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1.
We present petrographic and geochemical data on representative samples of the Devonian adakite, boninite, low-TiO2 and high-TiO2 basalts and associated rocks in the southern Altay areas, Xinjiang, NW China. These volcanic rocks mostly occur as tectonic blocks within suture zones between the Siberian and Junggar plates. Adakite occurs in the Suoerkuduke area ca. 40 km south of Fuyun, and actually represents a poorly-sorted massive volcaniclastic deposit, mostly consisting of a suite of hornblende andesite to pyroxene andesite. The geochemical features of the adakite suggest its generation by melting of subducted oceanic crust. Boninite occurs in the Saerbulake area ca. 20 km southwest of Fuyun, as pillowed lava or pillowed breccia. It is associated with high-TiO2 basalt/gabbro and low-TiO2 basalt. The boninites are metamorphosed, but contain relict clinopyroxene with Mg# (=100*Mg/(Mg+Fe)) of 90–92, and Cr2O3 contents of 0.5–0.7 wt% and chromian spinel with Cr/(Cr+Al) ratio of 0.84. The bulk rock compositions of the boninites are characterized by low and U-shaped REE with variable La/Yb ratios. They are classified as high-Ca boninite. The Cr-rich cpx phenocryst and Chromian spinel suggests that the boninites were formed by melting of mildly refractory mantle peridotite fluxed by a slab-derived fluid component under normal mantle potential temperature conditions. Basaltic rocks occur as massive flows, pillowed lavas, tuff breccia, lapilli tuff and blocks in tectonic mélanges. Together with gabbros, the basaltic rocks are classified into high-TiO2 (>1.7 wt%) and low-TiO2 (<1.5 wt%) types. They show variable trace element compositions, from MORB-type through transitional back-arc basin basalt to arc tholeiite, or within plate alkalic basalt. A notable feature of the Devonian formations in the southern Altay is the juxtaposition of volcanic rocks of various origins even within a limited area; i.e. the adakite and the boninites are associated with high-TiO2 and low-TiO2 basalts and/or gabbros, respectively. This is most likely produced by complex accretion and tectonic processes during the convergence in the Devonian–Carboniferous paleo-Asian Ocean between the Siberian and Junggar plates.  相似文献   

2.
The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.  相似文献   

3.
Chichi-jima, Bonin Islands, consists of dominant Eocene submarine volcanic rocks, comprising boninites, andesites and dacites, and subordinate sedimentary rocks. The dacites occur frequently in breccias and pillows overlying a boninite pillow lava sequence. The boninite pillows are intruded by a multiple dike, in which a core boninite is chilled against outer dacites. A density-stratified chamber may have been capped by a dacite magma. The dacites, which can be divided into quartz dacite and quartz-free dacite, are differentiates from the boninite-forming magmas, because they vary continuously in composition from boninites through andesites. The quartz dacites, corresponding to rhyolite in SiO2, are lower in Na2O and K2O than most orogenic dacites. Some of the dacites are characterized by ferropigeonite (Wo7–16En23–39Fs68-54) phenocrysts and are clearly ferrodacite, producing variable amounts of Fs-rich normative pyroxenes. The relation of SiO2 to total FeO/MgO ratio indicates that many of both types of dacites, with glasses in boninites, are enriched in total FeO despite the strong calc-alkalic affinity of boninites. The crystallization temperature of ferropigeonite with Mg value 30 in a quartz dacite is estimated to be 900° C and that in a quartz-free dacite to be 1050° C, which are unusually high for differentiated silicic rocks. Some Chichi-jima rocks are fresh, having a low ratio of Fe2O3 to FeO. On the basis of the experimental study of magmatic ferric-ferrous equilibria at 1 bar, the oxygen fugacities are calculated as 10–13.6 bars at 900° C for a ferropigeonite quartz dacite and 10–8.9 bars at 1200° C for a boninite with the lowest Fe3+/Fe2+. Both values lie below the quartz-fayalite-magnetite buffer line. The boninite series volcanic rocks have preserved low oxygen fugacities as well as high temperatures until the latest differentiation stage. The ferropigeonite phenocrysts have crystallized from the dacite magmas under the conditions of moderately high temperatures, very low oxygen fugacities and high total FeO and SiO2 concentrations.  相似文献   

4.
The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO2 (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO2 (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.  相似文献   

5.
In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4–10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north.The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (≤32). Boninites with similarly low ratios have not been reported from elsewhere.The Adola tholeiites have high Ti/Zr (150–300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80–126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20–40). Low Ti/Zr is characteristic of Tertiary boninites in the west Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.  相似文献   

6.
低硅埃达克岩(low-SiO2adakites,LSA,SiO2<60%)和高硅埃达克岩(high-SiO2adakites,HSA,SiO2>60%)的术语是Martin等(2005)提出来的。本文不赞同上述分类,因为,他们的低硅埃达克岩是赞岐岩而不是埃达克岩。赞岐岩是幔源的,埃达克岩是壳源的,虽然埃达克岩可以与地幔混合形成高镁的埃达克岩,但仍然是壳源的。不应当把壳源的岩浆和幔源的岩浆混淆起来。  相似文献   

7.
Boninitic rocks and associated high-magnesian basalt and high-iron tholeiite in the Xiangcheng area constitute the basal horizon of the arc volcanic sequence in the Triassic Yidun Island-Arc, southwestern China. The boninite occurs as pillow, massive and ocellar lavas; the last one possesses well-developed globular structure and alternates with the former two. The boninite is characterized by the absence of phenocrysts of olivine and low-Ca pyroxenes and by low CaO/Al2O3 ratios (<0.67) and high Cr (>1000 ppm) and Ni (>250 ppm). The normalized abundance patterns (NAP) of trace elements to primitive mantle are similar to the NAP of low-Ca modern boninites and SHMB in the Archaean and Proterozoic.As a mechanism of ocellar texture, liquid immiscibility in boninite is supported by the following lines of evidence: (a) sharp contact between ocelli and matrix, (b) constant volumetric ratios of ocelli/matrix and common coalescence of ocelli in ocellar rocks, (c) identical micro-spinifex textures and mineral asse  相似文献   

8.
The northern Noorabad area in western Iran contains several gabbro and basalt bodies which were emplaced along the Zagros suture zone. The basalts show pillow and flow structures with amygdaloidal textures, and the gabbroic rocks show massive and foliated structures with coarse to fine-grained textures. The SiO2 contents of the gabbros and basalts are similar and range from 46.1–51.0 wt.%, and the Al2O3 contents vary from 12.3–18.8 wt.%, with TiO2 contents of 0.4–3.0 wt.%. The Nb concentrations of some gabbros and basalts are high and can be classified as Nb-enriched arc basalts. The positive εNd(t) values (+3.7 to +9.8) and low 87Sr/86Sr(initial) ratios (0.7031–0.7071) of both bodies strongly indicate a depleted mantle source and indicate that the rocks were formed by partial melting of a depleted lithospheric mantle and interaction with slab fluids/melts. The chemical composition of trace elements, REE pattern and initial 87Sr/86Sr-143Nd/144Nd ratios show that the rocks have affinities to tholeiitic magmatic series and suggest an extensional tectonic regime over the subduction zone for the evolution of these rocks. We propose an extensional tectonic regime due to the upwelling of metasomatized mantle after the late Cretaceous collision in the Harsin-Noorabad area. These rocks can be also considered as Eocene back arc magmatic activity along the Zagros suture zone in this area.  相似文献   

9.
《International Geology Review》2012,54(13):1596-1615
ABSTRACT

Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.  相似文献   

10.
Tertiary basalt is widespread in the area south of Wadi Hodein, south Eastern Desert, Egypt. It is the youngest unit in the basement rocks of the Central Eastern Desert classification of El Shazly (Proc 22nd Intl Geol Congr, New Delhi 10:88–101, 1964) and El Ramly (Ann Geol Surv Egypt II:1–17, 1972), traversed all the previous succession of the basement rocks as well as the Nubia Sandstone of Cretaceous age, forming sheets, small hills, ridges, and dikes. This Tertiary basalt is strongly associated with the opening of the Red Sea. Geologic, petrographic, and petrochemical studies as well as microprobe and X-ray analyses were performed on samples from Wadi Hodein Tertiary basalt. Field and petrographic studies classified the Tertiary basalt in south Wadi Hodein into porphyritic olivine basalt, plagiophyric basalt, and doleritic basalt. Opaque minerals (magnetite and ilmenite) constitute 6–7.5% of this basalt. Petrochemical studies and microprobe analyses reveal that they are low-TiO2 basalt with low uranium and thorium contents, classified as being basaltic andesite to andesite, originated from calc-alkaline magma, and developed in within-plate tectonic environment. Scanning electron microscopy shows that magnetite and ilmenite are the prevalent opaque minerals in this Tertiary basalt. Field radiometric measurements of the Tertiary basalt in south Wadi Hodein reveals low uranium and thorium contents. Uranium contents range from 0.5 to 0.9 ppm, while thorium contents range from 1.2 to 3.2 ppm. Fractional crystallization and mass balance modeling indicate that the most-silica low-TiO2 Tertiary basalt in south Wadi Hodein can be derived from the relatively less-silica low-TiO2 Tertiary basalt of south Quseir and Gabal Qatrani through fractional crystallization of plagioclase, olivine, augite, and titanomagnetite oxides. Tertiary basalts in south Wadi Hodein and south Quseir have nearly the same age, 25 Ma (Sherif, The Fifth International Conference on the Geology of Africa, 2007), 24 Ma (Meneisy and Abdel Aal, Ain Shams Sci Bull 25(24B): 163–176, 1984), and 27 Ma (El Shazly et al., Egypt J Geol 1975), respectively. Finally, the fractionation modeling and geochemical characteristics of these basalts suggested their origination from one basaltic magma emplaced in late Oligocene.  相似文献   

11.
A thick sequence of mafic-ultramafic rocks, occurs along a major shear zone (Phulad lineament), running across the length of Aravalli Mountain Range for about 300 kms. It has been suggested, that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally, the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite (mantle component) followed by layered cumulus gabbroic rocks (crustal component). A complex of non-cumulus rocks comprising hornblende schists, gabbros, sheeted dykes and pillowed basalts structurally overlies layered gabbros. Huge bodies of diorite intrude volcanics.

Geochemical classification suggests that all non-cumulus mafic rocks are sub-alkaline basalts except one variety of dykes which shows mildly alkaline character. The sub-alkaline rocks are tholeiite to calc-alkaline with boninite affinity. Tectono-magmatic variation diagrams and MORB normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks.

The mafic rocks of Phulad Ophiolite Suite are zoned across the strike in terms of their distribution from west to east. The hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes. The alkaline dyke occurs at the easternmost part. The rocks of Phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction. The geochemical interpretation presented in this study, together with discussion of lithological association is used to decipher the tectonic evolution of the Mesoproterozoics of NW Indian shield.  相似文献   


12.
In the Heathcote Greenstone Belt of central Victoria, a sequence of boninites and low-Ti andesites is overlain and intruded by tholeiitic basalts with affinities to backarc basin basalts. Two suites of boninites have been identified: one (Type A) with Ti/Zr ratios of 63±4, (La/Yb)N of 2–3 and HREE 5 times chondritic levels. The other suite (Type B) overlies Type A boninites and has Ti/Zr ratios of 23±3 and lower TiO2 and HREE contents (2–3 × chondrite), but shows significantly greater LREE enrichment, with (La/Yb)N greater than 5. Fractionation within both suites was largely controlled by the low-Ca pyroxenes protoenstatite and enstatite. Plagioclase-phyric low-Ti, high-Mg andesites occur in fault contact with the boninites, and have Ti/Zr and (La/Yb)N ratios very close to those of Type B boninites, but at higher absolute abundances of TiO2 and HREE. They are not related to either boninite suite by any realistic fractionation scheme, but originated from the same source as Type B boninites by approximately half the degree of partial melting that generated the boninites.Type A boninites could have been generated when LILE-enriched hydrous fluids derived from a subducted slab invaded depleted, clinopyroxene-poor lherzolite at depths less than 30 km, and initiated H2O-undersaturated partial melting. In a later partial melting event at similar depths, continued influx of metasomatic fluids into by now highly-depleted peridotite could have generated Type B boninites and low-Ti andesites. The presence of boninites and low-Ti andesites in the Cambrian Heathcote and Mount Wellington Greenstone Belts in southeastern Australia suggests that the early history of the Lachlan Foldbelt took place in a subduction-related, intraoceanic setting.  相似文献   

13.
The 2724–2722 Ma Stoughton-Roquemaure Group (SRG) of the Abitibi greenstone belt (the Archean Superior Province, Canada) is a ≤ 2 km thick komatiite–basalt succession intermittently exposed for about 50 km along strike. The ultramafic and mafic rocks occur mainly as pillowed, brecciated, and massive flows with well preserved spinifex textures in the komatiites. Volcanological, comparative stratigraphic and geochemical studies of the group along a volcanic marker horizon at the base of the succession allow the assessment of magma emplacement processes and mantle source rocks. Major feeder channels, secondary distributary tubes surrounded by pillowed flows with minor breccias and hyaloclastites display facies architecture of small volume flow fields (1–2 km3). Within the SRG, Al-depleted (ADK; Barberton-type) and Al-undepleted (AUK; Munro-type) komatiitic lavas are intercalated with tholeiitic basalt flows at a m- to 10s of m scale. Basalts and komatiites are inferred to be mantle plume-related; both rock types form two groups with characteristics of ADK and AUK including Al2O3/TiO2 ~ 9–12 for ADK versus 17–22 for AUK, as well as (Gd/Yb)n with > 1.3 versus ~ 1, respectively. The interdigitation of compositionally different flow units, limited extent of SRG volcanic rocks and facies architecture with the prevalence of small volume flows argue for a relatively small, heterogeneous mantle plume during the incipient stage of the evolution of the Archean Abitibi belt. Assuming that the scale of heterogeneities is comparable to the field expression of compositional changes and stratigraphy, it can be suggested that geochemical plume ‘layering’ is on 10s to 100s of m-scale. The evolution of this Archean mantle plume from inception to demise compares favorably with the Yellowstone hotspot which is assumed to have developed over 17 m.y. and had a diameter of about 300 km.  相似文献   

14.
《Gondwana Research》2014,25(2):842-858
The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO2 and low K2O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al2O3/TiO2 ratios, but low TiO2 and SiO2 contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.  相似文献   

15.
High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al2O3 (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu*=1.2–3.8), and the lowest initial 87Sr/86Sr (as low as 0.7033) and highest initial Nd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An50–60) and mafic silicate (Fo54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.  相似文献   

16.
Western Ghats Belt of western Dharwar Craton is dominated by metavolcanic rocks (komatiites, high-magnesium basalts (HMBs), basalts, boninites) with occasional metagabbros. This rock-suite has undergone post-magmatic alteration processes corresponding to greenschist- to lower-amphibolite facies conditions. Komatiites are Al-depleted, characterized by lower Al2O3/TiO2 and high CaO/Al2O3. Their trace element distribution patterns suggest most of the primary geochemical compositions are preserved with minor influence of post-magmatic alteration processes and negligible crustal contamination. Chemical characteristics of Al-depleted komatiites imply their derivation from deeper upper mantle with/without garnet involvement. HMBs and basalts are differentiated based on their magnesium content. Basalts and occasionally associated gabbroic sills have similar geochemical characteristics. HMB are characterized by light rare earth element (LREE) enrichment, with significant Nb–Ta and Zr negative anomalies. Basalts and associated gabbros display tholeiitic affinity, with LREE-enriched to slightly fractionated heavy rare earth element (HREE) patterns. Boninites are distinctive in conjunction of low abundances of incompatible elements with respect to the studied komatiites. Chondrite-normalized REE patterns of boninites show relative enrichment in LREE and HREE with respect to MREE. Prominent island arc signatures are evident in HMB, basalts, boninites, and gabbros in terms of their Nb–Ta and Zr–Hf negative anomalies, LREE enrichment and HFSE depletion. It is suggested that these HMB–basalts (associated gabbros)–boninites are the products of arc magmatism. Their REE chemistry attests to a gradual transition in melting depth varying between spinel and garnet stability field in an arc regime. The close spatial association but contrasting elemental characteristics of komatiites and HMB–basalts–boninites can be explained by a plume-arc model, in which the ~3.0 Ga komatiites are considered to be the products of plume volcanism in an oceanic setting, while the HMB, basalts, boninites, and associated gabbros were emplaced in a continental margin setting around 2.8–2.7 Ga.  相似文献   

17.
18O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more 18O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a 18O value of up to 7.0 to which must be added a small amount of H2O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low Nd values typical of boninites do not correlate with the 18O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces 18O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater.  相似文献   

18.
青藏高原西部蛇绿岩中玻安岩(boninite)及其地质意义   总被引：8，自引：1，他引：7  
简要报道了在青藏高原西部班公湖蛇绿岩带和狮泉河蛇绿岩带之间的火山岩中识别出的玻安岩。玻安岩是俯冲岩浆作用的产物,在青藏高原东部及周边地区已有玻安岩或玻安岩系岩石存在的报道。结合西部出现的玻安岩说明青藏高原地区俯冲作用普遍存在。玻安岩的形成时代为J2-K1,埃达克岩的形成时代为K1,由J2-K1,的玻安岩至K1的埃达克岩反映冈底斯北部岛弧带由不成熟的洋内弧向成熟岛弧转变的过程,它们是中生代冈底斯北部大陆水平生长(侧向增生)的记录。  相似文献   

19.
西藏得几蛇绿岩体中玻安岩的地球化学特征及其地质意义   总被引：15，自引：0，他引：15  
西藏日喀则蛇绿岩是我国发育最好的蛇绿岩之一，目前有关日喀则蛇绿岩形成的构造背景存在多种不同的认识。作者在研究得几蛇绿岩剖面时发现存在玻安岩，从而为日喀则蛇绿岩具有弧前背景提供了直接证据。得几蛇绿岩中玻安岩类样品具有高硅、高镁、低铁的特征，岩石中Zr、Yb和Y的含量也较低，具有明显的玻安岩特征。根据这些岩石的地球化学性质，认为岩石是由早期经历过较高程度的部分熔融的残余在含水条件下再发生部分熔融形成的。得几蛇绿杂岩体部分形成于弧前环境。  相似文献   

20.
The Boa Vista Basin (BVB) is located approximately 60 km southwest of Campina Grande, Paraíba, northeastern Brazil. It has a half-graben geometry controlled by dip-slip normal faults striking NE–SW. From the base to the top, the BVB is composed of (1) a lower volcanic unit of altered basalts and basaltic andesites overlying Precambrian basement rocks, (2) an intermediate unit of bentonitic shales that pass upward to medium- to coarse-grained sandstones and conglomerates and downward to sandstones and siltstones, and (3) an upper volcanic unit of massive to vesiculated basaltic flows grading to pillowed or autobrecciated basalts. These basalts show porphyritic (olivine and augite microphenocrysts), glomeroporphyritic, intersetal, pilotaxitic, and variolitic textures. They are medium-K, Fe-rich tholeiites with SiO2 of 50.2–53.3 wt%, magnesium number of 50.54–60.21 wt%, total alkali of 2.15–3.92 wt%, and TiO2 of 1.8–1.9 wt% and are related by low-pressure fractionation of olivine, plagioclase, magnetite, ilmenite, and apatite. They are LREE-enriched (LaN/YbN=8.54–44.14) with no significant europium anomaly. Trace element modeling suggests a garnet-bearing metasomatised lherzolite as their source. The geological context and geochemistry of the basalts suggest a close connection between reactivated deep-rooted Precambrian shear zones, which channeled mantle-derived Tertiary tholeiitic magmas, and continental rifting in northeastern Brazil.  相似文献   

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