首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The basalt stratigraphy of the Deccan Trap between MahabaleshwarGhat and Belgaum over-steps the basement from north to south.Sr-isotope and Zr/Nb ratios, and Sr, Rb, and Ba concentrationscorrelate portions of the post-Poladpur stratigraphy over 250km along the Western Ghats, thereby confirming a southerly componentof dip of 0?06?. At the southwestern margin, the stratigraphyextends upwards from the compositionally uniform Ambenali Formation(Cox & Hawkesworth, 1984) into a sequence of grossly heterogeneousflow units which have been allocated to the Mahabaleshwar andPanhala Formations (Lightfoot & Hawkesworth, 1988). TheMahabaleshwar Formation is represented only by a sequence ofhighly fractionated flows (termed the Kolhapur unit) with similar87Sr/86Sr0 to the Mahabaleshwar (0?7045), but with Sr<240ppm and TiO2>2?25%. Succeeding the Kolhapur unit are a seriesof flows with high 87Sr/86Sr0 (0?7045-0?705), Zr/Nb > 13,and low Sr (< 200 ppm), which have been allocated to thePanhala Formation, and a group of flows with high 87Sr/86Sr0(0?707–0?708) and Sr (>230), but trace element concentrationssimilar to the Mahabaleshwar Formation; these have been allocatedto the Desur unit of the Panhala. Geochemical variations in flows overlying the Ambenali definetwo distinct trends: one is attributed to gabbro fractionation,and the other to variations in the compositions of the parentalmagmas, and arguably their source regions. There is little evidencefor significant crustal contamination in these flows, and thedegree of fractionation and the composition of the phase extractare shown to vary along strike within the Mahabaleshwar Formation.The high TiO2 content of Kolhapur unit flows is shown to bethe result of shallow-level gabbro fractionation, rather thanthe presence of a primitive high-Ti magma. Mahabaleshwar Formationbasalts exhibit a broad negative correlation between the degreeof fractionation and Sr-isotopic composition. The endmemberwith lower 87Sr/86Sr0 has different Zr/Y from the Ambenali basalts,and would appear to have been generated by lower degrees ofmelting of a similar source. The other endmember has more radiogenicSr, lower Zr/Nb, similar Zr/Y, but higher mg-number. The simplestinterpretation is that these magmas were more primitive andhence hotter and more able to interact with the lithosphereen route to the surface, and that they then mixed to producethe Mahabaleshwar array. The Panhala Formation basalts ploton the Sr-Nd array defined by the Mahabaleshwar Formation, andthe Desur unit basalts plot on an extension of this array; thissuggests that the source characteristics are also lithospheric.The absolute elemental abundances may then be a function ofmelting and fractionation. We are impressed by the apparentswitch from crustal lithospheric contributions to mantle lithosphericcontributions through the stratigraphy, and suggest that this,together with the more protracted fractionation of the magma,reflects a change in the availability of the lithospheric componentsaccompanying the southerly migration of the volcanic edifice. * Present address: Geoscience Laboratories, Ontario Geological Survey, 11th Floor, 77, Grenville Street, Toronto, Ontario, M7A 1W4, Canada  相似文献   

2.
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   

3.
The Serra Geral (Paraná) continental flood basalt (SG-CFB) province in Brazil is associated with the Jurassic-Cretaceous breakup of Gondwanaland and the transition of continental to oceanic magmatism during the opening of the South Atlantic Ocean. A suite of 24 samples representative of the SG-CFB in Rio Grande do Sul, southern Brazil, shows a compositional continuum from basalt (50–53 wt% SiO2, Mg# 60-45), to basaltic andesite, to andesite, rhyodacite, and rhyolite (73 wt% SiO2). Certain compositional aspects of the mafic rocks (e.g., TiO2, K2O, CaO, Zr/Nb, Zr/Y, Ti/Zr) resemble those of basaltic dikes and flows associated with the opening of the North Atlantic Ocean.Fractionation trends are apparent in MgO variation diagrams and calculations show that basalt-basaltic andesite continuums are largely due to removal of plagioclase and clinopyroxene. These mafic rocks can be categorized as (i) having higher or lower incompatible-element contents (e.g. K2O 0.6–1.5 wt%; Rb 12–43 ppm; Ba 125–240 ppm) due to incorporation of Brazilian Archean crust or rhyolitic magma by basalt from a particular source material or to partial-melting differences of that source, and (ii) as having higher or lower TiO2, Sr, Ba, and P contents due to source heterogeneities. Crustal components are obvious in certain basaltic samples, as where K2O > 1 wt%, SiO2 > 51%, and TiO2 1%, but are insignificant in others (e.g., compositions close to those of South Atlantic basin basalts). Calculations indicate origins for intermediate and silicic rocks by removal of pl, cpx, and Ti-magnetite from basaltic andesite, but crust and magma-mixing affecting basaltic-andesite fractionates were likely also involved. Where contamination is insignificant, Zr, Nb, and Y abundances indicate T-type MORE source material like that for certain S. Atlantic Ocean basalts. Source material was essentially a 91 hybrid of N-type and P-type MORB components. N-type MORB lithosphere followed SG-CFB because decompression due to rifting crust enabled partial melting of uppermost (depleted) mantle.  相似文献   

4.
Basaltic and basaltic andesitic flows have been identified from the Chhotaudepur area in the Deccan Large Igneous Province. The \(\hbox {SiO}_{2}\) content of these flows varies from 46.7 to 53.7 wt%. None of the samples have primary magma signatures as they exhibit low Mg# (0.42–0.68), Ni (4.8–33.4 ppm) and Cr (33.08–143.06 ppm). Highly variable concentrations of the LILE [Rb (2–74 ppm), Ba (52–351 ppm), Cs (0.1–1 ppm) and Sr (273–745 ppm)] and relatively enriched LREE are also noticed. The coherent chondrite normalized REE diagram and primitive mantle normalized multi-element diagram suggests a consanguinity among the flows. Low (Nb/Th)\(_{\mathrm{PM}}\) (0.30–1.09), high (Th/Yb)\(_{\mathrm{PM}}\) (3.09–16.58) ratios and marked variations in Rb concentration (2.4–74.11 ppm) with variable La/Yb (5.5–23.7) ratios suggests that magmas in the Chhotaudepur region were assimilated by the crustal components. The elevated Th/Ta and La/Yb relation further indicate concurrent assimilation and fractional crystallization process was involved in the genesis of the flows. Assimilation and fractional crystallization modelling of the flows was carried out with 20% olivine, 25% clinopyroxene, 45% plagioclase and 10% titano-magnetite as fractionating minerals and upper continental crust as the assimilant. The results reveal that all the flows were modified by AFC process.  相似文献   

5.
Summary Amphibole and mica lamprophyres and related dykes of Tertiary age from the Kreuzeck Mountains, Central Alps, Austria, have been investigated petrographically and geochemically. They intrude a sequence of early Palaeozoic metapelites, greenstones and amphibolites to the north of the Cretaceous Periadriatic Lineament, a major suture zone of 700 km E-W extent. The dykes are spatially associated with Sb, W, Hg, and Cu-Ag-Au deposits.Most lamprophyres are characterized by primitive chemistry (mg-numbers > 60 and Cr > 200 ppm) and have high contents of LIL elements (K, Rb, Sr and Ba). Geochemically, five different subgroups of calcalkaline/shoshonitic to alkaline affinity can be distinguished. These are: Group 1, amphibole-bearing shoshonitic lamprophyres (0.5–1.0 wt% Ti02, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10); Group 2, mica-bearing shoshonitic lamprophyres (1–1.5 wt% TiO2, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Group 3, alkaline lamprophyres (1.5–2.1 wt% TiO2, Zr > 250 ppm, Nb > 30 ppm, Ba/Rb 10–25); Group 4, low-MgO alkaline lamprophyres ( 2.5 wt% TiO2, mg-number < 57, Nb 20 ppm, Ba/Rb 20); Group 5, calc-alkaline basaltic dykes ( 2.2 wt% TiO2, mg-number <55, Nb < 10 ppm, Ba/Rb < 10). Group 2,3 and 4 dykes have NE-SW orientations and are of Oligocene age (K-Ar age 27–32 Ma); Group 1 and 5 dykes are of Lower Oligocene age (K-Ar age 36 Ma) but have mostly E-W orientations.The Kreuzeck lamprophyres were generated in post-collisional magmatic events, which were probably linked to extensional tectonics following oblique continent-continent collision between the African and Eurasian Plates during the Eocene. Group 1, 2 and 5 dyke rocks have typical calc-alkaline geochemical signatures indicating that they represent partial melting products of subduction-modified lithosphere. Group 3 and 4 alkaline lamprophyres have geochemical features transitional between calc-alkaline and within-plate alkaline igneous rocks (e.g. Ba/Nb 30–70) indicating that their mantle source-region includes both subduction-modified lithospheric and OIB-type asthenospheric components.There is no apparent relationship between mineralization in the Kreuzeck region, thought to be of Ordovician-Devonian age, and much later lamprophyre intrusion. Alteration of the dykes by late-magmatic fluids has resulted in the formation of secondary minerals, and has occasionally led to increased Au and PGE values in the 10–35 ppb range particularly in close proximity to Cu-Ag-Au deposits.
Shoshonitische und alkalische Lamprophyre mit erhöhten Au- und PGE-Gehalten aus der Kreuzeckgruppe, Ostalpen, Österreich
Zusammenfassung Die vorliegende Arbeit untersucht die Petrographie und Geochemie tertiärer Lamprophyre und genetisch verwandter Ganggesteine aus der zentralalpinen Kreuzeckgruppe, nördlich des Periadriatischen Lineamentes, in Kärnten, Österreich. Die Ganggesteine durchschlagen die altpaläozoischen Metapelite, Grünsteine und Amphibolite des Altkristallins diskordant und stehen in räumlichem Zusammenhang mit Sb, W, Hg und Cu-Ag-Au Lagerstätten, die bereits seit dem Mittelalter abgebaut wurden.Die meisten Lamprophyre zeigen primitiven Charakter (Mg-Zahlen >60 und Cr > 200 ppm) und besitzen hohe Gehalte an LILE (K, Rb, Sr und Ba). Geochemisch lassen sich fünf verschiedeneGruppen mit kalkalkalisch/shoshonitischem bis alkalischem Charakter unterscheiden: Gruppe 1, Amphibol-führende shoshonitische Lamprophyre (0.5–1.0 Gew% TiO2, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10);Gruppe 2, Glimmer-führende shoshonitische Lamprophyre (1–1.5 Gew% TiO2, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Gruppe 3, alkalische Lamprophyre (1.5–2.1 Gew% TiO2, Zr > 250 ppm, Nb > 30ppm, Ba/Rb 10–25); Gruppe 4, alkalische Lamprophyre mit geringen MgO-Anteil ( 2.5 Gew% TiO2, Mg-Zahl < 57, Nb 20 ppm, Ba/Rb 20); Gruppe 5, kalkalkalisch basaltische Ganggesteine ( 2.2 Gew% TiO2, Mg-Zahl < 55, Nb < 10 ppm, Ba/Rb < 10). Die Lamprophyre der Gruppen 2, 3 und 4 zeigen nordöstliches Streichen und oligozänes Intrusionsalter (K-Ar Alter 27–32 Ma), während die Ganggesteine der Gruppen 1 und 5 überwiegend östliches Streichen und UnterOligozänes Intrusionsalter (K-Ar Alter 36 Ma) aufweisen.Die Intrusionen erfolgten während einer tektonischen Dilatationsphaseim Oligozän nach der Kontinent-Kontinent Kollision zwischen derAfrikanischen und der Eurasischen Platte im unteren Eozän. Ganggesteine der Gruppen 1, 2 und 5 besitzen typisch kalkalkalischen Charakter und stellen vermutlich Produktevon aufgeschmolzener, subduzierter Lithosphäre dar. Die Geochemie der alkalischen Lamprophyre derGruppen 3 und 4 (e.g. Ba/Nb 30–70) deutet auf ihre genetische Zwischenstellung zwischen subduction-related und within-plate regime.Zwischen den tertiären Gangintrusionen und den vermutlich paläozoischen Vererzungen der Kreuzeckgruppe besteht kein genetischer Zusammenhang. Die Alteration der Ganggesteine durch postmagmatische Lösungen hat jedoch zur Bildung von sekundären Mineralen und teilweise zu überdurchschnittlich erhöhten Au und PGE Gehalten von bis zu 35 ppb geführt.

With 6 Figures  相似文献   

6.
Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO2, Al2O3, CaO, S, and Ga are lost from the basalt, whereas Fe2O3, total Fe, MnO, K2O, H2O, Cl, B, Rb, and Cs increase. Sr87/Sr86 ratios also increase during weathering. MgO, Na2O, P2O5, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO2, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.  相似文献   

7.
《Gondwana Research》2001,4(3):509-518
The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO2 (> 2 wt%), Ti/Y, Ti/Zr, TiO2/K2O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO2 (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km2 in the Lesser Himalaya provides an evidence of plume activity in the region.  相似文献   

8.
The Central Indian Ocean Basin (CIOB) basalts are plagioclase-rich, while olivine and pyroxene are very few. The analyses of 41 samples reveal high FeOT (~10–18 wt%) and TiO2 (~1.4–2.7 wt%) indicating a ferrobasaltic composition. The basalts have high incompatible elements (Zr 63–228 ppm; Nb ~1–5 ppm; Ba ~15–78 ppm; La ~3–16 ppm), a similar U/Pb (0.02–0.4) ratio as the normal mid-oceanic basalt (0.16±0.07) but the Ba/Nb (12.5–53) ratio is much larger than that of the normal mid-oceanic ridge basalt (~5.7) and Primitive Mantle (9.56). Interestingly almost all of the basalts have a significant negative Eu anomaly (Eu/Eu*=0.78–1.00) that may have been a result of the removal of feldspar and pyroxene during crystal fractionation. These compositional variations suggest that the basalts were derived through fractional crystallization together with low partial melting of a shallow seated magma.  相似文献   

9.
For the first time occurrence of Ti rich Al depleted ultramafic cumulates having komatiitic signature in the northwestern fringe of Chotanagpur Gneissic Complex is presented. These rocks exhibit intrusive relationship with metasedimentary rocks and metaultramafites. Geochemically they are characterized by Mg# 79.1–91.60, high TiO2 (1.29–1.54 wt%), significantly low Al2O3/TiO2 and (Gd/Yb)n >1. Major oxides, trace and REE content suggest low degree of fractional crystallization and lesser degree of partial melting. These Al depleted komatiites are characterized by high concentration of incompatible elements than most suites of Barberton type komatiites. High Ti content suggests less degree of majorite garnet melting, leaving behind garnet in the restite. The rock shows higher Ti/Sc (190),Ti/V (22), Zr/Y (3), Zr/Sc (4), V/Sc (8), Zr/Sm (28) and Zr/Hf (47) ratios than primitive mantle and REE distribution pattern shows gentle slope from LREE to HREE in most samples pointing towards mantle metasomatism and crustal contamination during emplacement. The observed chemical characteristics indicate derivation of komatiite from an enriched mantle source and represent plume activity in an extensional tectonic regime of intracratonic setting.  相似文献   

10.
The Wenquan ultramafic rocks, located in the East Kunlun Orogenic belt in the northeastern part of the Qinghai‐Tibet Plateau, consist of dunite, wehrlite, olivine‐clinopyroxenite and clinopyroxenite, and exhibit cumulate textures. Olivine from dunite has high Fo (forsterite, 90–92) and NiO (0.15–0.42 wt%) contents. Cr‐spinels from all of the rocks in this suite are characterized by high Cr# (100×[Cr/(Cr+Al)], 67–91), low Mg# (100×[Mg/(Mg+Fe2+)], 17–35) and low TiO2 contents (mostly < 0.5 wt%). Clinopyroxenes display high Mg# (92–98) and low TiO2 contents (0.002–0.099 wt%), similar to those in ophiolitic cumulates. Geochemically, the Wenquan ultramafic rocks show enrichment of LILE, Sr, and Ba, and depletion of Nb and Th. Mineral chemistry and geochemistry indicate that the Wenquan cumulates were generated from a depleted mantle and likely evolved from high‐Mg basaltic magmas (Mg#=78) that underwent fractional crystallization and crustal contamination. Zircons from clinopyroxenites yield a U–Pb weighted mean age of 331 ± 2 Ma, which is nearly coeval with the formation age of the Buqingshan and A'nyemaqen Carboniferous ophiolites. The Wenquan Carboniferous ophiolites are confirmed to exist in the Central East Kunlun Fault zone, whereas previous studies have considered them to be the Proterozoic ophiolites. The Wenquan ultramafic rocks might be an arc cumulates of the Paleo‐Tethyan ocean, indicating that there were two cycles of oceanic–continental evolution along the Central East Kunlun Fault zone. Keywords Ultramafic rock, Cumulate, Ophiolite, East Kunlun Orogenic belt.  相似文献   

11.
The Mandla lobe is a 900 m thick lava pile that forms a 29,400 km2 northeastern extension of the Deccan Traps. Earlier, combined field, petrographic, and major element studies have shown that this lobe comprises 37 lava flows. Using a combination of trace elements (Ba, Ti, Zr, Rb, Sr) and Nb/Zr values, we group the flows into six chemical types (A–F) that are separated stratigraphically. Combined trace element and Nd-Pb-Sr isotopic data, document the presence of lavas resembling those of the Poladpur Formation and less abundantly, the Ambenali Formation of the southwestern Deccan are in conformity with the earlier reconnaissance work. In addition, our data reveal several flows similar to those of the Mahabaleshwar Formation, the type sections of which are located?~?900 km to the southwest. Based on the isotopic data the superposition of Mahabaleshwar-like flows over flows with Ambenali- and Poladpur-like characteristics is in the same stratigraphic order seen in the southwestern Deccan type section. However, from the stratigraphy indicated by the Discriminant Function Analysis (DFA) results and the serious discrepancy between the DFA and isotopic data, it seems that few Mandla lobe flows are different and not in the same stratigraphic order as in the southwestern part of the province. To some extent the differences may be explained by faulting along four large post-Deccan normal faults near Nagapahar, Kundam, Deori, and Dindori areas across which offsets of ~150 m have been measured. This post-emplacement faulting accounts for the presence of several chemically Mahabaleshwar-like lavas at the base of the ~900 m thick Mandla lobe pile, at a lower elevation than a thick sequence dominated by chemically Poladpur-like flows. However, presence of common signature lavas (similar to that in the northeastern Deccan) cannot be ruled out in this area. They are similar to Poladpur-type lavas both chemically and isotopically. They appear in different formations and erupted at different times other than Poladpur Formation. Close similarities in petrogenetic processes between the two regions are indicated, although it is not clear whether any of the Mandla lobe lavas are far-traveled counterparts of flows cropping out in the southwestern Deccan, or whether some magma migrated laterally in dike systems over great distances. Feeder dykes have not been found in the study area except for Chakhla-Delakhari Intrusive Complex (CDIC) in Satpura region that shows major and trace elemental similarities with the Seoni lavas, although, long distance transport of magma is yet to be proved. The Poladpur-like Mandla lobe flows appear to be different flows from those of the Poldapur Formation in the southwest, as they are somewhat different in isotopic (higher 206Pb/204Pb) composition. They also differ from any known flows in the other southwestern formations, but are broadly similar to flows found in sections across the northern Deccan west of the Mandla lobe.  相似文献   

12.
Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO2). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO2), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO2) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An92?45), olivine (Fo88?51), orthopyroxene (En69?56), and titanomagnetite (15%–21% TiO2) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased87Sr/86Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.  相似文献   

13.
Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.  相似文献   

14.
Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1?206Pb/204Pb?207Pb/204Pb?208Pb/204Pb?相似文献   

15.
Migration of uranium and arsenic in aquatic environments is often controlled by sorption on minerals present along the water flow path. To investigate the sorption behaviour, batch experiments were conducted for uranium and arsenic as single components and also solutions containing both uranium and arsenic in the presence of SiO2, Al2O3, TiO2 and FeOOH at a pH ranging from 3 to 9. In solutions containing only U(VI) or As(V) with the minerals, the sorption of U(VI) was low at acidic pH range and increases with increasing pH, whereas As(V) showed opposite sorption behaviour to Al2O3, TiO2 and FeOOH from acidic pH range to alkaline condition. For the As(V)–SiO2 system, the sorption was low for almost all pH. Sorption of U(VI) and As(V) on SiO2 and FeOOH is almost similar in solutions containing either U(VI) or As(V) separately, or both together. In the U(VI)–As(V)–Al2O3 system, a significant retardation in uranyl sorption and an enhancement in arsenate sorption on Al2O3 were observed for a wide range of pH. The sorption behaviour of U(VI) and As(V) was changed when Al2O3 was replaced by TiO2, where an increase in sorption was observed for both elements. The sorption behaviour of uranyl and arsenate in the U(VI)–As(V)–TiO2 system gives evidence for the formation of uranyl–arsenate complexes. The change in sorption retardation/enhancement of U(VI) and As(V) could be explained by the formation of uranyl–arsenate complexes or due to the competitive sorption between uranyl and arsenate species.  相似文献   

16.
Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sør Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K2O+Na2O (7-13 wt%) and K2O/Na2O (1-2), low to intermediate Mg#, wide ranges of SiO2 (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb)N and LREE/HREE, lower A/CNK, SREE and initial 87Sr/87Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K2O, P2O5, TiO2, Fe2O3/FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al2O3, Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sør Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Mühlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.  相似文献   

17.
  相似文献   

18.
The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO2, ranging from 43.6 to 52.1 wt%, Al2O3 from 5.0 to 12.6 wt%, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (143Nd/144Nd = 0.51229 -0.51276 and 87Sr/86Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume.

We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.  相似文献   

19.
Electrical conductivities of Ni2SiO4, Fe2SiO4, and MgSiO3 were measured on synthetic powders in the temperature range 340° to 1,100° C and at pressures up to 20 kbars. For ternary compounds such as olivines and pyroxenes the control of two further variables, like the chemical activities of two components are needed, besides temperature and pressure. The activities of the corresponding binary oxides were controlled by equilibrating the samples with their neighbour-phases. Control of the oxygen partial pressure was achieved by buffer techniques. From the slopes of the lg σ vs. 1/T lines the activation energies were calculated for 10 kbar: 0.56 eV and 2.7 eV for Ni2SiO4 in equilibrium with SiO2 and Ni/NiO-buffer for the temperature range 500°–800°C and 800°–1,000°C resp. 0.52 eV for Fe2SiO4 in equilibrium with SiO2 and metallic iron, and 0.38 eV in equilibrium with SiO2 and magnetite; 1.11 eV for MgSiO3 in equilibrium with SiO2, and 1.25 eV in equilibrium with Mg2SiO4.  相似文献   

20.
A mineralogical nomenclature applicable to the natural binary isomorphous series ZrSiO4-HfSiO4 is proposed. It is suggested that (Hf, Zr)SiO4 with more than 90 mol.-% hafnium component be called hafnon. Zoned crystals ranging in composition from hafnian zircon to hafnon were found in the heavy mineral concentrates from the tantalum pegmatites of the Morrua area, Conco, Moneia and Muiane mines in the district of Zambézia, Mozambique. The density (Berman balance), Hf/Zr ratio (XRF analysis) and unit-cell parameters (X-ray powder pattern) of the samples are determined and discussed. Two microprobe analyses of hafnon from Muiane are presented.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号