SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En42–46Wo41–50Fs8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO2 (52.66–55.52%), and low Al2O3, FeOt and TiO2, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit (87Sr/86Sr)i = 0.70492–0.70539, εNd(t) = − 12.47–15.07, (206Pb/204Pb)i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO2-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains. 相似文献
Li contents and isotopic compositions were determined for a suite of well-characterized basaltic lavas from the Central American Volcanic Arc (CAVA). Variable Li/Y (0.2–0.5), Li/Sc (0.1–0.4), and δ6Li values (+2.6 to −7.7‰) attest to significant compositional heterogeneity in the subarc mantle. Within specific arc segments, these parameters correlate strongly with each other and with a number of other constituents (e.g., K, Rb, Ba, B/La, 10Be/9Be, 87Sr/86Sr, U/Ce, and 230Th/232Th, among others); these correlations are particularly strong for Nicaragua samples. Coupling of this particular set of constituents is best explained in terms of addition of ‘subduction components' to the subarc mantle. Moreover, their selective enrichment with respect to relatively fluid-immobile incompatible elements signifies the dominance of fluid vs. silicate melt transport of slab components to the subarc mantle. Several interesting nuances are revealed by the Li data. First, although Li and B are strongly correlated in both Costa Rica and Nicaragua, there are systematic along-strike variations in Li/B that are consistent with these elements having different ‘fluid release patterns' from subducted slab segments. For example, Li/B is highest in Costa Rica where auxiliary evidence indicates higher subduction zone temperatures; apparently B is preferentially depleted and Li retained in the slab under warmer conditions. The same relations are reflected in Li/10Be and other subduction tracer systematics, all of which point to larger subduction contributions below Nicaragua. Yet, even Nicaragua lavas vary widely in levels of subduction enrichment. High-Ti basalts from Nejapa are the least enriched and have the highest δ6Li (1.4 to 2.6‰); these values are greater than in fresh MORB (ca. −4‰) and are not easily explained by additions of subducted Li because most oceanic crustal rocks and marine sediments have lower δ6Li than MORB (with typical values between −8 and −20‰). Thus, it appears the Nejapa data may be representative of isotopically light mantle domains. Relatively light δ6Li values in an undepleted spinel lherzolite (+11.3‰) from Zabargad Is. (Red Sea) and in primitive backarc basalts (−1.6 to −0.5‰) from Lau Basin support this conclusion. Considering representative fluid and mantle endmember compositions, the CAVA data are consistent with limited (up to a few percent) additions of slab-derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources. In our view, the subarc mantle is heterogeneous on a small scale, but some arc sectors clearly received greater slab inputs than others. 相似文献
The Yagan area of the southernmost Sino–Mongolian border is characterized by an extensional structure where a large metamorphic core complex (Yagan–Onch Hayrhan) and voluminous granitoids are exposed. New isotopic age data indicate that the granitoids, which were previously regarded as Paleozoic in age, were emplaced in early and late Mesozoic times. The early Mesozoic granitoids have 228±7 Ma U–Pb zircon age, and consist of linear mylonitic quartz monzonites and biotite monzogranites. Their chemical compositions are similar to those of potassic granites and shoshonitic series, and show an intraplate and post-collisional environment in tectonic discrimination diagrams. Their fabrics reveal that they experienced syn-emplacement extensional deformation. All these characteristics suggest that the adjustment, thinning and extensional deformation at middle to lower crustal levels might have occurred in the early Mesozoic. The late Mesozoic granitoids have a U–Pb zircon age of 135±2 Ma, and are made up of large elliptical granitic plutons. They are high-K calc-alkaline, and were forcefully emplaced in the dome extensional setting. Both the early and late Mesozoic granitoids have Nd (t) values of −2.3 to +5, in strong contrast with the negative Nd (t) values (−11) of the Precambrian host rocks. This suggests that juvenile mantle-derived components were involved in the formation of the granitoids. The similar situation is omnipresent in Central Asia. This study demonstrates that tectonic extension, magmatism and crustal growth are closely related, and that post-collisional and intraplate magmatism was probably a significant process for continental growth in the Phanerozoic. 相似文献
In situ Sr-isotope and microchemical studies were used to determine the provenance of K-feldspar megacrysts hosted in mafic alkaline potassic, ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central Italy.
At Monte Cimino volcanic complex, mafic leucite-free ultrapotassic megacryst-bearing rocks of olivine latitic composition are associated with evolved latite and trachyte. Here, latites and trachytes straddle the sub-alkaline field. Age-corrected 87Sr/86Sr values (Sri) of the analysed Cimino olivine latites vary from 0.71330 and 0.71578 and strongly increase at constant Mg value. Latite and trachyte have lower Sri than olivine latites ranging between 0.71331 and 0.71361. Sri of K-feldspar megacrysts from olivine latites are between 0.71352 and 0.71397, but core and rim 87Sr/86Sr ratios within individual megacryst are indistinguishable. In all the mafic rocks, the megacrysts are not in isotopic equilibrium with the hosts. K-feldspar megacrysts from both the latite and trachyte have similar Sr-isotope compositions (Sri=0.71357–0.71401) to those in the olivine latites. However, Sri of megacryst in the trachyte vary significantly from core to rim (Sri from 0.71401 to 0.71383). As with the olivine latites, the K-feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte.
At Vico volcano, megacryst-bearing rocks are mafic leucite-free potassic rocks, mafic leucite-bearing ultrapotassic rocks and old trachytic rocks. The mafic leucite-bearing and leucite-free rocks are a tephri-phonolite and an olivine latite, respectively. A megacryst in Vico trachyte is isotopically homogeneous (Sri CORE=0.71129, RIM=0.71128) and in equilibrium with the host rock (Sri bulk ROCK=0.71125). Sri of megacryst from tephri-phonolite is clearly not in isotopic equilibrium with its host (Sri bulk ROCK=0.71158), and it increases from core (Sri=0.71063) to rim (Sri=0.71077). A megacryst in Vico olivine latite is isotopically homogeneous (Sri CORE=0.71066, RIM=0.71065), but not in equilibrium with the host rock (Sri bulk ROCK=0.71013).
The Sr isotope microdrilling technique reveals that Cimino megacrysts were crystallised in a Cimino trachytic magma and were subsequently incorporated by mixing/mingling processes in the latitic and olivine latitic melts. A model invoking the presence of a mafic sub-alkaline magma, which was mixed with the olivine latite, is proposed to justify the lack of simple geochemical mixing relation between Cimino trachytes and olivine latites. This magmatological model is able to explain the geochemical characteristics of Cimino olivine latites, otherwise ascribed to mantle heterogeneity.
The similarity of core Sri of megacrysts hosted in Vico tephri-phonolite and olivine latite suggests that the K-feldspar megacrysts are co-genetic. Isotopic equilibrium between megacryst and Vico host trachyte indicates that the trachyte is the parent of this megacryst. On the contrary, the megacrysts hosted in tephri-phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim Sr isotope increase of the xenocryst hosted in the tephri-phonolite. The megacrysts hosted in the Vico mafic rocks might derive from a trachytic melt similar in composition to the old Vico trachytes. 相似文献
Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world's most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing'an, Lesser Xing'an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region, aiming to understand their role in the building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities which suggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram.
The emplacement of the Xinhuatun pluton took place at 184±4 Ma as revealed by zircon SHRIMP U–Pb data. This is also supported by the slightly younger Rb–Sr whole-rock (WR) isochron age of 173±3 Ma. A whole-rock (WR) Rb–Sr isochron age of 154±3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U–Pb age of 137±2 Ma and WR Rb–Sr isochron age of 143±5 Ma. Biotite-WR Rb–Sr isochrons and 40Ar/39Ar ages of feldspars allow us to estimate the cooling rate of each pluton.
Geochemical data suggest that the rocks are highly fractionated I-type granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite. 相似文献
Surface water samples were collected from Langtang Lirung glacier outlet point to the Narayani river system in central Nepal
in order to investigate the role of elevation in the variation of chemistry along the drainage networks. The chemistry of
Langtang–Narayani river system was dominated by sulfide oxidation coupled with carbonate dissolution and weathering of silicate
minerals. Calcium and magnesium concentrations were relatively higher than other cations and the sum of both species strongly
correlated with alkalinity, supporting the dissolution of carbonate and dolomite as the dominant source for these ions. Aluminosilicate
minerals primarily as albite and anorthite appeared as dominant silicate minerals within the drainage basin. Bisiallitization
was the dominant type of weathering within the entire drainage system. Hydrogen ion concentration was lower in the low elevation
sites than in high elevation sites reflecting the more consumption of carbon dioxide in the low elevation sites due to enhanced
chemical weathering rates. Furthermore, major solutes like sum of base cations, silicon as well as alkalinity increased in
concentration in the lower elevation sites. All regulating factors appeared to be directly related to elevation and hence
elevation appeared to be the prime factor for the variation in chemical species along the Langtang–Narayani river system.
Toshiyuki Masuzawa: deceased. 相似文献