Assimilation And Fractional Crystallization Processes Research Articles

Petrogenesis of the granitoids related to skarn-type mineralization in the Nyainqentanglha Metallogenic Belt, Tibet

The Nyainqentanglha Metallogenic Belt (NMB) is an economically important lead-zinc ore province located in the Central and Southern Lhasa subterrane, Tibet. The NMB consists mainly of skarn-type lead-zinc polymetallic deposits that form at the contact between Late Cretaceous-Eocene intrusive rocks and carbonatite/volcanic-sedimentary strata. These deposits are generally considered to be related to S-type granites formed by the Indo-Asian continental collision. However, the εHf(t) (ave. −1.6) of zircons from the mineralization-related granitoids indicates that it is crust-mantle mixing products. In addition, the volcanic rocks of the Dianzhong Formation, which have a close spatio-temporal relationship with the skarn-type deposits, typically show mantle-derived features. Therefore, the occurrence of AFC processes in mantle-derived material passing through the thick basement of the Lhasa Terrane may better explain the genesis of the mineralization-related granitoids. In this study, we present results on zircon U–Pb ages, major and trace elements of the granitoids associated with skarn-type mineralization from the Narusongduo district. LA-ICP-MS zircon U–Pb dating shows that the granite formed in the Paleocene (58.6 ± 0.5 Ma). Combined with published regional data, we propose the mineralization age of the skarn-type deposits in the NMB ranges from the Late Cretaceous to Eocene. Detailed petrology and geochemistry of mineralization-related granitoids are evaluated to constrain the magmatic evolution process. The granitoids have high contents of SiO2 (mean 72.8 %), K2O (mean 4.0 %), Rb (mean 186.9 ppm), DI (differentiation index) 84.7, A/CNK (mean 1.3), and low contents of MgO, TFe, suggesting that the granitoids have undergone strong differentiation. In addition, the continuous decrease of P2O5 with progressive differentiation and the lower average P2O5 abundance suggest that the mineralization-related granitoids belong to the I-type granite. The results suggest that the mineralization-related granitoids in the NMB originated from the mantle and is the fractionated I-type granite formed by the process of AFC (Assimilation and Fractional Crystallization). Considering that Pb and Zn often coexist in deposits, we suggest that magmatic differentiation may play an important role in the formation of the granite-related Pb and Zn mineralization.

Monazite record of assimilation and differentiation processes in the petrogenesis of Himalayan leucogranites

The Himalayan leucogranites have received particular attention as they record the continental crust-building processes of a collisional orogenic system. Deciphering the magmatic evolution processes and potential sources is essential for understanding the origin of the Himalayan leucogranites. In-situ monazite Th-Pb ages, trace element geochemistry, and Nd isotope ratios have been determined with whole-rock geochemical data for the leucogranites and their country rocks in the Yadong area, southern Tibet. Monazites in most of the studied leucogranites are marked by multiple age populations with variable REE ratios and wide ranges of Nd isotope ratios, reflecting a complex history of magmatic processes. Abundant inherited monazites are identified in the Yadong leucogranites, and most of them are excluded from the source regions due to their distinguished Nd isotope compositions from the magmatic monazites. The inherited monazites share close affinities in geochronological dates and geochemical compositions with those in the country rocks, strongly suggesting a major origin of the country-rock contamination. This highlights the contamination-assimilation processes in the genesis of the Yadong leucogranites. It is worth noting that xenocrystal monazites from the metasedimentary country rock are less preserved in the leucogranites than those from the granitoid country rock, which probably reflects the different assimilation reactions for different types of country rock. Assimilation of the metapelitic rocks released low content of xenocrystic monazite and high volume of partial melt into the leucogranites, whereas assimilation of the granitoid rocks released high content of xenocrystic monazite. In addition to the contamination-assimilation processes, the uniform Nd isotope data with large variations in REE ratios for magmatic monazites in some leucogranite samples suggest a subsequent closed-system fractional crystallization process during the magma evolution. Besides, some magmatic monazites in the muscovite granites show a significant tetrad effect in the Chondrite-normalized REE patterns, indicating that the magma system has evolved into an aqueous fluid-available environment. This supports an important role of the magmatic differentiation process in the formation of the Yadong leucogranites. Taken together, a petrogenic model involving AFC (assimilation and fractional crystallization) processes is proposed, and the geochemical variations of the Yadong leucogranites are mainly attributed to the post-source processes. It is suggested that the rock geochemistry of the Himalayan leucogranites often dose not directly reflect the nature and the petrogenetic processes in the source regions.

Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

The Oytag pluton in the West Kunlun orogenic belt of China represents the rare exposed Carboniferous granitoids associated with the early-stage evolution of the Paleo-Tethys Ocean, and its origin is controversial. New zircon U-Pb data suggest an emplacement age of 327–324 Ma for trondhjemite, which slightly predated tonalite (∼314 Ma). These results, along with available published ages, indicate several episodes of magmatism to build the Oytag pluton. Both trondhjemite and tonalite have positive whole-rock Nd isotopic compositions (ε Nd (t) = +4.56 to +8.01 for trondhjemite; ε Nd (t) = +4.68 to +6.15 for tonalite), which positively correlate with whole-rock MgO values. Given the presence of Ordovician inherited zircon grains, variations of whole-rock Nd isotopic compositions are attributed to addition of <10% crustal contaminants during assimilation and fractional crystallization (AFC) processes. Interestingly, trondhjemite and tonalite follow distinct AFC trends, consistent with their different ages. Zircons from two trondhjemite samples have overlapping U-Pb ages but show distinctly different trace elements and internal structures (sector-zoned vs. oscillatory-zoned). The sector-zoned zircons have a less evolved signature of lower Hf contents, higher Eu/Eu* and Th/U ratios than the oscillatory-zoned ones, indicating significant crystal fractionation processes for their parental melts. The more variable Hf contents of the oscillatory-zoned zircons than those of the sector-zoned ones reflect a more dynamic environment where they crystallized. We propose that the sector-zoned zircons were crystallized from locked, interstitial melt in a mush and the oscillatory-zoned zircons from extracted melt. In situ crystal-liquid segregation in a mush chamber thereby could be an efficient approach to the differentiation of Oytag trondhjemite. On the other hand, zircons from tonalite have similar Hf contents to the sector-zoned zircons from trondhjemite, but display slightly higher Eu/Eu* ratios and much higher Th/U ratios. This indicates that tonalite should have experienced much different AFC processes from trondhjemite. Both the episodic magmatism feature and the distinct differentiation processes argue for rises of different batches of magmatism to generate trondhjemite and tonalite in Oytag. Along with the large variations of Hf isotopes (5–6 epsilon units) in zircons, we propose a partial melting of juvenile mafic crust model for the Oytag pluton. Heat from upwelling of asthenosphere may have triggered the partial melting of wet, mafic underplates during the opening of a Paleo-Tethyan back-arc basin. • The Oytag pluton comprises predominant trondhjemite with minor tonalite. • Oytag trondhjemites (327–324 Ma) formed earlier than tonalites (∼314 Ma). • The Oytag pluton was built by different batches of magmas via partial melting.

The role of assimilation and fractional crystallization in the evolution of the Mars crust

Recent findings of evolved martian crustal materials by orbiters and rovers challenged the longstanding notion of an only-tholeiitic basaltic crust. Models exploring intraplate magmatic processes like fractional crystallization show that unrealistic amounts of solid accumulation are sometimes required for the formation of martian evolved materials, including evolved materials analyzed by the Curiosity rover at Gale crater. Assimilation and fractional crystallization (AFC) play an important role in the diversification of magmas on Earth, but has not been explored for its role in the generation of evolved martian compositions. In this study, we conduct a series of thermodynamic AFC models using the Magma Chamber Simulator (MCS) model interfacing with the Rhyolite-MELTS algorithm to better understand the controls on assimilation and whether it contributed to the evolution of the martian crust. We find that AFC models replicate evolved Gale crater targets effectively at different pressures and water contents. AFC likely contributed to martian crustal formation, particularly evolved compositions. Higher crustal temperatures likely lead to a higher volume of evolved magmas formed by AFC processes on early (rather than modern) Mars. AFC may have also occurred during the Amazonian, especially in the fractured upper layers of crust, although in lower volume than during the Noachian.

Newly discovered Early Carboniferous and Late Permian magmatic rocks in eastern Myanmar: Implications for the tectonic evolution of the eastern Paleo-Tethys

Eastern Myanmar is located at the junction of the Changning-Menglian and Chiang Rai-Chiang Mai zone and is a crucial region for constraining the evolution of the eastern Paleo-Tethys. This study presents new zircon U-Pb geochronological, mineral and whole-rock geochemical, and Sr-Nd-Hf-O isotopic data for magmatic rocks from the Sukhothai arc in eastern Myanmar. The rock suites analyzed include 360–355 Ma basaltic rocks and trondhjemitic dikes, and 257–254 Ma volcanic rocks and gabbroic cumulates. The basaltic rocks were derived from partial melting of mélange pair with peridotite and experienced assimilation and fractional crystallization (AFC). The trondhjemitic dikes were formed by partial melting of the basaltic rocks and experienced fractional crystallization at shallow depth. We suggest that andesite and dacite were derived from partial melting of depleted mantle wedge and underwent AFC process. The gabbroic cumulates are a crystallizing phase associated with the melts that produced the coeval volcanic rocks. We propose that eastern Myanmar, Central Tibet, SW Yunnan and Southeast Asia share a similar three-staged magmatic history, forming a ∼ 4000 km long magmatic belt. Stage I records the magmatic events related to subduction of the eastern Paleo-Tethys Ocean during the Early Carboniferous. A back-arc basin was opened during the Late Carboniferous, and extensive subduction-related magmatism was followed since the Permian. Stage II records the igneous rocks formed during the final amalgamation between the Indochina and Sibumasu Blocks during the Late Permian to Middle Triassic. Stage III is defined by the post-collisional magmatism distributed across the suture zone during the Late Triassic.

Linking ocean subduction with early Paleozoic intracontinental orogeny in South China: Insights from the Xiaying complex in eastern Guangxi Province

The tectonic setting of the early Paleozoic orogeny in the South China Block (SCB) has been hotly controversial. Although most studies have ascribed it to an intracontinental orogeny, there are various views on the geodynamic drivers of the intracontinental orogeny. This study reports on the Ordovician Xiaying igneous complex in the eastern part of Guangxi province, southern South China. This complex is composed of granodiorite, monzogranite and micro-granular diorite enclosed by granodiorite. The zircon U–Pb dating suggests that these three types of rocks emplaced synchronously at around 464–442 Ma, significantly older than those S- and I-type granites (450–400 Ma) distributed in the interior of the SCB. One monzogranite collected in the border of the complex has an older age (480 Ma). The Xiaying hornblende–bearing granodiorites have low SiO2 (61.8–68.7 wt%), P2O5 (0.07–0.10 wt%), Rb/Sr (0.52–1.30) and Rb/Ba (0.13–0.34) and high Fe2O3t (3.84–5.57 wt%), MgO (1.48–2.35 wt%), CaO (2.28–4.54 wt%) and Na2O/K2O (0.90–1.91), with metaluminous to weakly peraluminous features (most A/CNK = 0.87–1.07), showing typical I-type granitoid characteristics. The Nd–Hf isotopic model ages (ca 1.6–1.8 Ga) indicate that the Xiaying granodiorites were probably formed by partial melting of a late Paleoproterozoic basement in the deep crust. The micro–granular dioritic enclaves (MMEs) have similar formation ages and Nd–Hf isotopic compositions to the host granodiorites, and show geochemical evolution relation with granodiorites, suggesting that they resulted from the mixing of mafic and granodioritic magmas, which derived from a depleted mantle and a Paleoproterozoic crust, respectively. Based on their similar formation ages and zircon Hf-isotope compositions and the assimilation and fractional crystallization (AFC) modelling, it is suggested that major monzogranites were generated from the granodioritic magma through the AFC process. The rock associations and arc-related geochemical characteristics of the granodiorites and dioritic MMEs suggest that they probably formed in a subduction-related tectonic setting. Combined with the spatial distribution of the rock types and formation ages of these early Ordovician to early Devonian magmatic rocks in the SCB from southeast to northwest, it is inferred that there was an Ordovician northwestward subduction of the proto–South China Ocean occurring to the south of the SCB, which may connect with the Tam Ky–Phuoc Son Suture Zone in the eastern Indochina Block. It was the subduction of the proto–South China Ocean and subsequent collision between the southern SCB and an unknown continent that triggered early Ordovician to early Devonian intracontinental orogeny in the interior of the SCB.

Three‐dimensional P‐wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

AbstractIn this study, we compiled and analyzed 69310 P‐wave travel‐time data from 6639 earthquake events. These events (M ≥ 2.0) occurred from 1980s to June 2019 and were recorded at 319 seismic stations (Chinese Earthquake Networks Center) in the study area. We adopted the double‐difference seismic tomographic method (tomoDD) to invert the 3‐D P‐wave velocity structure and constrain the crust‐upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt (MLYB). A 1‐D initial model extracted from wide‐angle seismic profiles was used in the seismic tomography, which greatly reduced the inversion residual. Our results indicate that reliable velocity structure of the uppermost mantle can be obtained when Pn is involved in the tomography. Our results show that: (1) the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning: a nearly E–W‐trending low‐velocity zone is present beneath the Dabie Orogen, in contrast to the mainly NE‐trending low‐velocity anomalies beneath the Jiangnan Orogen. They suggest the presence of thickened lower crust beneath the orogens in the study area. In contrast, the Yangtze and Cathaysia blocks are characterized by relatively high‐velocity anomalies; (2) both the ultra‐high‐pressure (UHP) metamorphic rocks in the Dabie Orogen and the low‐pressure metamorphic rocks in the Zhangbaling dome are characterized by high‐velocity anomalies. The upper crust in the Dabie Orogen is characterized by a low‐velocity belt, sandwiched between two high velocity zones in a horizontal direction, with discontinuous low‐velocity layers in the middle crust. The keel of the Dabie Orogen is mainly preserved beneath its northern section. We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen, which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low‐velocity layers observed there. Continuous deep‐level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks, forming the ‘sandwich’ velocity structure there; (3) high‐velocity updoming is widespread in the crust‐mantle transition zone beneath the MLYB. From the Anqing‐Guichi ore field northeastward to the Luzong, Tongling, Ningwu and Ningzhen orefields, high‐velocity anomalies in the crust‐mantle transition zone increase rapidly in size and are widely distributed. The updoming also exists in the crust‐mantle transition zone beneath the Jiurui and Edongnan orefields, but the high‐velocity anomalies are mainly stellate distributed. The updoming high‐velocity zone beneath the MLYB generally extends from the crust‐mantle transition zone to the middle crust, different from the velocity structure in the upper crust. The upper crust beneath the Early Cretaceous extension‐related Luzong and Ningwu volcanic basins is characterized by high velocity zones, in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression‐related Tongling ore field. The MLYB may have undergone a compressive‐to‐extensional transition during the Yanshanian (Jurassic–Cretaceous) period, during which extensive magmatism occurred. The near mantle–crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust. The magmas may have ascended through major crustal faults, undergoing AFC (assimilation and fractional crystallization) processes, became emplaced in the fault‐bounded basins or Paleozoic sequences, eventually forming the many Cu‐Fe polymetallic deposits there.

Shallow marine basaltic volcanism of the Machadodorp Member (Silverton Formation, Pretoria Group), Transvaal Basin, South Africa — An example of Paleoproterozoic explosive intraplate volcanic activity in an epeiric embayment

The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example of ancient submarine explosive volcanism in an intra-cratonic setting under incipient rifting conditions. The Silverton Formation consists of three different members, which include, in ascending order, the Boven Member, the Machadodorp Member and the Lydenburg Member. The formation is primarily characterized by the muddy sediments of the Boven and Lydenburg members, which formed in a relatively shallow submarine environment, predominantly as sub-storm wave-base pelagic deposits, within an epeiric embayment on the Kaapvaal Craton. The subaqueous Machadodorp Member volcanic activity is mostly distributed in the eastern part of the basin. So far, the volcanism of the Machadodorp Member has not been studied in detail. Therefore, a detailed facies analysis of its volcanism was initiated, allowing constraints on its volcanic evolution.The Machadodorp Member consists of a basal unit of basaltic sheet lava, followed by a fining-upward succession of volcaniclastic rocks, ranging from breccias, tuff-breccias and tuffs, rich in fluidal-shaped clasts, to laminated tuffs that are related to subaqueous mass flows, and a final sheet lava that characterized the end of volcanic activity in the section studied.The Machadodorp Member rocks are tholeiitic basalts in composition, with all samples plotting in the low-TiO2 suite. Fractional crystallization of clinopyroxene, olivine and plagioclase was the predominant evolutionary process as illustrated by mineral chemistry and major element geochemical modeling. AFC (Assimilation and Fractional Crystallization) processes could not account for all the trace element variations, nor for the arc-like signatures observed on the primitive mantle normalized plots. Variable ancient mantle enrichment beneath this area is a more plausible cause for the trace elements variations observed in the Machadodorp rocks. Trace element modeling indicates that the parental composition of these rocks was a variably enriched spinel lherzolite source. Such enrichment must have occurred before 3 Ga during episodes of arc collision and subduction.

Approach to trace hidden paleo-weathering of basaltic crust through decoupled Mg[sbnd]Sr and Nd isotopes recorded in volcanic rocks

On Earth, chemical weathering of basaltic rocks not only modulates the climate change, but also plays a crucial role on the ocean oxygenation and ecosystem evolution. Records of paleo-weathered crusts are a challenge to identify because they are obscured by subsequent alteration processes and buried by the newly formed volcanic lavas. However, late formed volcanic rocks theoretically could record previously weathered crust via their assimilation and fractional crystallization (AFC) processes. Here, we present a systematic study on major, trace elements and Sr-Nd-Mg isotopes in a suite of Changbaishan early formed basalts and late erupted trachytes from Northeast China. Detailed evidence of field observation, geochemical signature and rhyolite-MELTS simulations suggest that trachytes mainly originated from deep fractional crystallization of basaltic melts. The low abundant trachytes from the stratigraphic layer above the basalts display decoupled MgSr and Nd isotopes. δ26Mg and 87Sr/86Sr in trachytes range from −0.27 up to +0.94‰ (the heaviest isotopic composition ever reported for Mg in the igneous rocks) and 0.70490 to 0.71065, respectively, which are significantly elevated relative to that of their mantle source. However, they preserved mantle source-like 143Nd/144Nd (0.512584 to 0.512656) ratios. These unusual characteristics cannot be explained by thermal diffusion, fractional crystallization or simple crustal contamination processes. Specifically, δ26Mg correlates positively with SiO2 and 87Sr/86Sr, and negatively with MgO, CaO, Al2O3, Sr and Nb/U indicators. The highly elevated Mg and Sr isotopes but limited Nd isotopic changes in trachytes require a component to be the relatively younger weathering residue of basaltic rocks. Taking this component as a wall rock endmember, the well-developed above correlations can be best modelled by AFC processes. This reflects that some trachytes in the aftermath of AFC processes almost fully lost their primary source characteristics, but inherited the intense chemical weathering signature of basalts. This event probably occurred in the middle Miocene as studies of paleosols showed humid and warm climate during that period after the initial eruption of Changbaishan basalts. Therefore, our study as a specific case for the first time provides an efficient way to explore paleo-weathering of basaltic crusts buried in the Earth's surface, through comprehensive investigating Mg, Sr and Nd isotopes of volcanic rocks developed in the late stage of a volcano cone.

U–Pb geochronology, Sr–Nd isotopic compositions, geochemistry and petrogenesis of Shah Soltan Ali granitoids, Birjand, Eastern Iran

The Shah Soltan Ali area (SSA) is located in the eastern part of the Lut Block metallogenic province. In this area different types of sub-volcanic intrusions including diorite porphyry, monzonite porphyry and monzodiorite porphyry have intruded into basaltic and andesitic rocks. Zircon U–Pb dating and field observations indicate that intermediate to mafic volcanic rocks (38.9 Ma) are older than subvolcanic units (38.3 Ma). The subvolcanic intrusions show high-K calc-alkaline to shoshonitic affinity and are metaluminous. Based on mineralogy, high values of magnetic susceptibility [(634 to 3208) × 10−5 SI], and low initial 87Sr/86Sr ratios, they are classified as belonging to the magnetite-series of oxidant I-type granitoids and are characterized by an enrichment in LREEs relative to HREEs, with negative Nb, Ti, Zr and Eu anomalies. These granitoids are related to volcanic arc (VAG) and were generated in an active continental margin. Low initial 87Sr/86Sr ratios (0.7043 to 0.7052) and positive εNd values (+1.48 to +3.82) indicate that the parental magma was derived from mantle wedge. Parental magma was probably formed by low degree of partial melting and metasomatized by slab derived fluids. Then assimilation and fractional crystallization processes (AFC) produced the SSA rocks. This magma during the ascent was contaminated with the crustal material.All data suggest that Middle-Late Eocene epoch magmatism in the SSA area, occurred during subduction of Neo-Tethys Ocean in east of Iran (between Afghan and Lut Blocks).

Petrogenesis of plio-quaternary intra-plate continental alkaline lavas from the İskenderun Gulf (Southern Turkey): Evidence for metasomatized lithospheric mantle

The Quaternary alkaline volcanic field of Southern Turkey is characterized by intra-continental plate-type magmatic products, exposed to the north of the İskenderun Gulf along a NE-SW trending East Anatolian Fault, to the west of its intersection with the N–S trending Dead Sea Fault zone. The İskenderun Gulf alkaline rocks are mostly silica-undersaturated with normative nepheline and olivine and are mostly classified as basanites and alkaline basalts with their low-silica contents ranging between 43 and 48 wt.% SiO2. They display Ocean Island Basalt (OIB)–type trace element patterns characterized by enrichments in large-ion-lithophile elements (LILE) and light rare earth element (LREE), and have (La/Yb)N = 8.8–17.7 and (Hf/Sm)N = 0.9–1.6 similar to those of basaltic rocks found in intraplate suites. The basanitic rocks have limited variations Sr-Nd isotopic ratios (87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512918–0.521947), whereas the alkali basalts display more evolved Sr-Nd isotopic ratios (87Sr/86Sr = 0.70346-0.70365, 143Nd/144Nd = 0.512887–0.521896). The İskenderun Gulf alkaline rocks also display limited Pb isotopic variations with 206Pb/204Pb = 18.75–19.09 207Pb/204Pb = 15.61–15.66 and208Pb/204Pb = 38.65–39.02, indicating that they originated from an enriched lithospheric mantle source. Calculated fractionation vectors indicate that clinopyroxene and olivine are the main fractionating mineral phases. Similarly, based on Sr-Nd isotopic ratios, the assimilation and fractional crystallization (AFC) modeling shows that the alkali basalts were affected by AFC processes (r = 0.2) and were slightly contaminated by the upper crustal material.The high TiO2 contents, enrichments in Ba and Nb, and depletions in Rb can likely be explained by the existence of amphibole in the mantle source, which might, in turn, indicate that the source mantle has been affected by metasomatic processes. The modeling based on relative abundances of trace elements suggests involvement of amphibole-bearing peridotite as the source material. İskenderun Gulf alkaline rocks can thus be interpreted as the products of variable extent of mixing between melts from both amphibole-bearing peridotite and dry peridotite.

Significance of assimilation and fractional crystallization (AFC) process in the generation of basaltic lava flows from Chhotaudepur area, Deccan Large Igneous Province, NW India

Basaltic and basaltic andesitic flows have been identified from the Chhotaudepur area in the Deccan Large Igneous Province. The \(\hbox {SiO}_{2}\) content of these flows varies from 46.7 to 53.7 wt%. None of the samples have primary magma signatures as they exhibit low Mg# (0.42–0.68), Ni (4.8–33.4 ppm) and Cr (33.08–143.06 ppm). Highly variable concentrations of the LILE [Rb (2–74 ppm), Ba (52–351 ppm), Cs (0.1–1 ppm) and Sr (273–745 ppm)] and relatively enriched LREE are also noticed. The coherent chondrite normalized REE diagram and primitive mantle normalized multi-element diagram suggests a consanguinity among the flows. Low (Nb/Th)\(_{\mathrm{PM}}\) (0.30–1.09), high (Th/Yb)\(_{\mathrm{PM}}\) (3.09–16.58) ratios and marked variations in Rb concentration (2.4–74.11 ppm) with variable La/Yb (5.5–23.7) ratios suggests that magmas in the Chhotaudepur region were assimilated by the crustal components. The elevated Th/Ta and La/Yb relation further indicate concurrent assimilation and fractional crystallization process was involved in the genesis of the flows. Assimilation and fractional crystallization modelling of the flows was carried out with 20% olivine, 25% clinopyroxene, 45% plagioclase and 10% titano-magnetite as fractionating minerals and upper continental crust as the assimilant. The results reveal that all the flows were modified by AFC process.

Large Paleozoic and Mesozoic porphyry deposits in the Central Asian Orogenic Belt: Geodynamic settings, magmatic sources, and genetic models

The Central Asian Orogenic Belt (CAOB) or Altaids contains several large porphyry Cu(-Au-Mo) and Au(-Cu) deposits. These deposits are genetically associated with equigranular and porphyritic tonalite, granodiorite and diorite. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that these hydrous, mineralized calc-alkaline intrusions were emplaced from the Cambrian to the Triassic.Trace element geochemistry and Sr–Nd–Hf–O isotopic compositions show two different signatures of the mineralizing intrusions. Mineralizing intrusions at Bozshakol, Aktogai, Nurkazghan, and Tuwu-Yandong have depleted Sr–Nd–Hf–O isotopic compositions (Isr = 0.7033 to 0.7047, εNd(t) = +3.6 to +6.6, εHf(t) = +9.6 to +16.4, δ18O = 6.8 to 4.7‰) and variable high Nb/Ta (11.6 to 16.4) ratios, implying a dominantly juvenile lower crustal source with mantle involvement at Bozshakol and Aktogai and sedimentary rocks contamination at Nurkazghan. Mineralizing intrusions at Erdenet, Koksai and Yubileinoe have slightly depleted Sr–Nd–Hf isotopic compositions (Isr = 0.70403 to 0.7043, εNd(t) = +2.9 to +3.8, εHf(t) = +6.6 to +11.6) and low δ18O (3.1 to 2.5‰), slightly enriched Sr–Nd–Hf–O isotopic compositions (Isr = 0.7044 to 0.7049, εNd(t) = +0.04 to +1.9, εHf(t) = 3.2 to 12.0, δ18O = 5.2 to 6.4‰) and enriched Sr–Nd isotopic compositions (Isr = 0.7041 to 0.7081, εNd(t) = −2.6 to −2.9), respectively, as well as intermediate to low Nb/Ta ratios (13.4–8.1), implying a juvenile lower crustal source with wall rocks contamination at Erdenet and ancient crust contamination at Koksai and Yubileinoe. Therefore, Juvenile sources played a significant role in the generation of fertile magmas for porphyry-Cu and -Au style mineralization in the CAOB.Mineralizing intrusions include the adakitic, transitional, and normal arc rocks. Mineralizing intrusions in some porphyry Cu(-Mo) deposits (e.g., Aktogai, Erdenet) have adakitic affinities based on their high Sr/Y (30–193) and La/Yb (17–49) ratios coupled with low Y (1.5–10.3 ppm) and Yb (0.2–1.0 ppm) concentrations. Variable high Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a garnet-bearing amphibolite. These adakitic rocks were formed by partial melting of thickened lower crust at depths of >40 km. Mineralizing intrusions at most porphyry Cu(-Au) and Au deposits, including Bozshakol, Nurkazghan, Koksai, Tuwu-Yandong, and Yubileinoe, have variable intermediate Sr/Y (13–84) and La/Yb (7–23) ratios, intermediate to low Y (4.7–16.2 ppm) and Yb (0.5–1.3 ppm) concentrations, showing a transitional signature between adakitic and normal arc rocks. Variable moderate Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole. These transitional rocks were formed by the MASH (melting, assimilation, storage, and homogenization) processes at depths of <40 km and AFC (assimilation and fractional crystallization) processes in a mid-crustal magma chamber. One large porphyry Cu-Au deposit (e.g., Nurkazghan) is associated with intrusions that have calc-alkaline geochemical signatures, with enrichment of light rare earth elements (LREE), depletion of heavy REE, Ba, Nb, Sr and Ti and low Sr/Y, Sm/Yb and La/Yb ratios, suggesting that the lower crustal source could be a clinopyroxenite. They are formed by partial melting of lower crust at depths of 30–35 km with AFC important to magma evolution. Here, three genetical models are suggested for these mineralizing magmas in the CAOB: (1) thick (> 40 km) lower crust melting for adakitic magmas; (2) MASH and AFC for transitional magmas; and (3) thin (30–35 km) lower crust melting followed by AFC for normal arc magmas. A majority of transitional and normal arc magmas suggest that thinner crust was important to the formation of porphyry Cu(-Au) and Au deposits that dominate in the CAOB.Our geochronology and geochemistry results indicate that most of the mineralizing intrusions from the porphyry deposits in the CAOB formed in island arc settings from the Cambrian to Triassic. The intrusions at Erdenet, Koksai and Yubileinoe are exceptions, forming in continental arcs in the Permian, Silurian and Devonian, respectively. Paleozoic and minor Mesozoic porphyry deposits in the CAOB formed during six periods. The greatest metal endowment is associated with the Devonian porphyry deposits (374 to 372 Ma, e.g. Oyu Tolgoi). The largest number of porphyry deposits were emplaced in the Carboniferous (334 to 317 Ma, e.g., Aktogai, Kounrad, Tuwu-Yandong, Almalyk). The CAOB is a complex collage of Paleozoic and Mesozoic tectonic elements and has considerable potential for further discoveries of large porphyry Cu(-Au-Mo) and Au deposits.

Analysis of the zircon population and Sr-isotope data of the Paleogene igneous rocks from Kyustendil area, SW Bulgaria

In the present study, we apply a combination of Sr-isotope whole-rock analyses and LA-ICP-MS U-Pb zircon dating to understand the magmatic evolution of the igneous rocks in the Ruen Zone, western Bulgaria, and bracket the time of their formation. This approach reveals a general interval of magmatic activity from 35.57 ± 0.27 Ма (Pishtilski volcanics) to 28.63 ± 0.58 Ма (cross-cutting dykes in the Osogovo Mountain). The volcanic rocks from the Prekolnitsa Graben are formed with significant (50%) crustal contamination/assimilation. Three major sources of crustal rocks can be outlined, with Triassic, Ordovician and Cambrian age, respectively. For the subvolcanic to plutonic magmatic rocks of the Osogovo Mountain, we suggest maturation and zircon crystallization over 2–4 Ma. The magma was generated dominantly in the continental crust and evolved through mainly fractional crystallization (FC) in the upper crustal chamber, whereas the evolution of the Osogovo dykes requires additional involvement of AFC (Assimilation and Fractional Crystallization) and contamination processes.

Controls on the formation of Cu-rich magmas: Insights from the Late Triassic post-collisional Saishitang complex in the eastern Kunlun Orogen, western China

Petrogenesis of high Sr/Y andesitic to dacitic Cu-rich magmas remains highly controversial and has been attributed to be related to either particular sources or differentiation processes/conditions. This study provides a comprehensive petrological and geochemical dataset including Cu, Au and Pt systematics in the Saishitang complex in the eastern Kunlun Orogen to investigate the influence of the magmatic source and differentiation processes on Cu enrichment in a post-collisional, slab-derived magmatic system. The Saishitang complex contains a ~233Ma plutonic main body and tens of porphyritic dykes intruded at ~218Ma, and hosts skarn copper mineralization formed at 218–224Ma. The main intrusion consists of quartz diorite, granodiorite and minor amounts of monzogranite, whereas the porphyritic dykes vary from diorite to quartz diorite, granodiorite and granite. The porphyritic diorites, with high MgO, Ni and Cr contents and moderately high Sr/Y and La/Yb ratios, were most likely derived from partial melting of the subducted slab and overlying sediments and then interacted with mantle peridotite. Mineral compositions and geochemical data, as well as mass-balance, Rayleigh fractionation and isotopic mixing modeling show that the parent magmas subsequently experienced simultaneous assimilation and fractional crystallization (AFC) at both the lower and the mid-to-upper crustal levels, and were finally emplaced to different upper crustal levels. Systematic Cu and Au variations during the Saishitang magmatic evolution suggest that the lower crustal AFC process improved the Cu budget by approximately one order of magnitude and therefore was the most critical factor in controlling the formation of Cu-rich magmas, although other processes such as slab melting and sulfide saturation were also important.

Heterogeneous magnesium isotopic composition of the lower continental crust: A xenolith perspective

We report 26 high‐precision whole‐rock Mg isotopic analyses for two suites of well‐characterized granulite xenoliths from Chudleigh and McBride, North Queensland, Australia, in order to constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the lower continental crust. Previous studies suggest that the Chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower crust via combined assimilation and fractional crystallization (AFC). The δ26Mg values of the xenoliths range from −0.31 to −0.21‰ and correlate with radiogenic isotopes, reflecting mixing of mantle‐derived mafic magma (δ26Mg = −0.31‰) with preexisting isotopically heavy crustal materials (δ26Mg = ∼ +0.5‰) through the AFC process. The McBride granulites range compositionally from mafic to felsic, and originated as cumulates, solidified mafic/felsic melts, and restites that formed during basaltic underplating and reworking of preexisting lower crust. Their δ26Mg values vary widely from −0.72 to +0.19‰. The large Mg isotopic variation in the McBride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet, which is isotopically light. Based on these results, the lower continental crust has a heterogeneous Mg isotopic composition, with a weighted average δ26Mg of −0.18‰. The bulk continental crust, based on available data, has an average Mg isotopic composition of −0.19‰, and is slightly heavier than the mantle. The highly heterogeneous Mg isotopic distribution in the crust indicates that chemical weathering not only modifies the upper crust compositions but also significantly influences lower crust compositions through emplacement of upper crustal materials into the deep crust.

Multistage Evolution of Dolerites in the Karoo Large Igneous Province, Central South Africa

EarlyJurassic sheet-like intrusions (sills and dykes) are abundant in the Karoo Basin in South Africa, and were emplaced as a part of the Karoo Large Igneous Province. Here we discuss the evolutionary history of dolerite sills and dykes in different parts of the basin on the basis of new major and trace element analyses of dolerite samples collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the Golden Valley Sill Complex (GVSC). In addition, we present Sr^ Nd isotope data for selected samples.The dolerites are subalkaline tholeiitic basalts and basaltic andesites characterized by enriched trace element patterns, variable degrees of depletion in Nb^Ta relative to light rare earth elements, negative to positive Pb anomalies, and mild to moderate enrichment in initial Sr^Nd isotopic ratios. The aim of this study is to unravel the evolutionary history of the melts that gave rise to the dolerites. We propose that the primary melts were derived from sub-lithospheric mid-ocean ridge basalt (or ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb^Ta, mildly enriched Sr^Nd isotopic ratios) through interaction with metasomatized lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up to 10% assimilation of granulites with strong arc-type geochemical signatures. The AFC processes may alternatively have taken place in the uppermost mantle. Distinct geochemical characteristics among the GVSC and drill-core units reflect different amounts of AFC. During and/or after intrusion into the sedimentary rocks in the Karoo Basin the magmas underwent a second stage of fractional crystallization (50^60%) and local contamination by their sedimentary wall-rocks. High U concentrations and U/Th ratios in some dolerites in the southwestern part of the Karoo Basin were probably caused by fluids released from shales rich in organic material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect interaction withTa^Th^U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have interacted with such rocks. Considering that continental flood basalts are emplaced through continental crust and sedimentary basins, it is likely that other LIPs have similar evolutionary histories to that proposed for the Karoo Basin.

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from 238U– 230Th– 226Ra systematics

In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469–2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show 238U– 230Th– 226Ra disequilibria with excess 238U and 226Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest ( 230Th/ 232Th) ratio compared to older Miyakejima eruptives, yielding the youngest 238U– 230Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a ( 230Th/ 232Th)–( 238U/ 232Th) diagram and (2) a positive linear correlation in a ( 226Ra/ 230Th) 0 − 1/ 230Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230Th and 226Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in ( 230Th/ 232Th) and ( 226Ra/ 230Th) 0 ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr–Nd–Pb isotopic systems. In such cases, Th isotopes and ( 226Ra/ 230Th) ratio are excellent geochemical tracers of magmatic evolution.

Magma genesis and crustal contamination of continental intraplate lavas in northwestern Syria

The Miocene to Quaternary lavas of northwestern Syria range from basanite, alkali basalts, and tholeiites to basaltic andesites, hawaiites, and mugearites. Crustal assimilation and fractional crystallization processes (AFC) modified the composition of the mantle derived magmas. Crustal assimilation is indicated by decreasing Nb/U (52.8-17.9) and increasing Pb/Nd (0.09-0.21) and by variable isotopic compositions of the lavas (Sr-87/Sr-86: 0.7036-0.7048, Nd-143/Nd-144: 0.51294-0.51269, Pb-206/Pb-204: 18.98-18.60) throughout the differentiation. Modeling of the AFC processes indicates that the magmas have assimilated up to 25% of continental upper crust. The stratigraphy of the lavas reveals decreasing degrees and increasing depths of melting with time and the strongly fractionated heavy rare earth elements indicate melt generation in the garnet stability field. Modeling of melt formation based on trace element contents suggests that 8-10% melting of the asthenospheric mantle source produced the tholeiites, whereas basanite and alkali basalts are formed by 2-4% melting of a similar source.

Post-collisional, Potassic and Ultrapotassic Magmatism of the Northern Tibetan Plateau: Constraints on Characteristics of the Mantle Source, Geodynamic Setting and Uplift Mechanisms

Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to ∼15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan–Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India–Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau.