light on the origin of these magmas, given the contrast Li contents and isotopic compositions between the subducting continental crust and the mantle. Here, we present high-precision Li isotopes data for representative orthogneiss, continental eclogite, syn-exhumation and post-collisional mafic magmatic rocks from the North Qaidam orogen. The syn-exhumation mafic magmatic rocks have relatively higher Li contents (26.5–50.0 ppm) and lower δ7Li values (−1.01‰–1.48‰) than those of the post-collisional mafic magmatic rocks (Li = 11.1–22.7 ppm, δ7Li = 1.20‰–3.38‰), which are comparable to those of orthogneiss and continental eclogite, respectively. Dehydration and melting modelling results show that these mafic magmatic rocks have similar Li contents and δ7Li values to the continental eclogite- and orthogneiss-derived melts but are different from their derived fluids. Monte Carlo simulation for Li-Nd isotopes suggests the syn-exhumation and post-collisional mafic rocks could be derived from an enriched mantle source that contains ∼3–8% continental crustal components dominated by the orthogneiss and continental eclogite. The calculated results are consistent with the results from the previous study simulated by trace elements. Therefore, our results highlight Li isotopes as a potential tool to trace the nature of the continental crustal components recycling in continental subduction zones.

Identification of variable contributions to arc rocks by Li[sbnd]Mo and Sr–Nd–Hf–O isotope compositions: A study of the Mafan mafic rocks in the Hong'an orogen, Central China

The geochemistry of arc rocks provides a window to probe compositional variability in mantle wedge and slab-derived inputs. However, quantitative estimation of their contribution is a great challenge. Here we report an integrated study for determination of variable contributions to early Paleozoic arc plutons (the Mafan diorites and gabbros), from the northern Hong'an orogen, central China. Zircon grains from the diorites yielded crystallization age of 445 ± 3 Ma, whereas the gabbros have zircon UPb age of 432 ± 3 Ma. Although both the two mafic rocks suites exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li–Mo–O and radiogenic Sr–Nd–Hf isotope compositions, as well as major and trace element features. Geochemically, the diorites are medium-K calc-alkaline series with high and positive whole-rock εNd(t) (+3.8 to +4.5) and zircon εHf(t) values (+12.4 to +14.3), and normal mantle-like zircon δ18O values (5.1 ± 0.2 ‰). Contrarily, the gabbros are tholeiitic and have relatively low whole-rock εNd(t) (+ 2.3 to +3.1) and zircon εHf(t) (+5.0 to +8.5) values with normal mantle-like zircon δ18O values (5.4 ± 0.3 ‰). The diorites have δ98Mo values of −0.15 to −0.03 ‰, while the gabbros have δ98Mo values of −0.28 to −0.12 ‰. These values are slightly higher than those of the mid-ocean ridge basalts (MORB) (−0.20 ± 0.01 ‰). Whereas, δ7Li values of the studied diorites (2.15–4.49 ‰) and gabbros (2.89–5.33 ‰) are mostly consistent within the range of MORB. In combination with the above results, the primary magma of diorites was inferred to be derived from 1 to 5% partial melting of depleted lithospheric mantle metasomatized by slab-derived fluid with minor sediment-derived melts. While the gabbros were suggested to be generated by 5–10% partial melting of more depleted lithospheric mantle that had undergone perhaps extra 0.3% melt extraction, which had further been metasomatized by more subducted materials. In summary, both the slab-derived components and nature of the mantle wedge sources are quantitatively constrained for the mafic rocks in the Mafan plutons. Our new data suggest that the Mafan plutons exhibited the Mariana arc-affinity and represented products of an intra-oceanic subduction system in the northern Hong'an orogen. Combined with previously published data for the early Paleozoic mafic magmatic rocks along the Qinling-Tongbai-Hong'an orogenic belt, there exists an oceanic arc in the east (Hong'an orogen) and an Andean-type continental arc in the west (Qinling-Tongbai orogen).

Carbon-rich polyphasic inclusions in postcollisional mafic magmatic rocks from the Dabie Shan, China: Implications for the carbon cycle in continental subduction zones

Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.

Petrogenesis and Tectonic Implications of the Oligocene Dalongtan Shoshonitic Syenite Porphyry in Central Yunnan, Southeastern Tibetan Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd-Hf Isotopes

Shoshonitic rocks are widely distributed in post-collisional settings and provide key information on deep geodynamic mechanisms and magmatic evolution. In this paper, we present petrographic, zircon U-Pb age-related, trace elemental, Hf isotopic, bulk-rock elemental, and Sr-Nd isotopic data of the Dalongtan shoshonitic syenite porphyries (DSSPs) in central Yunnan, southeastern Tibet. The DSSPs formed at 33.2 ± 0.3 Ma in a post-collisional setting. They define linear trends on Harker diagrams, and they display similar trace element patterns and enriched bulk-rock Sr-Nd isotopes [(87Sr/86Sr)i = 0.70964–0.70968, εNd(t) = −12.9 to −12.7] and zircon Hf isotopes (εHf(t) = −15.7 to −13.1) to the coeval mantle-derived potassic mafic rocks. This suggests that the DSSPs were fractionated from the lithospheric mantle-derived mafic magmas. The DSSPs, along with the coeval felsic and mafic magmatic rocks (37.2–32.3 Ma), exhibit a planar distribution on the SE Tibet and predate the left-lateral shearing of the Ailaoshan–Red River shear zone (ARSZ) (32–22 Ma), suggesting that there are no genetic relationships between them. The DSSPs have geochemical characteristics similar to those of A-type granites, with high total alkalinity (10.39–11.17 wt.%), HFSE concentrations (Zr + Nb + Ce + Y = 890.2–1054.3 ppm), Ga/Al ratios (10,000 × Ga/Al = 2.95–3.46), whole-rock zircon saturation temperatures (906–947 °C), and oxygen fugacity (ΔFMQ = +3.30–+4.65), indicating that they are products of the high-temperature melting of the lithosphere as a result of asthenosphere upwelling in extensional settings. Based on our data and regional observations, it is proposed that the generation of the DSSPs may be linked to the convective thinning of the thickened lithospheric mantle following the India–Asia collision.

Application of aeromagnetic survey to mineral exploration of Jinping, Yunnan, China by using multirotor UAV

Jingping, Yunnan, China, is an unfriendly area for ground survey due to the high altitude, rough terrain and dense vegetation. To detect the concealed magnetite ore bodies in this area, the aeromagnetic survey in a 7.8 km2 region was carried out. The field experiments were conducted to investigate the source and level of the noise. Then, the proton magnetometer was placed 3 m under the multirotor unmanned aerial vehicle (UAV) to decrease the static interference. The inverse distance weighting interpolation algorithm with a nonlinear filter was proposed to suppress the noise and dynamic interference caused by the strong wind. The results show that these methods can alleviate the interference of the UAV rotor and strong wind effectively. The patterns of the data from our aeromagnetic survey agree well with the horizontal distribution of the magnetic strata deduced from the geological background. Furthermore, the concealed mafic magmatic rocks and the titanium magnetite inferred from the aeromagnetic survey are confirmed by 4 drill logs in the study area, which supports the validity of the UAV aeromagnetic survey.

Heterogeneous orogenic lithospheric mantle beneath the North Qaidam orogen: Geochemical evidence from syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U Pb ages and Hf O isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon U Pb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn -exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ 18 O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn -exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn -exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn -exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone. • Syn-exhumation/post-collisional mafic magmatic rocks in the North Qiadam orogen. • They were derived from a heterogeneous orogenic lithospheric mantle. • The mantle source was metasomatized by melts from the subducted continental crust. • These rocks record crust-mantle interaction in the continental subduction channel.

Geochronological constraints of high-grade metasedimentary rocks of the Italva and Costeiro basins: Reconstructing the Outer Magmatic Arc System of the ribeira belt, SE Brazil

U-Pb provenance studies of the magmatic arc-related basins of the Ribeira Orogen are essential to unravel the depositional history and the paleogeography prior to the diachronic collision that has resulted in Neoproterozoic to Cambrian Western Gondwana assembly. This work brings new geology and LA-ICPMS data from metasedimentary units and felsic to mafic magmatic rocks that compose the Outer Magmatic Arc System (OMAS) in the Ribeira segment of the Araçuaí-Ribeira Orogenic system (AROS), contributing to unravel the construction of the West-Gondwana supercontinent. The OMAS evolution started in Tonian time (860–790 Ma) as the juvenile intra-oceanic Serra da Prata arc and evolved into the immature continental Rio Negro arc during the Cryogenian-Ediacaran times (650–620 Ma). The long-term subduction revealed the coeval stages development of arc-related basins. In the first Tonian stage, basin development is associated with the Italva group, divided into two sequences representing both fore and back-arc settings. The forearc sequence (Euclidelândia unit) comprises meta-psammitic sedimentary rocks with a single provenance source from the SP arc (peaks at 812 Ma and 828 Ma). The back-arc sequence (São Joaquim unit) consists of marbles with MORB-like interlayered amphibolites (ca. 831 Ma) bracketing the maximum age of carbonatic platform deposition. On the other hand, the younger Cryogenian to Early Ediacaran Costeiro back-arc basin is represented by three units of the São Fidélis group. The proximal Macuco unit, comprising meta psammitic rocks with marble, yielded a bimodal provenance distribution (852–815 Ma; 643–634 Ma), which reflects only the SP and RN magmatic arcs sources. The pelitic distal Upper and Lower units point out multimodal sources, coming from the SP-RN arc-rocks (ca 950–734 Ma; ca. 693–621 Ma) and Archean to Mesoproterozoic sources (ca. 3.12–3.0 Ga; 2.98–2.49 Ga; 2.29–1.50 Ga; 1.49–1.01 Ga), derived from ancient cratonic blocks located eastern wards, in African side. All units of the OMAS recorded two metamorphic episodes, with the oldest M1 (ca. 600–590 Ma) related to the collision of the OMAS against the Inner Magmatic Arc System (IMAS) and the younger M2 (ca. 545–533 Ma) that resulted from the final docking of the Cabo Frio terrane (Angola block) and finally closed the OMAS back-arc basin. Data obtained reinforces the long-term E-verging subduction that built the OMAS.

The Role of Heavy Minerals in Understanding the Provenance of Sandstone: An Example from the Upper Cretaceous Tanjero Formation, Surdash Region, Northeastern Iraq

The Tanjero Formation is the Upper Cretaceous (Campanian-Maastrichtian) crops out in northeastern Iraq at the High-Folded and Imbricate Zones. To study heavy minerals, twenty sandstone samples were obtained from selected sites of the Tanjero Formation exposed at the southwestern limb of the Surdash anticline in the northeast part of Sulaimanyia city. The analyzed data indicates that the most abundant heavy minerals are opaque, followed by pyroxene, epidote, amphibole, chlorite, rutile, and garnet. The significant abundance of unstable and metastable minerals in the heavy mineral suites shows their direct derivation from the adjacent main basic igneous and metamorphic source rocks, which are situated in the High-Folded and Imbricate Zones in the northern and northeastern parts of Iraq. The estimated zircon-tourmaline-rutile index reveals that the majority of the sandstone samples have less than 75% zircon-tourmaline-rutile index, indicating that the sediments are generally immature. Furthermore, the relationship between an ultra-stable and a meta-stable heavy mineral indicated the immaturity of the Tanjero Formation sandstone and its moderate stability, proving that these minerals cannot be transported over extremely long distances close to the source rocks area. The heavy mineral assemblage demonstrated that the studied area is distinguished by high tectonic activity. In addition, according to the MF- MT-GM ternary diagram, the analyzed sandstone samples lie within the active continental margins field, which is distinguished by a higher percentage of MF minerals than GM minerals generated from mafic magmatic source rocks.

Geochemical evidence for a widespread Paleoproterozoic continental arc-back-arc magmatism in the Lesser Himalaya during the Columbia supercontinent assembly

The geodynamic evolution of the proto-northern Indian continental margin during Paleoproterozoic is widely debated. The proposed models include (i) a mantle plume triggered rift-related tectonic setting at a passive continental margin and (ii) an Andean type active continental margin. A widespread quasi-continuous belt of Paleoproterozoic (∼1.9–1.8 Ga) granitoids and orthogneisses, as well as mafic volcanic and intrusive rocks intercalated with metasedimentary rocks, occur all along the strike of Lesser Himalaya. A geochemical re-investigation is presented here using the existing geochemical data on mafic volcanic and intrusive rocks from the Lesser Himalayan Sequence of Himachal, Garhwal, and Nepal sectors to address the tectonic affiliation of these rocks and to infer the geodynamic setting of the proto-northern continental margin of the Indian Shield. These mafic magmatic rocks are classified as subalkaline basalt and basaltic andesite with calc-alkaline magmatic affinity to transitional character between calc-alkaline and tholeiites. Broad initial ɛNd range reveals a heterogeneous mantle source developed by mixing between a depleted mantle and a recycled component. Mantle source enrichment of these rocks is attributed to the interaction between the siliceous partial melt from the subducted sediment-laden oceanic crust and the mantle wedge, which is further supported by compositional similarity with experimental hybrid partial melts of eclogite-peridotite reaction. Estimated mantle potential temperature from the composition of these mafic rocks is comparable to that of mid-oceanic ridge basalts and does not invoke a mantle plume in their genesis. Geochemical proxies based on the major oxide abundances reveal the generation of these rocks from olivine-poor lithologies. It is suggested that these rocks were generated in a back-arc setting during the assembly of the Columbia supercontinent at the proto-northern continental margin of the Indian Shield. Distinct Nd isotopic systematics in these rocks compared to the Paleoproterozoic metamafic rocks of Cathaysia Block of South China Craton questions their purported linkage in the Columbia configuration.

Geochronology, petrogenesis and tectonic significance of two episodes of Neoproterozoic diabasic magmatism in South China: from orogenesis to intracontinental rifting

ABSTRACT Neoproterozoic mafic magmatic rocks are widely distributed in the Jiangnan Orogenic Belt. Understanding the petrogenesis and tectonic settings of these mafic rocks has significant implications for the regional tectonic evolution and the assembly and breakup of supercontinent Rodinia. In this research, two Neoproterozoic diabase samples (Baotan and Huaihua) were investigated by zircon and baddeleyite U–Pb dating with the ages of 805 Ma and 788 Ma, respectively. The Baotan samples are depleted in HFSE, enriched in LILE and have enriched Sr–Nd isotope compositions (87Sr/86Sr(i) = 0.731620 to 0.745440, εNd(t) = – 3.7 to – 7.6). We suggest that they were derived from an enriched lithospheric mantle in a convergent tectonic setting, which was metasomatized by contemporaneous slab–derived hydrous fluids. On the contrary, the Huaihua diabase samples have trace element patterns similar to oceanic island basalts but little HFSE depletion and slightly enriched Sr–Nd isotopes (87Sr/86Sr(i) = 0.703101 to 0.705006, εNd(t) = – 0.1 to – 1.6). We infer that they originated from partial melting of a convective asthenospheric mantle under an extensional environment, which was previously metasomatized by slab–derived melts. In combination with other geological observations, we proposed a geodynamic evolution model for the South China Block from the mid–Neoproterozoic orogenesis to the late Neoproterozoic intracontinental rifting.

About this title - Lamprophyres, Lamproites and Related Rocks: Tracers to Supercontinent Cycles and Metallogenesis

Paleoproterozoic to Cenozoic lamprophyres, lamproites and related rock types (e.g., orangeites, kimberlites) are volatile-rich mafic magmatic rocks with a unique potential for the investigation of processes affecting mantle reservoirs. They originated from primary mantle-derived melts that intruded both cratons and off-craton regions, which were parts of former supercontinents – Columbia, Rodinia and Gondwana–Pangea. Well-known for hosting economic minerals and elements such as diamonds, base metals, gold and platinum-group elements, they are also significant for our understanding of deep-mantle processes, such as mantle metasomatism and mantle plume–lithosphere interactions, as well as large-scale geodynamic processes, such as subduction-related tectonics, and supercontinent amalgamation and break-up. This book aims to provide a timely overview of the state-of-the-art and recent advances as achieved by various research groups around the world. Mineralogical, geochemical, geochronological and isotope analyses are used to decipher the complex petrogenesis and metallogenesis of these extraordinary rocks, and unravel a complete history of tectonic events related to individual supercontinent cycles.

Hf-Nd isotopes from ultramafic and mafic rocks in the western Dharwar Craton, India, record early Archean mantle heterogeneity

Both primary mantle differentiation and early extraction of oceanic and continental crust can have resulted in a chemically heterogeneous Archean mantle. Evidence for heterogeneity of the Archean mantle is preserved in the correlated Hf and Nd isotopes in mafic (basaltic) magmatic rocks and their ‘decoupling’ in ultramafic rocks, especially in komatiites, from the Archean Dharwar Craton, India. The komatiites and komatiitic basalts from the western Dharwar Craton were emplaced at ~3.25 Ga and have large spread in initial radiogenic isotope compositions with initial ɛHf (ranging from +0.9 to +21.3) and initial ɛNd from +0.2 to +7.4 that do not correlate. In contrast, the ɛHfi and ɛNdi in contemporaneous mafic (ɛHfi = +5.9 ± 11.2; ɛNdi = +3.2 ± 7.6) and felsic (ɛHfi = +4.3 ± 4.6; ɛNdi = +2.7 ± 4.2) rocks in the same area correlate, as is typical of rocks that are the product of magmatic processes involving melting and magmatic differentiation. The high initial ɛHf and the lack of correlation (‘decoupling’) of ɛHf and ɛNd in the komatiites and komatiitic basalts indicate that the source had superchondritic Lu/Hf which is attributed to early mantle differentiation at great depth, possibly in the perovskite stability field. This finding indicates that the primordial mantle was mineralogically and chemically layered and this layering was the result of fractionation processes very early in Earth's history, probably during magma ocean solidification. During crystallization of a primordial magma ocean, fractionation of Mg-perovskite with minor Ca-perovskite component could have resulted in a melt with high Lu/Hf and nearly unfractionated Sm/Nd. The correlation of the initial Hf isotope composition of the komatiites and komatiitic basalts with their MgO content, and thus melting temperatures, indicates the presence of a highly fractionated component derived from the deep mantle. The coupling of ɛHf-ɛNd in mafic to felsic igneous rocks and the decoupling in contemporaneous ultramafic rocks is thus the result of mantle material originating from different depths involving different ancient fractionated reservoirs. The existence of a chemical and isotopically heterogeneous Archean mantle that supplied material to form the continental crust has to be accommodated in crustal growth and preservation models that commonly assume a nearly homogeneous and gradually depleting mantle reservoir as the source of the continental crust.

Coexisting Early Cretaceous arc-type and OIB-type mafic magmatic rocks in the eastern Jiangnan Orogen, South China Block: Implications for paleo-Pacific plate subduction

Mafic magmatic rocks can provide important information about the nature of mantle source, material recycling, and mechanism of crust–mantle interaction. However, owing to the scarcity of such rocks, the nature and evolution of the Late Mesozoic mantle source beneath southeastern China have not been well constrained. Here, we present data on zircon UPb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopes for two types of simultaneous mafic intrusive rocks from the eastern Jiangnan Orogen (EJNO), with the aim of providing insights into their petrogenesis and the relationship of extensive Late Mesozoic magmatism in southeastern China and subduction of the paleo-Pacific plate. The Qukou diabases (~ 128 Ma) show a subalkaline affinity and have a mean Mg# value of 53.4. They exhibit arc-like trace-element distribution patterns, namely, enrichment in light rare earth elements (LREEs), Ba, Th, and U, and depletion in Nb, Ta, P, and Ti, as well as moderately radiogenic Sr and chondritic Nd isotopic compositions, with 87Sr/86Sr(i) = 0.7057–0.7063 and εNd(t) = −1.01 to 1.13. In contrast, the Yangboping gabbros (~ 125 Ma) belong to alkaline basalt and have a mean Mg# value of 56.8. They present OIB-like trace-element characteristics, show enrichment in LREEs, Sr, and Ba, and no depletion in Nb, Ta, Zr, Hf, and Ti, and possess highly radiogenic Sr and weakly nonradiogenic Nd isotopes of 87Sr/86Sr(i) = 0.7096–0.7114 and εNd(t) = −0.97 to −0.11. These geochemical features suggest that the Qukou diabases were generated by partial melting of a pyroxenitic source region, and this pyroxenitic mantle source was formed via the metasomatism by HFSE-depleted hydrous melts of subducted slab-derived terrigenous sediments, whereas the Yangboping gabbros were generated from a metasomatite that formed by metasomatic reaction between depleted MORB mantle and subducted slab-derived HFSE-enriched hydrous melts, in which the ratios of altered oceanic crust-derived melts to sediment-derived melts were 90:10–60:40. The differences in trace-element and isotopic compositions of these two types of mafic magmatic rocks suggest that their mantle sources were metasomatized by hydrous melts from different depths: the HFSE-depleted hydrous melts were from the shallow depths (<80 km), whereas the HFSE-enriched melts were from the greater depths (>200 km), indicating that the subducted oceanic slab would have founded/broken off to provide enough heat to melt the subducted oceanic crust. Consequently, we propose that the flat-slab subduction and slab break-off model of the paleo-Pacific plate was the most likely geodynamic mechanism for Late Mesozoic magmatism in the South China Block.

Experimental study on soda granites of the Singhbhum Craton, eastern India, and its petrogenetic significance

The soda granite (~Paleoproterozoic) of the Singhbhum Craton (SC) of eastern India occurs along the entire stretch of the Singhbhum Shear Zone (SSZ; or Copper Belt Thrust Zone). The SSZ (marking a thrust contact between the adjacent North Singhbhum mobile belt and SC) serves as the important locale to host apatite‐magnetite, copper sulphide, and uraniferous mineralization. Typical field occurrence of soda granite is either as tongues (or patches) within a shear zone or as lit‐per‐lit style injections into surrounding country rocks which are generally highly sheared and mylonitized. Experimental studies on granite bodies can potentially throw light on their mineralogical and petrogenetic aspects; however, these studies are hitherto unreported in case of soda granites for almost the last 60 years. In this view, during the present research, experimental studies on soda granites have been carried out and these decipher the nature of the stability relationship of K‐feldspar (and transgression to more disordered anorthoclase), amphibole (both Ca‐amphibole and Fe–Mg–Mn amphibole and breaking down of Ca‐amphibole to Fe–Mg–Mn amphibole + non‐quad pyroxene + quartz at an appropriate temperature), biotite, muscovite, and albite (high albite‐ low albite displacive transformation). Whole‐rock chemistry (deduced from homogenized glass compositions) indicates its peraluminous, S‐type nature with an affinity towards Caledonian post‐collisional plutons. Characteristic geochemical plots suggest a pelitic source material for melting (and subsequent internal differentiation of the melt) and this has given rise to both migmatite as well as patchy soda granite exposures in the field. It is suggested that partial melting of mainly metapelitic (and minor mafic magmatic rocks) source (belonging to Singhbhum Series) gave rise to soda granites in this region.

Rhaetian tectono-magmatic evolution of the Central Atlantic Magmatic Province volcanism in the Betic Cordillera, South Iberia

Volcanic rocks of the Zamoranos Formation exposed in the Betic Cordillera of southern Spain reveal Late Triassic CAMP volcanism extending from southwest to southeast of the Iberian massif. Comparison with volcanic rocks from the Upper Triassic of the Algarve region in Portugal enables correlation between these different magmatic provinces based on the striking geochemical similarity. The studied mafic magmatic rocks in the Betic Cordillera reveal three volcanic episodes during the deposition of the Zamoranos Formation with two types of geochemical affinity: alkaline (Group 1) and sub-alkaline (Group 2 and 3). The sub-alkaline rocks resemble the studied Triassic rocks of Portugal and the magmatism described as CAMP on a global scale. In the present study, the identification of alkaline rocks preceding the main CAMP sub-alkaline phase by ca. 15 Ma, is the first evidence of alkaline precursors prior to CAMP magmatism and indicates a change from a prior deeper enriched mantle source to a subsequent, shallower one. The temporal evolution of alkaline magmatism followed by a CAMP-type sub-alkaline magmatism is best explained in a context of initial rifting during Norian-Rhaetian times. It is inferred that rifting gradually developed from west to east in the southern area of Iberia.

Lower Paleozoic rifting event in Central Iberian Zone (central-north Portugal): Evidence from elemental and isotopic geochemistry of metabasic rocks

The occurrence of Lower Paleozoic mafic magmatic rocks in the Central Iberian Zone (CIZ) of the Variscan Orogen is rare. Amphibolites and metagabbros embedded in the metasediments of the Douro-Beiras Supergroup outcrop at Farminhão, Viseu (central-north Portugal). The protoliths of these two rock types are tholeiites presenting different isotopic signatures (εNd480 = +4.63 to +4.93 and +5.74 to +7.67) and incompatible element ratios (normalized La/Lu up to 4.5 and down to 0.7), respectively which suggests they are not cogenetic. The closely related meta-ultramafic rocks are considered as cumulates generated from the magmas that originated the metagabbros. The elemental and isotopic features of the metagabbros are similar to those reported for amphibolitic occurrences in Tenzuela (near Segovia, Spain) which allows the proposition of a similar age for the mafic rocks of Farminhão. Despite the depleted characteristics of the studied rocks, they are interpreted as having been formed during a continental rifting process characterized by variable degrees of stretching, some 100 Ma after the end of the deposition of the Douro-Beiras Supergroup. This rifting event marks the onset of the Variscan Cycle (s.l.) in the Central Iberian Zone. The occurrence of these metabasic rocks near the confluence between the Porto-Viseu Metamorphic Belt and the Juzbado-Penalva do Castelo Shear Zone suggests that these first-order structures may have worked as weakness zones constraining the ascent of magmas during the Ordovician. The Lower Ordovician metabasic rocks here studied are chemically similar to the abundant lower to medium Cambrian magmatic rocks of the Ossa Morena Zone, also in Iberia, further reinforcing the diachronous character of the opening of the Rheic Ocean that later propagated to the eastern sectors of the European Variscan Belt.

Heavy Minerals Study of Sandstone from the Late Miocene-Early Pliocene Mukdadiya Formation; Kirkuk, Iraq: Implications for Provenance

Mukdadiya Formation (Late Miocene-Pliocene) exposed in the northeastern limb of Baba anticline fold in Kirkuk structure. The selected section was located in the Shoraw area, northeastern Kirkuk city, Iraq. Twenty sandstone samples were collected to study heavy minerals. The study indicates that opaque and epidote group minerals forming the main heavy minerals, followed by amphibole, pyroxene, garnet, and chlorite. According to heavy minerals assemblage, the source rocks are interpreted to be composed essentially of sedimentary followed by igneous and metamorphic rocks and the high contents of unstable and metastable minerals confirm their direct derivation from the adjacent primary source. Ultra-stable and metastable heavy minerals relationship indicated that the sandstone of the Mukdadiya Formation is immature and moderate stability and showed that these minerals couldn't be transported for very long distances close to the source area and not represents polycyclic grain. MF-MT-GM Ternary diagram showed that the studied samples fall within the field of active continental margins which is characterized by a relatively high percentage of minerals (MF˃GM) derived from mafic magmatic rocks.

Nature of the pre-collisional lithospheric mantle in Central Tibet: Insights to Tibetan Plateau uplift

The properties of accreted terranes in a large collisional orogen determine, in large part, the nature and subsequent development of the orogen. In this paper we present a detailed study of Late Mesozoic sodic mafic magmatic rocks from Central Tibet. These sodic mafic rocks from Bandatso provide an excellent opportunity to assess the nature and evolution of pre-collisional lithospheric mantle in the Himalayan–Tibetan region, and will thus aid our understanding of the evolution of the orogeny. The Bandatso sodic mafic rocks are characterised by marked oceanic island basalt (OIB)-like enrichment in incompatible elements, such as Nb (up to 139 ppm) and light rare-earth elements. This Late Mesozoic (ca. 148 Ma and 90 Ma) mafic magmatism is likely to be the result of two-stage mantle evolution in the northern Qiangtang. The late Jurassic rocks were derived from carbonated lithosphere metasomatized by subducted oceanic sediments during flat subduction. Whereas the late Cretaceous basanites were likely derived from interaction of lithosphere with upwelling asthenosphere induced by removal of lithosphere that had previously been thickened by relamination. Our study suggests the presence of an inherited carbonated lithospheric mantle beneath the northern Qiangtang prior to continental collision. The conclusions from this work suggest that the enrichment of the mantle source of Cenozoic potassic rock closely related to plateau uplift, more likely result from continental subduction after the late Cretaceous (ca. 90 Ma). These conclusions will help our understanding of the Cenozoic geology of this multi-terrane collision system and uplift mechanism of the Tibetan Plateau.

Depositional environment - The original control on gold processing

SYNOPSIS Methods for treating and processing refractory gold ores are well established. However, what is less well understood is how the formation of the gold-bearing deposits affects gold processing and extraction. In order to evaluate the effect of ore genesis on gold extraction a number of South African gold deposits were studied. These included complex Pilgrim's Rest samples as well as refractory Fairview, Barbrook, and Consolidated Murchison samples. We found the refractory nature of gold ores is controlled by a limited number of well-understood mineralogical factors. Solid solution gold is linked to low temperature and pressure conditions in fine-grained sedimentary lithologies, while reactive pyrrhotite can form from hydrothermal fluids associated with mafic magmatic rocks. These formational controls can be used to identify and avoid complex deposits at a desktop study phase, or address and reduce complications further along the pipeline using early mineralogical studies. Keywords: gold processing, refractory gold, ore mineralogy.

Geochemical and geochronological record of the Andaman Ophiolite, SE Asia: From back-arc to forearc during subduction polarity reversal?

Ophiolites are widely studied to unravel how new subduction zones form. They may contain crustal and mantle rocks that formed during juvenile stages of intra-oceanic subduction, modifying the pre-existing oceanic lithosphere within which subduction started, and in which a magmatic arc formed upon subduction maturation. Previous geochemical work on the Cretaceous Andaman Ophiolite and its metamorphic sole, located in the forearc of the Sunda subduction zone of south-east Asia, has revealed geochemical signatures that are difficult to reconcile in a single tectonic setting but may rather record different stages in a longer evolution. A recent kinematic reconstruction proposed that the Andaman Ophiolite may have formed during subduction initiation in a former back-arc basin, when the former (Woyla) arc collided with the Sundaland continent. Here, we evaluate whether such a scenario may provide a feasible context to explain the enigmatic geochemical signatures preserved in the Andaman Ophiolite. To this end, we provide new, and review existing geochemical as well as geochronological constraints on the formation of its crustal rocks, as well as the evolution of its mantle portion. We identify mafic magmatic rocks and cogenetic plagiogranites that are consistent with formation in a magmatic arc, whereas other magmatic rocks, as well as metamorphic sole protoliths, have characteristics indicative of a back-arc origin. Three new, and two previous zircon U/Pb ages of arc magmatic rocks give a 99–93 Ma age range, but we also identify an inherited ~105 Ma age. This latter age coincides with Ar/Ar cooling ages of the Andaman metamorphic sole, and with a plagioclase xenocryst age from recent Barren Island volcanics east of Andaman. The geochemical and geochronological constraints of the Andaman ophiolites are straightforwardly explained in the context of the regional kinematic history: (1) The original lithosphere formed in the back-arc basin of the Woyla intra-oceanic arc; (2) Subduction initiation and SSZ ophiolite formation within this basin occurred around or slightly before 105 Ma; (3) This was followed by arc magmatism between 99 and 93 Ma upon subduction maturation.