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1.
Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO2 and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO2-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of 87Sr/86Sr, leaving 143Nd/144Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001  相似文献   

2.
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.  相似文献   

3.
A metallogenic survey of alkalic rocks of Mt. Somma-Vesuvius volcano   总被引：2，自引：0，他引：2  
Summary Somma-Vesuvius is an alkaline volcano whose products (pumice, scoria and lava) have alkaline (Na2O + K2O) contents between 6 and 16 wt%, Mg number <50, SiO2 59–47 wt% and MgO 0–7.8 wt% (more than 50% of the samples have a content <2 wt%). Immobile-element ratios (Th/Yb, Ta/Yb, Ce/Yb) indicate a shoshonitic character, while the K2O content (4–10 wt%) is characteristic of ultrapotassic rocks. The behavior of selected metals is discussed by grouping them on the basis of the stratigraphic sequence and differentiating the volcanic activity between plinian and interplinian (Rolandi et al., 1998; Ayuso et al., 1998). This allows observation of the variation within each formation from 25.000 y. BP to the last historic eruptive cycle (1631–1944 AD). The main processes to explain the wide distribution of the data presented are fractional crystallization of a mantle-derived magma, magma mixing, and contamination with heterogeneous lower and/or upper crust. Variation diagrams distinguish different behavior for groups of metals: Ag (0.01–0.2 ppm), Mo (1–8.8 ppm), W (1.3–13 ppm), Pb (16–250 ppm), Sb (0.2–2.6 ppm), Sc (0.2–61 ppm), Li (15–140 ppm) and Be (1–31 ppm) increase with increasing differentiation and tend to correlate with the incompatible trace elements (Th, Hf, etc). Cu (10–380 ppm), Au (2–143 ppb), Co (0.7–35.1 ppm) and Fe (1.3–6.2 wt%) decrease towards advanced stage of differentiation. Iron also identifies three magmatic groups. The ratio Fe3+/Fe2+ ranges between 0.2 and 1.8, and Fe2O3/(Fe2O3 + FeO) ranges between 0.2 and 0.8, giving rise to an oxidized environment; exceptions are in the samples belonging to the interplinian formations: I, II, medieval and 1631–1994 AD. Fluorine ranges between 0.1 and 0.4 wt% for the complete Mt. Somma-Vesuvius activity, except for the Ottaviano and Avellino plinian (0.8 wt%) events. Chlorine has a wider range, from 0.1 wt% to 1.6 wt%. Mt Somma-Vesuvius has some features similar to those of mineralized alkaline magmatic systems which coincide with the transition between subduction-related compression and extension-related to continental rifting. We infer that a prospective time for the formation of mineralization at Mt Somma-Vesuvius was during the 1631–1944 eruptive period. Received March 27, 2000; revised version accepted February 28, 2001  相似文献   

4.
The lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed by basalts and basaltic andesites having a significant variation in TiO2 (from 1.2 to 3.3 wt%), Zr (from 84 to 253 ppm), Nb (from 5 to 16ppm) and Ba (from 63 to 407 ppm), at MgO ranging from 10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in a temperature range from 1220° to 1125°C. These rocks resemble those belonging to the lower-most formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi) as well as those of the Poladpur formation. Samples analyzed for87Sr/86Sr give a range of initial ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO2 (1.2–1.5 wt%) and Zr (84–105 ppm) at moderate MgO (5.2–6.2 wt%), matching the composition of low-Ti basalts of Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the north-western outcrops. The mildly enriched high field strength element contents of the samples with TiO2 > 1.5 wt% make them products of mantle sources broadly similar to those which generated the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized diagrams, and relatively high87Sr/86Sr, make evident a crustal input with crustally derived materials at less differentiated stages than those represented in this sample set, or even within the sub-Indian lithospheric mantle.  相似文献   

5.
The Tuwu–Yandong porphyry copper belt lies in the eastern Tianshan mountains, eastern section of the Central Asian orogenic belt. The copper mineralization is mainly hosted in plagiogranite porphyries intruded into early Carboniferous volcanic rocks of the Paleozoic Dananhu island arc between the Tarim and Siberian plates. The plagiogranite porphyries have contents of 65–73 wt% SiO2, 14–17 wt% Al2O3, 0.9–2.2 wt% MgO, 3–16 ppm Y, 0.4–1.40 ppm Yb, 347–920 ppm Sr, and positive Eu anomalies. The rocks also exhibit positive ɛ Nd(t) values (+5.0 to +9.4) and low initial 87Sr/86Sr values (0.70316–0.70378). Such features are similar to those of adakites derived from partial melting of a subduction-related oceanic slab. The mineralization age is early Carboniferous (350–320 Ma), which is close to that of the porphyries. The close relationship between the Cu mineralization and the porphyry is also indicated by their similar Sr-Nd-Pb isotopic compositions. We suggest that the copper porphyry (magma) system in the Dananhu island arc was formed by direct melting of an obliquely subducting early Carboniferous oceanic slab.  相似文献   

6.
Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.  相似文献   

7.
A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif (380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier phoscorites and carbonatites (group 1) are characterized by relatively low87Sr/86Sr (0.70330–0.70349) and143Nd/144Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from the second group has higher87Sr/86Sr (0.70350–0.70363) and143Nd/144Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6 ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from the earlier group fits into neither of the two groups and is characterized by the highest initial87Sr/86Sr (0.70385) and lowest143Nd/144Nd (0.51229) of any of the apatites. Within both groups initial87Sr/86Sr and143Nd/144Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ18O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted quadrant of an εNd versus εSr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring the Sr−Nd evolution of carbonatites. Editorial responsibility: W. Schreyer  相似文献   

8.
A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < 3 wt%), Al-rich hornblende (Al2O3 ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO2 of 60–63 wt%, low MgO (<2.0 wt%), K2O (<1.3 wt%), but high Al2O3 (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.  相似文献   

9.
We report major and trace element composition, Sr–Nd isotopic and seismological data for a picrite–basalt–rhyolite suite from the northern Tarim uplift (NTU), northwest China. The samples were recovered from 13 boreholes at depths between 5,166 and 6,333 m. The picritic samples have high MgO (14.5–16.8 wt%, volatiles included) enriched in incompatible element and have high 87Sr/86Sr and low 143Nd/144Nd isotopic ratios (εNd (t) = −5.3; Sri = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO2 (2.1–3.2 wt%, volatiles free), have high FeOt abundances (11.27–15.75 wt%, volatiles free), are enriched in incompatible elements and have high Sr and low Nd isotopic ratios (Sri = 0.7049–0.7065; εNd (t) = −4.1 to −0.4). High Nb/La ratios (0.91–1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U–Pb zircon ages between 291 ± 4 and 272 ± 2 Ma. Group 1 rhyolites are enriched in Nb and Ta, have similar Nb/La, Nb/U, and Sr–Nd isotopic compositions to the associated basalts, implying that they are formed by fractional crystallization of the basalts. Group 2 rhyolites are depleted in Nb and Ta, have low Nb/La ratios, and have very high Sr and low Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite–basalt–rhyolite suite from the NTU, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a Large Igneous Province (LIP) that is characterized by large areal extent, rapid eruption, OIB-type chemical composition, and eruption of high temperature picritic magma. The Early Permian magmatism, which covered an area >300,000 km2, is therefore named the Tarim Flood Basalt.  相似文献   

10.
Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events. Received: 14 August 1998 / Accepted: 1 June 1999  相似文献   

11.
Pre-collisional Eocene–Oligocene arc diorites, quartzdiorites, granodiorites, and volcanic equivalents in the Kerman arc segment in central Iran lack porphyry Cu mineralization and ore deposits, whereas collisional middle-late Miocene adakite-like porphyritic granodiorites without volcanic equivalents host some of the world’s largest Cu ore deposits. Petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment. Ore-hosting Kuh Panj porphyry intrusions exhibit high Sr (>400 ppm), low Y (<12 ppm) contents, significant REE fractionation (La/Yb > 20), no negative Eu anomalies (Eu/Eu* ≥ 1), and relatively non-radiogenic Sr isotope signatures (87Sr/86Sr = 0.7042–0.7047), relative to Eocene–Oligocene granitoids (mainly Sr < 400 ppm; Y > 12; La/Yb < 15; Eu/Eu* < 1; 87Sr/86Sr = 0.7053–0.7068). Trace element modeling indicates peridotite melting for the barren Eocene–Oligocene intrusions and a hydrous garnet-bearing amphibolite source for middle-late Miocene ore-hosting intrusions. The presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30–35 to 40–45 km or 12 kbar. The changes in residual mineralogy in the source of Eocene to Miocene rocks in the Kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel. Underplating of Cu and sulfur-rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust–mantle boundary, and dehydration melting under oxidizing conditions produced syn- and post-collisional ore-hosting intrusions, while the lack of post-collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur-rich and oxidized adakitic magmas. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

12.
Late Mesozoic dioritic and quartz dioritic plutons are widespread in the Daye region, eastern Yangtze craton, eastern China. Detailed geochronological, geochemical, and Sr–Nd isotopic studies have been undertaken for most of these plutons, in an attempt to provide a comprehensive understanding in the age, genesis and geodynamical control of the extensive magmatism. SHRIMP and LA-ICP-MS zircon U–Pb dating indicate that the plutons were emplaced in the range of latest Jurassic (ca. 152 Ma) to early Cretaceous (ca. 132 Ma), which was followed by dyke emplacement between 127 and 121 Ma and volcanism during the 130–113 Ma interval. Both diorites and quartz diorites are sodic, metaluminous, high-K calc-alkaline, and characterized by strongly fractionated, sub-parallel REE patterns without obvious Eu anomalies. The rocks are enriched in highly incompatible elements and large ion lithophile elements, but depleted in high field strength elements. Samples of diorite and quartz diorite have similar Sr–Nd isotopic compositions that are consistent with the early Cretaceous basalts and mafic intrusions throughout the eastern Yangtze craton. The geochemical and isotopic data, together with results of geochemical modeling, indicate an enriched mantle source for the plutonic rocks. The quartz diorites have geochemical signatures resembling adakites, such as high Al2O3 (15–19 wt.%), Sr (630–2,080 ppm), Na2O (>3.5 wt.%), negative Nb–Ta anomalies, low Y (7–19 ppm), Yb (0.5–1.8 ppm), Sc (5–15 ppm), and resultant high Sr/Y (45–200) and La/Yb (31–63) ratios. Genesis of the adakitic quartz diorites is best explained in terms of low-pressure intracrustal fractional crystallization of cumulates consisting of hornblende, plagioclase, K-feldspar, magnetite, and apatite from mantle-derived dioritic magmas. Mantle-derived magmatism broadly coeval with that of the Daye region also is widespread in other regions of the eastern Yangtze craton, reflecting large-scale melting of the lithospheric mantle during the Late Mesozoic. The large-scale magmatism was most likely driven by lithospheric extension associated with thinning of lithospheric mantle beneath the eastern China continent.  相似文献   

13.
We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dyn/Ybn and Lan/Ybn. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial 87Sr/86Sr and 206Pb/204Pb ratios, and the highest 144Nd/143Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.  相似文献   

14.
The problem of mantle metasomatism vs. crustal contamination in the genesis of arc magmas with different potassium contents has been investigated using new trace element and Sr–Nd–Pb isotopic data on the island of Vulcano, Aeolian arc. The analysed rocks range in age from 120 ka to the present day, and cover a compositional range from basalt to rhyolite of the high-K calc-alkaline (HKCA) to shoshonitic (SHO) and potassic (KS) series. Older Vulcano products (>30 ka) consist of HKCA–SHO rocks with SiO2=48–56%. They show lower contents of K2O, Rb and of several other incompatible trace element abundances and ratios than younger rocks with comparable degree of evolution. 87Sr/86Sr ranges from 0.70417 to 0.70504 and increases with decreasing MgO and compatible element contents. 206Pb/204Pb ratios display significant variations (19.31 to 19.76) and are positively correlated with MgO, 143Nd/144Nd (0.512532–0.512768), 207Pb/204Pb (15.66–15.71) and 208Pb/204Pb (39.21–39.49). Overall, geochemical and isotopic data suggest that the evolution of the older series was dominated by assimilation–fractional crystallisation (AFC) with an important role for continuous mixing with mafic liquids. Magmas erupted within the last 30 ka consist mostly of SHO and KS intermediate and acid rocks, with minor mafic products. Except for a few acid rocks, they display moderate isotopic variations (e.g. 87Sr/86Sr=0.70457–0.70484; 206Pb/204Pb=19.28–19.55, but 207Pb/204Pb=15.66–15.82), which suggest an evolution by fractional crystallisation, or in some cases by mixing, with little interaction with crustal material. The higher Sr isotopic ratios (87Sr/86Sr=0.70494–0.70587) of a few, low-volume, intermediate to acid rocks support differentiation by AFC at shallow depths for some magma batches. New radiogenic isotope data on the Aeolian islands of Alicudi and Stromboli, as well as new data for lamproites from central Italy, are also reported in order to discuss along-arc compositional variations and to evaluate the role of mantle metasomatism. Geochemical and petrological data demonstrate that the younger K-rich mafic magmas from Vulcano cannot be related to the older HKCA and SHO ones by intra-crustal evolutionary processes and point to a derivation from different mantle sources. The data from Alicudi and Stromboli suggest that, even though interaction between magma and wall rocks of the Calabrian basement during shallow level magma evolution was an important process locally, a similar interpretation can be extended to the entire Aeolian arc. Received: 27 September 1999 / Accepted: 24 May 2000  相似文献   

15.
Summary Major, trace element and isotopic (Sr, Nd, Pb) data are reported for representative samples of interplinian (Protohistoric, Ancient Historic and Medieval Formations) activity of Mt. Somma-Vesuvius volcano during the last 3500 years. Tephra and lavas exhibit significant major, trace element and isotopic variations. Integration of these data with those obtained by previous studies on the older Somma suites and on the latest activity, allows to better trace a complete petrological and geochemical evolution of the Mt. Somma-Vesuvius magmatism. Three main groups of rocks are recognized. A first group is older than 12.000 yrs, and includes effusive-explosive activity of Mt. Somma. The second group (8000–2700 yrs B.P.) includes the products emitted by the Ottaviano (8000 yrs. B.P.) and Avellino (3550 yrs B.P.) plinian eruptions and the interplinian activity associated with the Protohistoric Formation. Ancient Historic Formation (79–472 A.D.), Medieval Formation (472–1139 A.D.) and Recent interplinian activity (1631–1944 A.D.) belong to the third group of activity (79–1944 A.D.). The three groups of rocks display distinct positive trends of alkalis vs. silica, which become increasingly steeper with age. In the first group there is an increase in silica and alkalis with time, whereas an opposite tendency is observed in the two younger groups. Systematic variations are also evident among the incompatible (Pb, Zr, Hf, Ta, Th, U, Nb, Rb, Cs, Ba) and compatible elements (Sr, Co, Cr). REE document variable degrees of fractionation, with recent activity displaying higher La/Yb ratios than Medieval and Ancient Historic products with the same degree of evolution. N-MORB normalized multi-element diagrams for interplinian rocks show enrichment in Rb, Th, Nb, Zr and Sm (> *10 N-MORB). Sr isotope ratios are variable, with Protohistoric rocks displaying 87Sr/86Sr =  0.70711–0.70810, Ancient Historic 87Sr/86Sr = 0.70665–0.70729, and Medieval 87Sr/86Sr = 0.70685–0.70803. Neodymium isotopic compositions in the interplinian rocks show a tendency to become slightly more radiogenic with age, from the Protohistoric (143Nd/144Nd = 0.51240–0.51247) to Ancient Historic (143Nd/144Nd = 0.51245–0.51251). Medieval interplinian activity (143Nd/144Nd: 0.51250–0.51241) lacks meaningful internal trends. All the interplinian rocks have virtually homogeneous compositions of 207Pb/204Pb and 208Pb/204Pb in acid-leached residues (207Pb/204Pb ∼15.633 to 15.687, 208Pb/204Pb ∼38.947 to 39.181). Values of 206Pb/204Pb are very distinctive, however, and discriminate among the three interplinian cycles of activity (Protohistoric: 18.929–18.971, Ancient Historic: 19.018–19.088, Medieval: 18.964–19.053). Compositional trends of major, trace element and isotopic compositions clearly demonstrate strong temporal variations of the magma types feeding the Somma-Vesuvius activity. These different trends are unlikely to be related only to low pressure evolutionary processes, and reveal variations of parental melt composition. Geochemical data suggest a three component mixing scheme for the interplinian activity. These involve HIMU-type and DMM-type mantle and Calabrian-type lower crust. Interaction between these components has taken place in the source; however, additional quantitative constraints must be acquired in order to better discriminate between magma characteristics inherited from the sources and those acquired during shallow level evolution. Received May 5, 2000; revised version accepted June 19, 2001  相似文献   

16.
Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.  相似文献   

17.
Isotope geochemistry and fluid inclusion study of skarns from Vesuvius   总被引：3，自引：0，他引：3  
Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals. Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a) primary silicate melt inclusions (THOM = 1000–1050 °C), b) CO2 ± H2S-rich fluid inclusions (THOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (THOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing saline-melt inclusions (THOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below 700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable 206Pb/204Pb (19.047–19.202), 207Pb/204Pb (15.655–15.670), and 208Pb/204Pb (38.915–39.069) and relatively low 143Nd/144Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ13CV-PDB = −5.4 to −1.1‰; δ18OV-SMOW = 11.7 to 16.4‰) indicate formation from a 18O- and 13C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution of CO2-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns. Received March 1, 2000; revised version accepted November 2, 2000  相似文献   

18.
Summary ?To enhance the ability to distinguish tholeiitic from alkalic magma parentages by mineral compositions, I determined trace-element abundances in plagioclase separated from xenolithic gabbros of Mauna Kea volcano. These gabbros have origins in tholeiitic and alkalic magmas of the Hamakua postshield stage of Mauna Kea volcanism. Chondrite-normalized rare-earth element (REE) patterns for plagioclase show that highly calcic plagioclase, ≥ An78, from alkalic magma has greater light-REE/heavy-REE (LREE/HREE) ratios than less calcic plagioclase, An64–75, from tholeiitic magma (ratios, 22–33 vs < 20), suggesting that higher LREE/HREE ratios are inherent to plagioclase of alkalic magmas. However, with compositional evolution (i.e., to lower An), plagioclase REE patterns are of limited use for distinguishing tholeiitic from alkalic parentage because LREE/HREE ratios within each group increase and overlap in the range of ∼ 20–90. Sr, Ba, Hf, and Ta can also discern parentages as their abundances in plagioclase largely reflect abundances inherent to their parental magmas. The best expressions for identifying parentage use Sr abundances (Sr vs An; vs Ce/Yb; vs Sr/Ce), although Hf, Ba, and Ta abundances vs An and vs Ce/Yb are also useful – the distinctions due to tholeiitic plagioclase having relatively low Sr (∼ 500–1000 ppm), Ba (< 100 ppm), Hf (< 0.10 ppm), and Ta (< 0.05 ppm). These relationships help to distinguish between the effects of differentiation on trace-element abundances in plagioclase and their abundances owed to intrinsic concentrations in their magmas. They create compositional fields for tholeiitic and alkalic parentages that remain graphically separated even though differentiation may have enriched the plagioclase in incompatible elements.
Zusammenfassung ?Plagioklas aus tholeitischen und alkalischen Magmen von Hawaii: Unterscheidung aufgrund von REE, Sr, Ba, Hf und Ta Um die M?glichkeit der Unterscheidung tholeitischer von alkalischer Magmaherkunft durch Mineralzusammensetzungen zu verbessern, habe ich die Spurenelementverteilung in Plagioklasen, die von xenolithischen Gabbros des Mauna Kea Vulkans abgetrennt wurden, untersucht, Diese Gabbros entstammen tholeitischen und alkalischen Magmen des Hamakua “Post-Schild” Stadiums des Mauna Kea Vulkanismus. Chondritisch normalisierte Seltene Erd (SEE) Verteilungs-Muster für Plagioklase zeigen, dass stark kalzische Plagioklase, > An78, aus alkalischen Magmen h?here leichte SEE/schwere SEE (LSEE/HSEE) Verh?ltnisse zeigen, als weniger kalzische Plagioklase, An64–75 aus tholeitischem Magma (Verh?ltniszahlen 22–33 gegenüber < 20). Dies weist darauf hin, dass h?here LSEE/HSEE-Verh?ltnisse typisch für Plagioklase aus alkalischen Magmen sind. Im Zuge der Evolution der Zusammensetzungen (d.h. zu niedrigeren An-Werten hin), sind die SEE Verteilungsmuster von Plagioklasen weniger hilfreich um tholeitische von alkalischer Herkunft zu unterscheiden. Dies ist deshalb so, weil die Verh?ltniszahlen innerhalb jeder Gruppe zunehmen und im Bereich von 20–90 überlappen. Sr, Ba, Hf und Ta k?nnen auch dazu dienen, um die Herkunft der Plagioklase zu definieren, da ihre H?ufigkeit gro?teils auf H?ufigkeiten, die für die Ursprungsmagmen typisch sind, zurückgehen. Die besten Herkunft-Parameter sind die Sr H?ufigkeiten (Sr vs An; vs Ce/Yb; vs Sr/Ce), obwohl die H?ufigkeit von Hf, Ba und Ta gegen An und gegen Ce/Yb auch nützlich sind. Diese Unterscheidungen gehen darauf zurück, dass tholeitische Plagioklase relativ niedrige Sr (∼ 500–1000 ppm), Ba (< 500 ppm) Hf (< 0.10 ppm) und Ta (< 0.5 ppm) führen. Diese Beziehungen erleichtern die Unterscheidung zwischen den Auswirkungen der Differenzierung auf die Spurenelement-Verteilungsmuster in Plagioklasen und auf ihre H?ufigkeiten, die auf die intrisischen Konzentrationen in den Ursprungsmagmen zurückgehen. Sie definieren charakteristische Felder für tholeitische und für alkalische Herkunft, die graphisch separiert bleiben, auch wenn die Gehalte der Plagioklase an inkompatiblen Elementen durch Differenzierung zugenommen haben mag.

Received July 22, 1999;revised version accepted December 7, 1999  相似文献   

19.
The intraplate Cameroon Volcanic Line (CVL) straddles the African-South Atlantic continent-ocean boundary and is composed mainly of alkaline basic volcanic rocks. Voluminous silicic volcanics characterize the continental sector of the CVL. We present here new geochemical, isotopic (Sr-Nd-O) and 40Ar/39Ar geochronological data on the main silicic volcanic centres of the Western (Mt. Oku, Sabga and Mt. Bambouto) and Eastern (Ngaoundere plateau) Cameroon Highlands. The silicic volcanism of Mt. Oku, Sabga and Mt. Bambouto occurred between 25 and 15 Ma and is represented by voluminous quartz-normative trachytes and minor rhyolitic ignimbrites. At Mt. Bambouto central volcano about 700 m of silicic volcanics erupted in less than 2.7 million years. These silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites. In general, onset of the silicic volcanism migrated from NE (Oku: 25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and Mt. Manengouba: 12 Ma). The silicic volcanism of the Ngaoundere plateau (eastern branch of the CVL) is instead dominated by nepheline-normative trachytes which are associated with strongly alkaline basalts and basanitic rocks. These Ne-trachytes are younger (11-9 Ma) than the Q-trachytes of the Western Highlands. The least differentiated silicic volcanics are isotopically similar (87Sr/86Sr < 0.70380; 143Nd/144Nd > 0.51278) to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials. Such interactions may, however, have played some role in the genesis of the most evolved silicic volcanics which have 87Sr/86Sr as high as 0.705–0.714. Fractional crystallization is the preferred mechanism for genesis of the silicic melts of both Western and Eastern Highlands, as shown by modeling major and trace element variations. The genesis of the least evolved Q-trachytes from the Western Highlands, starting from slightly to moderately alkaline basalts, is compatible with fractionation of dominantly plagioclase, clinopyroxene and magnetite. Crystal fractionation may have occurred at low pressure and at QFM buffer f O2conditions. Parental magmas of the Ngaoundere Ne-trachytes are likely instead to have been strongly alkaline basalts which evolved through crystal fractionation at higher P (6-2 kbar) and f O2 (QFM + 2). The migration (25 to 12 Ma) of the silicic volcanism from NE to SW in the continental sector of the CVL is reminiscent of that (31-5 Ma) of the onset of the basic volcanism in the oceanic sector (Principe to Pagalu islands) of the CVL. These ages, and that (11-9 Ma) of the silicic volcanism of the Ngaoundere plateau, indicate that the Cameroon Volcanic Line as a whole may not be easily interpreted as the surface expression of hot-spot magmatism. Received: 24 February 1998 / Accepted: 22 September 1998  相似文献   

20.
Magnesian andesites in north Xinjiang,China   总被引：1，自引：0，他引：1  
Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan and west- and east-Tianshan in the north Xinjiang region. These MAs are andesitic rocks with 53–65% SiO2,<1% (0.21–1.08%; average of 0.72%) TiO2, and ≥50 Mg#. Magnesian dacites and diorites, with 52.38–66.91% SiO2, <0.30% TiO2 and ≥42 Mg# commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na. They have characteristic flat chondrite-normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65–1.15), low (La/Yb)N (0.98–6.4, mostly 4±) and low total REE contents (15–95 ppm). They also have high contents of compatible elements Cr and Ni (72–790 and 29–276 ppm, respectively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams. If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high Y contents (>15 ppm), low Sr/Y ratios (4.4–6.2), low (La/Yb)N, and high Mg# of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle wedge peridotite. New petrographic, chemical and isotopic [(143Nd/144Nd)I = 0.51221–0.51255 (εNd(t) +0.28 to +7.2); (87Sr/86Sr)I = 0.7029–0.7065] data suggest that the petrogenesis of the MAs in the north Xinjiang region may have involved: (1) multiple source materials including subducted oceanic slab, juvenile crustal materials (mainly volcanic-volcanoclassic rocks with low maturity and clear mantle geochemical signatures) coming from the forearc accretionary prism and mantle wedge peridotite; (2) a combination of different petrogenetic processes including partial melting of subducted oceanic slab and juvenile crustal materials, followed by interaction of slab melts with the mantle wedge peridotite; (3) high geothermal gradient creating a high temperature (>1,000°C) environment in a volatile-rich source region; (4) unique tectonic settings including oblique subduction, slab break off resulting in slab window formation and asthenosphere upwelling, and subduction erosion resulting in transfer of forearc accretionary materials into the source region of MA magma.  相似文献   

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