Adakitic Affinity Research Articles

Geochronology, geochemistry and petrogenesis of Miocene adakitic rocks in Milashan, Southern Tibet

中新世是青藏高原隆升、增厚的重要时期，并且在这一时期内拉萨地块广泛发育碰撞后岩浆岩。本文对南拉萨地块米拉山地区的钙碱性钾质火山岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素的测定与系统研究。米拉山中新世火山岩为粗面英安岩、英安岩和流纹岩（SiO₂=59.89%~71.78%）。锆石U-Pb定年结果为16.1±0.2Ma~20.4±0.3Ma，表明其喷发时代为中新世。岩石具有较高的Al₂O₃含量（13.54%~16.31%），低MgO（0.46%~1.95%）、高Sr（388×10^-6~804×10^-6）、低Y（6.55×10^-6~11.20×10^-6）和Yb（0.70×10^-6~1.07×10^-6）的特征，具有较高的Sr/Y值（51~80）、低相容元素（Cr=4.26×10^-6~32.53×10^-6，Ni=4.16×10^-6~25.75×10^-6）和弱Eu负异常。岩石具有轻稀土元素和Rb、Th、U、K等元素富集、重稀土元素和高场强元素Nb、Ta、Ti亏损的特征。米拉山中新世火山岩显示出埃达克质岩石的地球化学特征，可能来自于的镁铁质加厚下地壳的部分熔融，推测下地壳源区是石榴石角闪岩。锆石ε_Hf（t）值为+2.2~+7.8，表明源区为新生地壳物质，有俯冲板片熔体加入。米拉山中新世火山岩的喷发时代与米拉山断裂活动时间一致，二者可能同为拉萨地块岩石圈拆沉的结果。;Miocene is an important period for the uplifting and thickening of the Tibetan Plateau, and meanwhile, post-collisional igneous rocks are widely developed. The zircon U-Pb dating and Lu-Hf isotopes, and whole rock major and trace element compositions were analyzed on the calc-alkaline potassic volcanic rocks in the Milashan area, Southern Lhasa terrane. Milashan Miocene volcanic rocks are trachydacite, dacite and rhyolite, with intermediate to acid composition and SiO₂ ranging from 59.89% to 71.87%. The zircon U-Pb dating yielded ages from 16.1±0.2Ma to 20.4±0.3Ma. All samples exhibit high Al₂O₃ (13.54%~16.31%), low MgO (0.46%~1.95%), high Sr(388×10^-6~804×10^-6), low Y (6.55×10^-6~11.20×10^-6) and Yb (0.70×10^-6~1.07×10^-6), high Sr/Y ratios, low compatible element concentration (Cr=4.26×10^-6~32.53×10^-6, Ni=4.16×10^-6~25.75×10^-6) and slight negative Eu anomaly. The samples are characterized by enrichment of light rare earth elements and large ion lithophile element (LILEs, Rb, Th, U, K), and depletion of heavy rare earth elements and high field strength element (HFSEs, Nb, Ta, Ti). These Miocene volcanic rock samples show adakitic affinity, and may have been formed by partial melting of mafic thickened lower crust. The source region is probably garnet amphibolite. Lu-Hf isotopic determination yielded positive zircon ε_Hf(t) values ranging from +2.2 to +7.8, indicating that the source was juvenile crust with the addition of subducted slab melt. Milashan Miocene volcanic rocks were formed at the same time as the action of Milashan fault, which probably suggests that both Milashan Miocene volcanic rocks and Milashan fault are caused by the delamination of Lhasa terrane lithosphere.

Geochemistry, geochronology, and zircon Hf isotopes of Late Jurassic–Early Cretaceous granitoids in the Xing'an Massif, NE China: Implication for the Late Mesozoic tectonic evolution and crustal growth

AbstractThis paper presents new zircon U–Pb geochronological, Hf isotopic and whole‐rock geochemical data for the granitic plutons in the Xing'an Massif, Northeast China, to constrain the Late Mesozoic tectonic evolution of the Mongol‐Okhotsk Ocean and the Paleo‐Pacific Ocean. The zircon U–Pb ages indicate that the granitoids emplaced during the Late Jurassic–Early Cretaceous. The granodiorites show an adakitic affinity with high Sr/Y ratios and low Yb (< 1.30 μg/g) contents. The monzogranites exhibit high SiO2, low MgO contents, enrichment in LILEs (Rb, K, and Th), and depletion in HSFEs (Ta, Nb, Zr, P, and Ti). Petrological and geochemical features of these monzogranites suggest that they are highly fractionated I‐type granitoids. In addition, the zircon εHf(t) values and two‐stage model ages (TDM2) are in the range of +2.6 to +8.1 and 669–1011 Ma, respectively, indicating that primary magma was generated by partial melting of juvenile lower‐crustal materials, and there was a significant crustal growth in the Phanerozoic in the Northeast China. Combined with the coeval granitoids widely exposed in the Xing'an Massif, we conclude that the Late Jurassic magma in Northeast China was generated in an extensional setting related to the closure of the Mongol‐Okhotsk Ocean, but the Early Cretaceous magma was related to the subduction of the Paleo‐Pacific Plate.

Geochemical compositions of apatites from the Xuejiping and Disuga porphyries in Zhongdian arc: Implications for porphyry Cu mineralization

Triassic calc-alkaline granitic rocks are widely distributed in the Zhongdian arc, SW China. Some of these rocks host porphyry Cu deposits of different sizes, but most of them are barren. In order to understand the key factors of mineralization, and to provide fertility discriminators for future mineral deposit exploration in the region, this study presents new whole-rock and apatite geochemical data of the ore-bearing porphyries from the large Xuejiping deposit and Disuga prospecting area. These data have been compared with and integrated with previously published data of the ore-bearing porphyry from the giant Pulang deposit and the barren Triassic Xiuwacu biotite granite. The four intrusions are I-type granitoids with adakitic affinities, characterized by high whole-rock Sr/Y and La/Yb ratios and high Sr/Y and EuN/EuN* ratios in apatites. Coherent whole-rock geochemical variations and changes in Sr contents and REE ratios in apatites indicate that the four intrusions underwent similar fractional crystallization. All the rocks show enrichment of LILEs and LREEs and depletion of HFSEs with relatively wide ranges of whole-rock Sr-Nd isotopes ((87Sr/86Sr)t = 0.7043–0.7070; εNd(t) = -6.8–0.2). These geochemical features together with apatite Sr-Nd isotopes suggest that their parental magma may originate from partial melting of the slab-derived fluid metasomatized mantle wedge and mixing with melt of lower crust. Compared to the barren Triassic Xiuwacu biotite granite, three ore-bearing porphyries have higher EuN/EuN*, lower CeN/CeN* ratios of apatites, indicating higher oxygen fugacity. Besides, the ore-bearing porphyries have higher magmatic S contents than that of the barren one. The differences in magmatic oxygen fugacity and S content are key factors controlling the economic potential of magmas in this arc. Further, the Disuga porphyry from prospecting area shares similar characteristics with the other two ore-bearing porphyries in magma source, fractionation process, oxidation state, and S content. Therefore, the Disuga porphyry may have the potential to form a large porphyry Cu deposit. No large deposit has been found in the Disuga area is probably due to the intensive denudation in this area.

Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

AbstractThe nature and timing of post-collisional crustal thickening and its link to surface uplift in the eastern Lhasa block of the southern Tibetan plateau remain controversial. Here we report on Cenozoic magmatism in the Wuyu area of the eastern Lhasa block. The Eocene (ca. 46 Ma) trachyandesites and trachydacites show slight fractionation of rare earth elements (REE), slightly negative Eu and Sr anomalies, and relatively homogeneous Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7050–0.7063, εNd(t) = −0.92 to −0.03, εHf(t) = +2.6 to +4.8). Previous studies have suggested Neo-Tethys oceanic slab break-off at 50–45 Ma; thus, the Wuyu Eocene magmatism could represent a magmatic response to this slab break-off and originate from relatively juvenile Lhasa crust. The Miocene (ca. 15–12 Ma) dacites and rhyolites have adakitic affinities, e.g., high Sr (average 588 ppm), Sr/Y (29–136), and La/Yb (30–76) values, low Y (4–12 ppm) and Yb (0.4–0.9 ppm) contents, and variable Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7064–0.7142, εNd(t) = −11.7 to −3.7, εHf(t) = −3.2 to +4.5). Their more enriched Sr-Nd-Hf isotopes relative to the Eocene lavas indicate that they should be derived from mixed Lhasa lower crust comprising juvenile crust, ultrapotassic rocks, and probably Indian lower crust-derived rocks. This study has also revealed the transformation from Eocene juvenile and thin crust with a thickness of <40 km to Miocene mixed and thickened crust with a thickness of >50 km. Combined with published tectonic data, we suggest that both lithospheric shortening and magma underplating contributed to eastern Lhasa block post-collisional crustal thickening. Given the spatial-temporal distribution of eastern Lhasa block magmatism and regional geology, we invoke a post-collisional tectonic model of steep subduction of the Indian plate and subsequent westward-propagating plate break-off beneath the eastern Lhasa block, which caused the surface uplift.

Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton

Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U–Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250 Ma) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri = 0.70637 to 0.70659, εNd(t) = −16.7 to −10.2, zircon εHf(t) = −18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240 Ma) granites show a more juvenile isotope signature (εNd(t) = −11.8 to −4.7, zircon εHf(t) = −14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223 Ma) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (εNd(t) = −3.7 to +1.1) and zircon Hf (εHf(t) = +0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri = 0.70374 to 0.70388, εNd(t) = +3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218 Ma for the flare-up and its further differentiation into three secondary pulses including ca. 260–250 Ma appinite and granite, ca. 242–238 Ma ferroan granites and ca. 227–218 Ma A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.

Ore genesis and fluid evolution of the Qiaomaishan Cu–W deposit, in the Middle-Lower Yangtze River Metallogenic Belt: Evidence from in situ analyses of apatite and scheelite

The newly exploited Qiaomaishan skarn-type Cu–W deposit, located in the Xuancheng ore-cluster district in the Middle-Lower Yangtze River Metallogenic Belt (MLYRB), is one of the few tungsten polymetallic deposits in this region. This study conducted a series of in situ analyses of elements and isotopes for apatite, scheelites and sulfides, aiming to provide new insights into the source, hydrothermal ore-forming processes, fluid evolution history and genesis of this deposit and important knowledge to investigate regional tungsten mineralization events in the MLYRB. Apatite geochemistry shows that the early magma source of the ore-related rocks in Qiaomaishan obviously displayed adakitic affinity with high oxygen fugacity and featured fluorine-rich fluids originating from a W-rich mixed source. The tungsten mineralization is dominated by scheelite in the Qiaomaishan deposit. Based on microscopic observations and in situ analysis, two types of scheelite with different geochemical characteristics were recognized. Two types of scheelite in the Qiaomaishan have controlled by different mechanism of REE substitutions. Scheelite-I occurs in the sulfide stage and displays high Na and ΣREE concentrations, downwards-sloping REE fractionation trends with negative Eu anomalies, and 87Sr/86Sr ratios (0.70945–0.71033) similar to those of ore-related granodiorite porphyry. However, scheelite-II shows relatively low ΣREE concentrations, high Na contents, MREE-rich patterns with positive Eu anomalies, and high 87Sr/86Sr ratios (0.71477–0.72352). Distinct Eu anomalies and Sr concentrations of the two types of scheelite and S isotopes of sulfides indicate that the early-stage fluids were inherited from the primary granodioritic magmatic-hydrothermal fluids; however, the late-stage fluids were modified by intense fluid-rock interactions and inputs from the host gypsum-bearing carbonates. Collectively, the F- and W-rich sources, intense fluid-rock interactions and changes in oxidation–reduction conditions favoured tungsten mineralization, which resulted in the formation of the Qiaomaishan tungsten deposit.

Halogens and trace elements of apatite from Late Mesozoic and Cenozoic porphyry Cu-Mo-Au deposits in SE Tibet, China: Constraints on magmatic fertility and granitoid petrogenesis

Abstract Tethyan orogenesis and Indian–Eurasia collision facilitate the development of abundant Late Mesozoic and Cenozoic porphyries and associated Cu ± Mo ± Au deposits in the SE Tibetan plateau. Typical deposits including Beiya with the largest Au reserve of 350 t, Tongchanggou with the largest Mo reserve of 142.5 Mt and Pulang with the largest Cu reserve of 804 Mt were mostly concerned by economic geologists. Porphyries occurred in these three typical deposits were classified as Au-fertile (Wandongshan monzogranite porphyry in Beiya), Mo-fertile (granodiorite porphyry in Tongchanggou), Cu-fertile (quartz monzogranite porphyry in Pulang) and ore-barren (Baliancun biotite monzogranite porphyry in Beiya) systems. In this study, apatite geochemistry from four porphyry systems with distinctive metal endowments was investigated to further constrain magmatic fertility indicators and granitoid petrogenesis. All the studied apatite grains belong to magmatic fluorapatite and are generally wrapped within plagioclase and biotite phenocrysts, suggesting that apatite occurs as early-crystallization phase during crystal fractionation. Apatite grains with porous texture, irregular patchy zoning and secondary rare earth mineral inclusions were eliminated to make the minimal effect of hydrothermal alteration. The F and Cl contents in melt are also estimated based on analyzed F–Cl contents and corresponding F–Cl partition coefficients between apatite and melt under different P–T conditions in this study. Apatite from the Wandongshan (WDS) Au-fertile monzogranite porphyry (MP) has the highest Sr content (average of 1530 ppm) and (F/Cl)melt ratios (44 to 125), indicating that the WDS MP experienced the highest degree of melt fractionation. Additionally, the WDS Au-fertile porphyry is more oxidized than ore-barren, Mo-fertile and Cu-fertile porphyries, evidenced by its highest apatite δEu (0.56–0.83) and lowest δCe (0.98–1.05) values. Rare earth elements (REEs), Y and Sr in apatite were found to be most effective for discriminating magma types. All the apatite grains from typical Late Mesozoic and Cenozoic porphyries in SE Tibet show relatively high Sr/Y (0.5–23.6) and δEu (0.41–0.83) values, pointing to an adakitic affinity of apatite-bearing rock, which is consistent with the whole-rock geochemistry. These porphyries formed under distinctive tectonic settings show distinguishable Clmelt and (F/Cl)melt ratios. The Pulang Cu-fertile quartz monzonite porphyry formed under a subduction setting has relatively high Clmelt contents of 100–1000 ppm and low (F/Cl)melt ratios of 2–22, suggesting derivation of the parental magma from a metasomatized hydrous mantle wedge. Whereas, the WDS Au-fertile MP and Bailiancun ore-barren biotite monzonite porphyry generated under intracontinental setting show low Clmelt contents of 23–180 ppm and variable (F/Cl)melt ratios of 12–125, because F-bearing minerals were decomposing at greater depths under higher pressures during the collision.

Paleoproterozoic Adakitic Rocks in Qingchengzi District, Northeastern Jiao-Liao-Ji Belt: Implications for Petrogenesis and Tectonism

Herein, zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock major and trace element geochemistry are presented for two Palaeoproterozoic granitic rocks in Qingchengzi district, northeastern Jiao-Liao-Ji Belt (JLJB). These new geochronological and geochemical data provide reference clues for exploring the petrogenesis and tectonic setting of Paleoproterozoic magmatic rocks in the Qingchengzi district, which further constrain the tectonic nature of the JLJB. Our zircon U-Pb dating denotes that the Paleoproterozoic magmatic events in the Qingchengzi district were emplaced at ~2163 Ma and ~1854 Ma, represented by granite porphyry and biotite granite, respectively. Geochemically, these Palaeoproterozoic rocks are characterized by high Sr (760–842 ppm), SiO2 (69.72–70.89 wt.%), and Al2O3 (15.53–16.78 wt.%) contents, low Y (2.1–9.0 ppm) and Yb (0.25–0.80 ppm) contents, which indicate an adakite affinity. Combined with Hf isotopic composition (εHf(t) = −1.5~+4.8; TDM2 = 3109~2560 Ma), we believe that the Paleoproterozoic adakitic magma originated from partial melting of the thickened lower crust material in the Meso-Neoarchean. Moreover, these rocks are enriched in light rare earth elements and large ion lithophilic elements (e.g., K, Rb, and Cs), and depleted in heavy rare earth elements and high field strength elements (e.g., Nb and Ta). These features are similar to magmatic rocks formed in an arc environment (either island arc or active continental margin) and are not consistent with an intraplate/intracontinental environment. According to this study and previous research results, we conclude that the arc–continent collision model is conducive to the Paleoproterozoic tectonic attribute of the JLJB, and the oceanic crust subduction between the Namgrim and Longgang blocks may have induced the widespread occurrence of magmatic events in the region.

Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

AbstractSignificant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ∼42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ∼19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.

Thermotectonic evolution of the Paleozoic granites along the Shangdan suture zone (central China): Crustal growth and differentiation by magma underplating in an orogenic belt

Abstract The nature of source rocks and the pressure-temperature-hydration (P-T-H2O) condition are the two main factors that control the geochemical properties of granites. Therefore, the evolution of P-T-H2O conditions can be used to deduce the tectonic setting of granites. In this paper, we report on three Paleozoic granite plutons along the Shangdan suture that revealed increasing melting temperature and decreasing pressure from 437 to 403 Ma, suggesting a crustal thinning process. The Tieyupu granodiorites (437 ± 4 Ma) display Na-rich adakite affinity, i.e., SiO2 = 69.1–70.1 wt%, Na2O/K2O = 1.9–2.26, positive zircon εHf(t) values (+4.29 to +12.04), and high Sr/Y (137–160) and Y/Yb (9.89–10.25) ratios, implying a garnet-rich residue in their source. In combination with moderate zircon saturation temperatures (814–822 °C), we infer that the Tieyupu granodiorites were formed by melting of Neoproterozoic metabasites under high-pressure (>1.5 GPa) and moderate-temperature (HP-MT) conditions. The Liangchahe granodiorites (415 ± 8 Ma) also display Na-rich adakite affinity, i.e., higher Na2O/K2O (2.16–3.11) and lower Sr/Y (77–88) ratios, and higher zircon saturation temperatures (854–874 °C), and they are interpreted to have been derived from melting of metabasites under moderate-pressure (>1.0 GPa) and high-temperature (MP-HT) conditions. Their variable zircon εHf(t) values (−14.97 to +9.80) and the existence of zircon xenocrysts suggest that the primitive adakitic melts were assimilated by evolved crustal components. The Yaogou monzogranites (403 ± 4 Ma) have the highest K2O/Na2O (0.81–1.00) ratios and total rare earth element (ΣREE; 105–191 ppm) contents, lowest Sr/Y (14–43) ratios, positive zircon εHf(t) values (+6.79 to +12.22), and highest zircon saturation temperatures (891–973 °C), showing they were formed by high-temperature melting of intermediate rocks under low-pressure conditions (<1.0 GPa). The evolution of P-T conditions revealed by these three granites suggests that crustal growth and differentiation were related to gradual extensional and melting of mafic protoliths in the orogenic belt.

Molybdenite Re–Os dating, petrology, and geochemistry of granitoids in the Bondar Hanza porphyry Cu deposit (Urumieh‐Dokhtar magmatic arc), Iran: Insight into petrogenesis, mineralization, and tectonic setting

The Bondar Hanza porphyry Cu deposit, hosted by a granitoid stock, is located 120 km south of Kerman city in the elongated NW–SE trending Urumieh‐Dokhtar magmatic arc (UDMA), Iran. The granitoid stock is a multiphase intrusive body, 2 km2 in surface area, which comprises microdiorite–microquartz diorite and granite–granodiorite. Petrological and geochemical analyses show that granitoids are peraluminous, magnesian, calcic to alkali‐calcic, and non‐adakitic intrusions characterized by negative Eu* anomalies. Thermobarometric and oxygen fugacity calculations indicate that magma emplacement occurred at 719–784°C, <100 MPa, and at a log fO2 of NNO +1.6 to +3.2. Sulphur isotopic composition of sulphide minerals (δ34S = 5.0–6.8 ‰) are similar to other porphyry Cu deposits worldwide. Molybdenite separates yield Re–Os ages of 28.22–28.03 Ma, indicating that the Cu mineralization and associated magmatism occurred during the Oligocene. The Bondar Hanza is one of the several granitoids from small porphyry Cu deposits in the UDMA that is related to island arc sub‐productive granitoids. It seems that the largest ore‐hosting porphyry systems within the UDMA are generally restricted to Miocene intrusions with adakitic affinity (e.g., Sarcheshmeh granitoids) and that the temporally discrete non‐adakitic magmatic systems are sub‐productive to barren. These data are interpreted to show that magmatism in the Bondar Hanza region was likely associated with partial melting of juvenile lower crust induced by north‐eastward subduction of the Neo‐Tethys oceanic lithosphere.

Melting of the juvenile lower crust in a far-field response to roll-back of the southern Neotethyan oceanic lithosphere: the Oligocene adakitic dacites, NE Turkey

Late Cenozoic tectono-thermal events and associated magmatism in the Sakarya Zone (SZ) are still contested. Although documented in the western part of the SZ, thus far, no magmatic activity has been identified in the eastern part of the Oligocene SZ. Here, we report a newly identified Oligocene magmatism to interpret the genesis with tectonic setting and gain new insight into the geological evolution of the eastern SZ. We present extensive geochemical, bulk-rock SrNd and zircon Hf isotope, and zircon UPb chronological analyses for the Tepebaşı dacites in the Artvin area, NE Turkey. Zircon UPb dating analyses revealed a dacite formation age of ~29.8 ± 0.3 Ma. Geochemically, with a K2O/Na2O ratio of 0.5 to 0.6, they are composed of rocks of a medium-K calc-alkaline adakitic affinity. The samples are further characterized by low Y (6–7 ppm), and high Sr (362–588 ppm) and Sr/Y ratios (58–98), with low Mg# (41–45) values, demonstrating a close affinity with the crustal source of adakitic rocks. They have slightly radiogenic isotope concentrations (87Sr/86Sr(t) = 0.70460–0.70544, εNd(t) = +1.7 to +2.0), and single-stage Nd model ages of TDM1 = 0.61–0.63 Ga, as well as uniform and positive εHf (t) of 8.2–10.5, with young Hf depleted mantle ages (TDM1 = 0.31–0.41 Ga). These isotopic features, in combination with the geochemical signature, preclude a mantle origin. Instead, they most likely originated from a juvenile mafic lower crustal material by low degree partial melting (<%5) rather than through partial fusion of a subducting slab or thickened lower crust. Trace element modeling reveals that the mafic juvenile lower crust is composed of <10% garnet-bearing amphibolite. Furthermore, trace element compositions imply that adakitic melts formed in an extensional setting without delamination of a thick mafic lower continental crust. We conclude that the Oligocene adakitic magmatism originated in an intracontinental setting, which was subjected to far-field extensional forces induced by roll-back of south Neotethyan oceanic lithosphere just before its detachment in the collision zone. We believe that hot asthenospheric upwelling due to the far-field extension induced by the roll-back of the southern branch of the Neotethyan oceanic lithosphere triggered adakitic magmatism. The heat induced by the upwelling of the asthenosphere likely led to the heat-fluxed melting of juvenile mafic crustal material in such an extensional tectonic setting during the Oligocene epoch in the eastern SZ.

Distinct sources for high-K and adakitic magmatism in SE Iran

Research into Arabia-Eurasia collision zone magmatism in Kerman Province, SE Iran, has largely focused on Late Cenozoic adakitic stocks or domes, with debate around lower crustal or subducted slab origins. Contemporary hawaiite-trachyandesite lava flows have been overlooked. New analyses for domes and lavas from near Dehaj show major and trace element distributions relating to two distinct compositional series. One contains medium-K domes with SiO2 > 60 wt%, high Sr/Y and La/Yb and generally low MgO, Ni and Cr, showing high-silica adakite affinity. The other series has high-K affinity and includes both lavas and dome samples. The two suites partially mixed in the shallow crust, confirmed by fieldwork and petrography. Isotopically the two suites are indistinguishable, implying a geologically ‘young’ age for the source of the adakites. Given its geochemical signatures and non-relationship with the more mafic, mantle-derived high-K series, we consider the adakite series to be derived from melting of eclogitized mafic lower crust. The high-K series relates to dehydration melting of mantle deeper within the ~220 km thick lithosphere. We also explore adakitic magmas across Iran and their relationship to porphyry copper deposits. At Dehaj and several other Iranian centres, adakites are chemically controlled by garnet as a source or fractionating phase, and are barren, whereas the presence of amphibole as a key phase seems to correlate with Cu mineralisation. This study also shows the need for evidence from multiple datasets to constrain adakite genesis and warns of avoiding sampling bias towards felsic lithologies.

Intraplate adakite-like rocks formed by differentiation of mantle-derived mafic magmas: A case study of Eocene intermediate-felsic porphyries in the Machangqing porphyry Cu-Au mining district, SE Tibetan plateau

Intraplate intermediate-felsic adakite-like rocks are widely considered to be generated via partial melting of the mafic lower crust. However, the genesis of phenocrysts of mafic minerals from some adakite-like rocks suggests that they can originate from mantle-derived magmas through assimilation and fractional crystallization (AFC) processes. In this study, we investigated the petrogenesis of three phases of Eocene porphyries, namely quartz diorite porphyry (QDP-I and QDP-II), granodiorite porphyry (GDP), and granite porphyry (GP), exposed in the Machangqing area of the western Yangtze Craton in the southeastern Tibetan plateau. These porphyries were formed at 34.2–35.2 Ma in an intracontinental extension setting and present adakitic affinities of high Sr/Y (50–95) and La/Yb (50–113) ratios. The similar characteristics of these Machangqing porphyries in terms of whole-rock trace elements and Sr–Nd isotopes, as well as in the mineralogy of amphibole phenocrysts and clinopyroxene antecrysts, indicate that these intermediate-felsic porphyries were derived from the same source. The important observation for investigating the parental magma of these intermediate-felsic porphyries is that they have close temporal and spatial correlations with mantle-derived mafic volcanic rocks. These intermediate-felsic porphyries have slightly enriched Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7068–0.7077, εNd(t) = −5.1 to −6.6], affected by limited crustal assimilation, as compared to those of contemporaneous mafic volcanic rocks. The high-Al amphibole phenocrysts and clinopyroxene antecrysts in QDP-I have similar rare earth element (REE) patterns with clinopyroxene phenocrysts in the mafic volcanic rocks, which suggest that these rocks originated from the same mantle-derived mafic magma. In addition, the major elements of these intermediate-felsic and mafic igneous rocks in the Machangqing area show fractional crystallization trends with an increasing magma silica component. Simulations of the major and trace elements of these igneous rocks show that the sequence of fractional crystallization of olivine, clinopyroxene, garnet, amphiboles, biotite, feldspar, and apatite with time promoted the evolution of the magma series. As revealed by the calculated physicochemical conditions during crystallization of the phenocrysts, the initial mafic magmas were derived from depths of greater than 64 km through the partial melting of the metasomatic lithosphere mantle induced by Eocene intraplate extension in the western Yangtze Craton. These initial mafic magmas underwent three stages of fractional crystallization at three levels of the thickened crust: the first at the base of the lower crust (49–55 km); followed by fractional crystallization at the middle crust (24–32 km); and finally at the upper crust (4–5 km). These fractional crystallization processes caused the initial mafic magmas to evolve through the intermediate (roughly represented by QDP-I) to the felsic compositions (GDP and GP). Whole-rock and mineral geochemistry evidence demonstrates that fractional crystallization of mantle-derived magmas, with a small amount of crustal assimilation, generated these Eocene intraplate intermediate-felsic adakitic porphyries in the Machangqing area.

Shoshonitic enclaves in the high Sr/Y Nyemo pluton, southern Tibet: Implications for Oligocene magma mixing and the onset of extension of the southern Lhasa terrane

Post-collisional potassic and high Sr/Y magmatism in the Lhasa terrane provides critical constraints on the timing and mechanism of subduction of Indian lithosphere and its role in the uplift of the Tibetan Plateau. Here, we report whole-rock geochemistry, mineral geochemistry, zircon UPb ages, and in situ zircon Hf isotope ratios for the Nyemo pluton, a representative example of such magmatism. The Nyemo pluton is composed of high Sr/Y host rocks and coeval shoshonitic mafic microgranular enclaves (MMEs). Whole-rock compositions of the host rocks and MMEs form linear trends in Harker diagrams, consistent with modification of both end-members by magma mixing. Although the main high Sr/Y phase of the pluton formed by partial melting of the lower crust of the thickened Lhasa terrane, the MMEs display abnormally enriched light rare earth elements, low whole-rock εNd(t) and low zircon εHf(t) that suggest derivation from low degree melting of hydrous and enriched mantle. Based on the occurrence of shoshonitic magma and high La/Yb and high Sr/Y with adakitic affinity host rocks around 30 Ma, the Nyemo pluton is best explained as a record of onset of extension that resulted from convective removal of the mantle lithosphere beneath Tibet in the Oligocene.

Formation of the Cretaceous skarn Cu–Au deposits of the southern Gangdese belt, Tibet: Case studies of the Kelu and Sangbujiala deposits

Mesozoic subduction of Neo-Tethyan oceanic crust caused magmatism that formed numerous Jurassic and Cretaceous intrusions within the southern Gangdese belt of Tibet. These intrusions are associated with the formation of giant Jurassic Xiongcun porphyry Cu deposits, but the prospectivity of other Cretaceous intrusions in this area remains unclear. This study presents new molybdenite Re–Os ages for the Kelu and Sangbujiala Cu–Au skarn deposits within the southern Gangdese belt as well as new geochemical, zircon U–Pb geochronological, and Sr–Nd–Hf isotopic data for the intrusions associated with these deposits. These new data indicate that (1) the Kelu deposit formed at ca. 92 Ma, coeval with emplacement of the diorite and biotite granodiorite intrusions (ca. 92–90 Ma) in this area, and (2) the Sangbujiala deposit formed at ca. 95 Ma, contemporaneous with emplacement of the mineralized biotite granodiorite (ca. 95–92 Ma) in this area. These data suggest that the skarn Cu–Au mineralization in this area formed as a result of Cretaceous magmatism in the southern Gangdese belt. The intrusions associated with the Kelu and Sangbujiala deposits have adakitic affinities with low contents of Y and heavy rare-earth elements and high Sr/Y ratios (22.4–123). These intrusions also have more primitive Sr–Nd isotopic compositions (εNd(t) = 3.99–4.81, (87Sr/86Sr)i = 0.7040–0.7044), lower εHf(t) values (8.0–13.18), and higher zircon EuN/Eu* values (mean of 0.50) than those of contemporaneous but barren adakitic intrusions in this region. This suggests that the mineralization-related intrusions were oxidized and formed from magmas that were generated predominantly by partial melting of subducted Neo-Tethyan slab material without any significant crustal contamination or assimilation of sedimentary material. However, these intrusions contain elevated Th, Ni, and Cr contents, and have high Mg# ratios (43.4–56.9), suggesting that they formed from magmas that interacted with metasomatized mantle wedge peridotite material. Combining our new data with the results of previous research suggests that the southern Gangdese belt records two stages of Cu–Au mineralization related to Neo-Tethyan subduction. The first stage generated the large Xiongcun porphyry Cu–Au deposit and was associated with normal subduction during the Jurassic, whereas the second stage was associated with the formation of the Kelu and Sangbujiala skarn deposits during Cretaceous slab rollback.

Geochronology and geochemistry of the Late Mesozoic magmatic rocks in the Chizhou Area, Lower Yangtze River Metallogenic Belt: Constraints on petrogenesis and tectonic setting

Late Mesozoic magmatism and its associated polymetallic mineralization are widespread in the Chizhou area, forming one of the ore clusters in the Lower Yangtze River Metallogenic Belt (LYRMB). The mineralization in the Chizhou area, however, is not as same as the other ore clusters (e.g., Tongling, Edong, and Jiurui) in age, source, and mechanism. In this study, we used zircon UPb dating, LuHf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes to investigate six ore-bearing intrusions in the Chizhou area. These intrusions comprise high-K calc-alkaline series pyroxene diorite, quartz diorite (porphyry), and granodiorite (porphyry). LA-ICP-MS zircon UPb dating results indicate that these intrusions formed between 150 and 141 Ma. The mafic intrusion, the Xiaodingchong pyroxene diorite, has low SiO2 and MgO contents, arc-like trace element characteristics, as well as enriched Sr-Nd-Hf isotopic compositions and high radiogenic Pb isotopes, indicating that it was derived from an enriched lithospheric mantle source. The acid intrusion, the Pailou granite, has high SiO2, enriched Sr-Nd-Hf isotopic compositions, continental crust-like geochemical characteristics, and adakitic affinities, such as high Sr, low Y, and low Yb, suggesting that the Pailou granite was derived from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust. The remaining intermediate-acid ore-bearing intrusions were produced by magma mixing of these two end members. The metallogenic deposits in the Chizhou area were mainly controlled by two different magma sources: an enriched mantle and the accreted Meso-Neoproterozoic crust. Furthermore, the low Ti-in-zircon temperatures and the variable amounts of inherited zircons indicate that the Late Mesozoic Chizhou ore-bearing intrusions were formed in hydrous conditions, which is consistent with the tectonic evolution of the low-angle paleo-Pacific plate subduction with slab foundering and roll-back.

Adakitic rocks and A‐type felsic dykes in the Changlingzi area, NE China: Constraints on multistage tectonism in the southern Great Xing'an Range

New LA–ICP–MS zircon U–Pb geochronology, whole‐rock major and trace element geochemistry data, and petrographic observations are presented for three granitic dykes in the Changlingzi area of NE China. These data provide precise age and petrogenesis information with respect to the late Palaeozoic–Mesozoic intrusions in the area, which further constrain the geodynamic evolution of the southern Great Xing'an Range. Our zircon U–Pb data denote that the felsic dykes were emplaced during the Late Permian (~254 Ma) and Late Jurassic (143–142 Ma). Geochemically, the Late Permian monzonite porphyries are characterized by high Al2O3 (16.52–16.92 wt%) and Sr (490–579 ppm) contents and low Yb (0.52–0.57 ppm), Y (5.7–6.3 ppm), and heavy rare earth element (HREE) contents, which indicate an adakitic affinity. The Late Jurassic felsic dykes are characterized by high FeOt/MgO (8.2–11.9) and Ga/Al (3.3–3.8) ratios, high zircon saturation temperatures (888–929°C), and remarkable negative Eu (Eu/Eu* = 0.25–0.42) anomalies, indicating they are akin to A‐type granites. The Late Permian adakitic magma resulted from the partial melting of thickened lower crust, and was formed during collisional orogenesis related to subduction of Palaeo‐Asian Ocean crust. The Late Jurassic A‐type magmas were probably generated by the partial melting of amphibolite‐facies lower crustal materials. The melting occurred in response to the upwelling of asthenospheric material caused by collapse of thickened crust, due to gravitational instability during closure of the Mongol–Okhotsk Ocean. Subsequently, the latest stage of magmatic activity in this area and elsewhere in NE China was controlled mainly by subduction of Palaeo‐Pacific Ocean crust.

Geochemistry, geochronology and petrogenesis of Maya Block granitoids and dykes from the Chicxulub Impact Crater, Gulf of México: Implications for the assembly of Pangea

The Late Paleozoic tectono–magmatic history and basement of the Maya block are poorly understood due to the lack of exposures of coeval magmatic rocks in the region. Recently, IODP–ICDP Expedition 364 recovered drill core samples at borehole M0077A from the peak ring of the Chicxulub impact crater, offshore of the Yucatán peninsula in the Gulf of México, have been studied comprehensively. In the lowermost ~600 m of the drill core, impact–deformed granitoids, and minor felsite and dolerite dykes are intercalated with impact melts and breccias. Zircon U-Pb dating of granitoids yielded ages of around 326 ± 5 Ma, representing the first recovery of Late Paleozoic magmatic rocks from the Maya block, which could be genetically related to the convergence of Laurentia and Gondwana. The granitoids show the features of high K2O/Na2O, LaN/YbN and Sr/Y ratios, but very low Yb and Y contents, indicating an adakitic affinity. They are also characterized by slightly positive ԑNd(326Ma) of 0.17–0.68, intermediate initial 87Sr/86Sr(326Ma) of 0.7036–0.7047 and two–stage Nd model age (TDM2) of 1027–1069 Ma, which may indicate a less evolved crustal source. Thus, the adakitic granitoids were probably generated by partial melting of thickened crust, with source components similar to Neoproterozoic metagabbro in the Carolina block (Pan–African Orogeny materials) along Peri–Gondwana. Felsite dykes are shoshonitic with typical continental arc features that are sourced from a metasomatic mantle wedge by slab–fluids. Dolerite dykes display OIB–type features such as positive Nb and Ta anomalies and low ThNpm/NbNpm. In our interpretation, the Chicxulub adakitic granitoids of this study are formed by crustal anatexis due to asthenospheric upwelling resulting from slab breakoff. Through comparing sources and processes of Late Paleozoic magmatism along the Peri–Gondwanan realm, a tearing slab breakoff model may explain the discontinuous magmatism that appears to have occurred during the convergence of Laurentia and Gondwana.

The genesis of high Ba‐Sr adakitic rocks: Insights from an Early Cretaceous volcanic suite in the central North China Craton

The origin of high Ba‐Sr magmatic suites with adakitic features has remained controversial, although they provide important clues on the evolution of continental lithosphere. Here, we present petrological, geochemical, zircon U–Pb and Lu‐Hf data from an Early Cretaceous volcanic suite of andesitic‐dacitic composition in the Laiyuan complex of the central North China Craton, with a view to gain insights on the magma source, petrogenesis, and tectonic implications. The volcanic suite represented by rocks of andesitic‐dacitic composition shows enrichment in light rare earth elements (LREE) and large‐ion lithophile elements (LILE), and depletion in high‐field‐strength elements (HFSE) with no obvious Eu anomalies. They also show typical adakitic features such as high Sr/Y and La/Yb ratios, as well as high Ba‐Sr concentrations. Zircon U–Pb geochronology indicates that these rocks were formed in Early Cretaceous during 131 to 127 Ma, with εHf(t) values ranging from −23.5 to −19.4, suggesting enriched lithospheric mantle source. The geochemical and isotopic data show that the enriched lithospheric mantle source experienced fluid‐related subduction metasomatism. The distinct geochemical features of these identified in our study including high Ba‐Sr concentrations and adakitic affinities were not only inherited from their magma source, but are also a result of fractional crystallization during magma evolution including amphibole‐dominated fractional crystallization at depth and limited plagioclase removal. The volcanism in the Taihang Mountains occurred under extensional regime triggered by the subduction and rollback of the Paleo‐Pacific slab during Late Jurassic to Early Cretaceous and consequent thermal‐mechanical erosion of the lithospheric mantle.