Mineral chemistry of Plio-Quaternary subvolcanic rocks,southwest Yazd Province,Iran

The NW–SE-trendingLate Cretaceous–Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA) in southwest Iran hosts numerous Plio-Quaternary subvolcanic porphyritic andesitic to rhyodacitic domes intruded into a variety of rock sequences. Bulk-rock geochemical data show that the calc-alkaline dacitic to rhyodacitic subvolcanic rocks share compositional affinities with high-silica adakites, including high ratios of Na₂O/K₂O >1, Sr/Y (most >70), and La/Yb (>35), high Al₂O₃ (>15 wt.%), low Yb (<1.8 ppm) and Y (<18 ppm) contents, no significant Eu anomalies, and flat to gently upward-sloping chondrite-normalized heavy rare-earth element (HREE) patterns. All analysed rocks are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). They also display typical features of subduction-related calc-alkaline magmas. In chondrite-normalized rare-earth element patterns, the light rare-earth elements (LREEs) are enriched ((La/Sm) _N = 3.49–7.89) in comparison to those of the HREE ((Gd/Yb) _N = 1.52–2.38). Except for the G-Aliabad Dome, plagioclase crystals in the Shamsabad, Ostaj, Abdollah, and Bouragh Domes are mostly oligoclase to andesine (An_19–49). Amphibole and biotite are abundant ferromagnesian minerals in the subvolcanic rocks. Calcic amphiboles are dominantly magnesiohornblende, magnesiohastingsite, and tschermakite with Mg/(Mg + Fe_tot) ratios ranging from 0.58 to 0.78. In all the studied domes, amphiboles are typically ferric iron-rich, but that those the Shamsabad Dome have the highest Fe³⁺/(Fe³⁺ + Fe²⁺) ratios, between 0.69 and 0.98. Amphiboles from the Ostaj and Shamsabad Domes are relatively rich in F (0.39–1.01 wt.%) in comparison to the other studied domes. This phase commonly shows pargasitic and hastingsitic substitutions with a combination of tschermakitic and edenitic types.

Temperature-corrected Al-in-hornblende data show that amphibole phenocrysts from the Ostaj, Abdollah, and G-Aliabad Domes crystallized at pressures ranging from 2.14 to 3.42 kbar, 3.49 to 3.96 kbar, and 2.02 to 3.47 kbar, respectively. Temperatures of crystallization calculated with the amphibole–plagioclase thermometer for the Ostaj, Abdollah, and G-Aliabad subvolcanic domes range from 735°C to 826°C (mean = 786 ± 29), 778°C to 808°C (mean = 791 ± 13), and 866°C to 908°C (mean = 885 ± 12), respectively. In the annite–siderophyllite–phlogopite–eastonite quadrilateral, biotite from the G-Aliabad, Bouragh, and Ostaj Domes are characterized by relatively low total Al contents with variable Fe_tot/(Fe_tot + Mg) values from 0.26 to 0.43. All biotite analyses define a nearly straight line in the X _Mg versus Fe_tot plot, with r = –0.96 correlation coefficient. In comparison to other domes, the F content of biotite from the G-Aliabad Dome shows high concentrations in the range of 1.80–2.57 wt.% (mean = 2.20). Inferred pre-eruptive conditions based on the calcic amphibole thermobarometric calculations for the Shamsabad, Abdollah, and Ostaj Domes show that the calc-alkaline subvolcanic magma chamber, on average, was characterized by a water content of 6.10 wt.%, a relatively high oxygen fugacity of 10^–10.66 (ΔNNO + 1.28), a temperature of 896°C, and a pressure of 2.75 kbar.     

Crustal thickness and adakite occurrence in the Philippines: Is there a relationship?

Adakites are increasingly being recognized worldwide in a variety of tectonic settings. Models on the formation of this geochemically distinct class of volcanic rocks have evolved from partial melting of subducted young, hot oceanic slabs to magmatism resulting from oblique subduction, low‐angle or flat subduction, or even slab‐tearing. Some workers have also pointed to the partial melting of thickened crust to explain the generation of adakitic melts. Rare earth element ratios from adakites and adakitic rocks in the Philippines were used in this study to obtain approximations of the levels where they were generated. These were tied to available geophysical data that defines the crustal thickness of the areas where the samples were collected. High Sm/Yb and La/Yb ratios denote the involvement of amphiboles, and in some cases garnet, in the generation of adakites and adakitic magmas. The presence of amphibole and garnet as residual phases suggests high pressures corresponding to thicker crust (～30 to 45 km). Adakites and adakitic rocks formed through processes other than melting of subducted young oceanic crust would need ≥30 km to account for the heavy rare earth element signatures. If mantle fractionation is not the process involved, crustal thickness is critical to generate adakites and adakitic rocks.     

Newly discovered Late Cretaceous adakites in South Fujian Province: Implications for the late Mesozoic tectonic evolution of Southeast China

The Yongchun pluton is a Late Cretaceous adakitic intrusion in South Fujian Province, Southeast China, with associated metal mineralization. An understanding of the Yongchun pluton is helpful in tectono‐magmatic evolutionary processes, and is important in explaining the origin of related porphyry‐type deposits. Zircons from three samples of the pluton were analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), yielding U–Pb ages of 99.50 ±0.87 Ma, 97.74 ±0.59 Ma, and 99.65 ±0.92 Ma. These ages are similar to those of the Sifang, Luoboling, and Sukeng plutons, all of which are related to Cu–Pb–Zn–Mo mineralization within the study area. The Yongchun pluton comprises high‐potassium, calc‐alkaline, metaluminous rocks, with average A/CNK values of 0.91, ⁸⁷Sr/⁸⁶Sr ratios of 0.705 51 to 0.706 83, εNd(t) values of ?4.63 to ?5.90, and two‐stage Nd model (T_2DM) ages of 1.49–1.39 Ga, indicating the magmas were generated by partial melting of Mesoproterozoic continental crust mixed with mantle‐derived magmas. The pluton has geochemical characteristics typical of adakites, such as a high Sr content (average 553 ppm), and low Y (average 15.2 ppm) and Yb (average 1.61 ppm) contents, indicating that the parental magma was formed under high‐pressure conditions. The magmatism was associated with thickening of the lower crust during a change in subduction angle and convergence rate of the paleo‐Pacific Plate at 100 Ma. The compression was limited to South Fujian Province.     

Temporal Geochemical Evolution of Neogene Magmatism in the Baguio Gold–Copper Mining District (Northern Luzon, Philippines)

Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene–Pliocene–Quaternary calc‐alkaline and adakitic intrusions. Systematic sampling, K‐Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2–18.7 Ma), a Middle–Late Miocene phase (15.3–8 Ma) and finally a Pliocene–Quaternary event (3–1 Ma). The first phase emplaced evolved calc‐alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward‐dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced first petrogenetically related and relatively unradiogenic low‐K calc‐alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to reflect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversified rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area.     

Adakitic magmas in the Ecuadorian Volcanic Front: Petrogenesis of the Iliniza Volcanic Complex (Ecuador)

The Iliniza Volcanic Complex (IVC) is a poorly known volcanic complex located 60 km SSW of Quito in the Western Cordillera of Ecuador. It comprises twin peaks, North Iliniza and South Iliniza, and two satellite domes, Pilongo and Tishigcuchi. The study of the IVC was undertaken in order to better constrain the role of adakitic magmas in the Ecuadorian arc evolution. The presence of volcanic rocks with an adakitic imprint or even pristine adakites in the Ecuadorian volcanic arc is known since the late 1990s. Adakitic magmas are produced by the partial melting of a basaltic source leaving a garnet rich residue. This process can be related to the melting of an overthickened crust or a subducting oceanic crust. For the last case a special geodynamic context is required, like the subduction of a young lithosphere or when the subduction angle is not very steep; both cases are possible in Ecuador. The products of the IVC, made up of medium-K basaltic andesites, andesites and dacites, have been divided in different geochemical series whose origin requires various interactions between the different magma sources involved in this subduction zone. North Iliniza is a classic calc-alkaline series that we interpret as resulting from the partial melting of the mantle wedge. For South Iliniza, a simple evolution with fractional crystallization of amphibole, plagioclase, clinopyroxene, magnetite, apatite and zircon from a parental magma, being itself the product of the mixing of 36% adakitic and 64% calc-alkaline magma, has been quantified. For the Santa Rosa rhyolites, a slab melting origin with little mantle interactions during the ascent of magmas has been established. The Pilongo series magma is the product of a moderate to high degree (26%) of partial melting of the subducting oceanic crust, which reached the surface without interaction with the mantle wedge. The Tishigcuchi series shows two stages of evolution: (1) metasomatism of the mantle wedge peridotite by slab melts, and (2) partial melting (10%) of this metasomatized source. Therefore, the relative ages of the edifices show a geochemical evolution from calc-alkaline to adakitic magmas, as is observed for several volcanoes of the Ecuadorian arc.     

Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)

In the Northern Volcanic Zone of the Andes, the Cayambe VolcanicComplex consists of: (1) a basal, mostly effusive volcano, theViejo Cayambe, whose lavas (andesites and subordinate dacitesand rhyolites) are typically calc-alkaline; and (2) a younger,essentially dacitic, composite edifice, the Nevado Cayambe,characterized by lavas with adakitic signatures and explosiveeruptive styles. The construction of Viejo Cayambe began >1·1Myr ago and ended at 1·0 Ma. The young and still activeNevado Cayambe grew after a period of quiescence of about 0·6Myr, from 0·4 Ma to Holocene. Its complex history isdivided into at least three large construction phases (Angurealcone, Main Summit cone and Secondary Summit cone) and compriseslarge pyroclastic events, debris avalanches, as well as periodsof dome activity. Geochemical data indicate that fractionalcrystallization and crustal assimilation processes have a limitedrole in the genesis of each suite. On the contrary, field observations,and mineralogical and geochemical data show the increasing importanceof magma mixing during the evolution of the volcanic complex.The adakitic signature of Nevado Cayambe magmas is related topartial melting of a basaltic source, which could be the lowercrust or the subducted slab. However, reliable geophysical andgeochemical evidence indicates that the source of adakitic componentis the subducted slab. Thus, the Viejo Cayambe magmas are inferredto come from a mantle wedge source metasomatized by slab-derivedmelts (adakites), whereas the Nevado Cayambe magmas indicatea greater involvement of adakitic melts in their petrogenesis.This temporal evolution can be related to the presence of thesubducted Carnegie Ridge, modifying the geothermal gradientalong the Wadati–Benioff zone and favouring slab partialmelting. KEY WORDS: adakites; ⁴⁰Ar/³⁹Ar dating; Cayambe volcano; Ecuador; mantle metasomatism; Andes     

中国东部埃达克岩及成矿作用

埃达克岩(adakite)最初是指与俯冲洋壳部分熔融有关的一类高铝的中酸性侵入岩或火山岩。主要地球化学特点有:高Sr、低Y和Yb,轻重稀土元素强烈分异,不具有明显的Eu负异常,K2O/Na2O比值低。埃达克岩从发现、命名以来,一直是国际地学界关注的前沿课题之一。埃达克岩研究之所以方兴未艾,一则是其特殊的地球化学性质,二则是其成矿专属性。中国东部是目前国内发现埃达克岩最多的地区,主要为中生代侵入岩和火山岩,分布在长江中下游区、胶东地区及晋京辽区(或京冀区、东北区)。其中,长江中下游是重要的铜金多金属成矿带,胶东是重要的金矿带。目前,对中国东部埃达克岩成因的解释有四种模式:加厚古老下地壳的直接部分熔融、底侵玄武质下地壳部分熔融、拆沉下地壳部分熔融以及洋中脊俯冲。埃达克岩与铜金成矿的机制目前的研究还不是很透彻。一般认为埃达克质岩浆富流体、高氧逸度和基性源岩,这些特点均有利于Cu、Au等深源金属元素的萃取与富集成矿。     

辽西彰武大四家子地区中生代火山岩锆石U-Pb年代学及地球化学特征

通过对辽西彰武县以东大四家子乡高城窝堡村义县组标准剖面中生代火山岩锆石U-Pb年代学研究表明, 其火山岩年龄为122.4±0.4Ma, 属早白垩世.对该区域23件典型火山岩样品的地球化学研究表明, 除3件流纹岩样品外, 其余样品具有高镁埃达克岩地球化学特征(SiO₂=56.46%~65.14%、Al₂O₃=14.60%~17.19%、Mg#=50~59、Sr=501~700μg/g、Yb=1.04~1.54μg/g、Y=12.0~17.5μg/g、Eu/Eu*=0.85~0.97、Sr/Y=29~46、La_N/Yb_N=13~28), 同位素上具有高的初始87Sr/86Sr(0.705464~0.705812)比值, 低的ε_Nd(122Ma)(-6.12~-12.80)值特征, 同时样品中存在具有反环带的辉石斑晶, 辉石中稀土元素含量分布存在着从核部到边部逐渐降低的趋势, 且存在负铕异常(Eu/Eu*=0.64~0.76).结合前人对彰武义县组下部火山岩的研究, 笔者倾向认为该套火山岩的成因是拆沉作用与岩浆混合作用共同作用的结果, 即拆沉作用导致软流圈地幔物质上涌加热下地壳形成的长英质岩浆, 与来自地幔由拆沉作用形成的埃达克质高镁安山岩浆混合形成.      

Geochemistry of Adakites from the Philippines: Constraints on Their Origins

Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum-mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post-collision setting.
All these samples show a significant depletion in Y and HREE with respect to their "normal" calc-alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these "typical adakites" is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed "intermediate adakites", display major and trace element chemical features intermediate between those of the former group and those of normal calc-alkaline lavas. We show that magma mixing between slab-derived adakitic magmas and mafic mantle-derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.     

俯冲带复杂的壳幔相互作用

俯冲带除俯冲板片脱水形成的富大离子亲石元素流体、交代地幔楔形成的岛弧钙碱性玄武岩安山岩-英安岩-流纹岩及相应侵入岩组合外，还存在由俯冲扳片熔融形成的埃达克质熔体交代地慢楔形成的埃达克岩-富铌玄武岩-富镁安山岩组合，从而构成了俯冲带的流体交代与熔体交代两大类壳慢相互作用体系及相应的岩石组合。熔体交代作用的显著特点是Mg、高场强元素Nb、Ti、P等含量增加，Nd／Sr值增高，而Si、K、Na及La／Yb降低。洋壳板片或洋脊俯冲、玄武质岩浆底侵使地壳增厚，或板片断离、撕裂等作用均可产生埃达克质熔体并随之产生熔体交代作用。流体和熔体与地幔橄揽岩的相互作用构成了俯冲带复杂的地球化学体系。