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1.
The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~?51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U–Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U–Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29–30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes?~?17–16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5?±?0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17–16 Ma calc-alkaline and the 12–13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust. 相似文献
2.
阿尔金山北缘喀腊大湾地区早古生代中酸性侵入岩岩石地球化学特征及构造意义 总被引：1，自引：0，他引：1
喀腊大湾地区位于阿尔金山脉东北缘,当地早古生代时期构造环境复杂并且伴随剧烈的花岗质岩浆活动.通过对区域内早古生代中酸性侵入岩5个岩体（大平沟岩体、阿北银铅矿岩体、阿北岩体、4337北花岗岩体、喀腊大湾南岩体）的岩石学、岩相学、地球化学特征研究,探讨早古生代区域重大构造事件和构造环境.南部的喀腊大湾南岩体表现低钾性质,其余4个岩体样本为中高钾钙碱性中酸性岩,并且有向高钾演变的趋势,铝饱和指数在1.0~1.1之间,数据统计和分析大部分样品具有I型花岗岩特征,少部分显示了I型向S型过渡的性质,代表了洋壳-陆壳活动性大陆边缘的环境.结合喀腊大湾周边地质背景,本地区应该是阿尔金古洋壳和塔里木地块南北向碰撞作用的火山岛弧区域. 相似文献
3.
Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan 总被引：1，自引：0，他引：1
Saito Satoshi; Arima Makoto; Nakajima Takashi; Misawa Keiji; Kimura Jun-Ichi 《Journal of Petrology》2007,48(9):1761-1791
The Miocene Kofu Granitic Complex (KGC) occurs in the Izu CollisionZone where the Izu–Bonin–Mariana (IBM) arc has beencolliding with the Honshu arc since the middle Miocene. TheKGC includes rocks ranging in compositions from biotite-bearinggranite (the Shosenkyo and Mizugaki plutons), and hornblende–biotite-bearinggranodiorite, tonalite, quartz-diorite, and granite (the Shiodaira,Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonaliteand trondhjemite (the Ashigawa–Tonogi pluton), indicatingthat it was constructed from multiple intrusions of magma withdifferent bulk chemistry. The Sr-isotopic compositions correctedto sensitive high-resolution ion microprobe (SHRIMP) zirconages (SrI) suggest that the primary magmas of each pluton wereformed by anatexis of mixed lower crustal sources involvingboth juvenile basalt of the IBM arc and Shimanto sedimentaryrocks of the Honshu arc. After the primary magmas had formed,the individual plutons evolved by crystal fractionation processeswithout significant crustal assimilation or additional mantlecontribution. SHRIMP zircon U–Pb ages in the KGC rangefrom 16·8 to 10·6 Ma and overlap the resumptionof magmatic activity in the IBM and Honshu arcs at c. 17 Maand the onset of IBM arc–Honshu arc collision at c. 15Ma. The age of the granite plutons is closely related to theepisodic activity of arc magmatism and distinct granitic magmabatches could be formed by lower crustal anatexis induced byintrusion of underplated mantle-derived arc magmas. Based onpressures determined with the Al-in-hornblende geobarometer,the KGC magmas intruded into the middle crust. Thus, the KGCcould represent an example of the middle-crust layer indicatedthroughout the IBM arc by 6·0–6·5 km/s seismicvelocities. This granitic middle-crust layer acted buoyantlyduring the IBM arc–Honshu arc collision, leading to accretionof buoyant IBM arc middle crust to the Honshu arc. KEY WORDS: arc–arc collision; crustal anatexis; granite; Izu–Bonin–Mariana (IBM) arc; Izu Collision Zone 相似文献
4.
祁连山在构造上是一条经历了多期构造旋回叠加的早古生代复合型造山带,花岗质岩浆作用研究对揭示其构造演化具有重要意义。锆石U-Pb年代学统计结果表明,祁连地区花岗质岩浆活动可以分为7个大的阶段,包括古元古代早期(2 470~2 348 Ma)、古元古代晚期(1 778~1 763 Ma)、中元古代晚期-新元古代早期(1 192~888 Ma)、新元古代中期(853~736 Ma)、中寒武世-志留纪(516~419 Ma),泥盆纪-早石炭世(418~350 Ma)以及中二叠世-晚三叠世(271~211 Ma)。其中古元古代早期发育强过铝质高钾钙碱性S型和准铝质低钾拉斑-高钾钙碱性I型花岗岩,记录了早期的陆壳增生及改造事件。古元古代晚期为准铝质-弱过铝质高钾钙碱性-钾玄质A型花岗岩,是Columbia超大陆裂解事件的产物。中元古代晚期-新元古代早期以过铝质-强过铝质钙碱性-钾玄质S型花岗岩为主,新元古代中期以准铝质-强过铝质钙碱性-高钾钙碱性A型花岗岩为主,分别对应Rodinia超大陆的汇聚和裂解事件。中寒武世-志留纪花岗岩是洋陆转换过程中的产物,约440 Ma加厚基性下地壳部分熔融形成的低Mg埃达克岩的广泛出现指示祁连地区全面进入碰撞造山阶段。泥盆纪-早石炭世花岗岩代表后碰撞伸展阶段岩浆岩组合,发育准铝质-强过铝质低钾拉斑-钾玄质等一系列花岗岩。中二叠世-晚三叠世花岗岩以准铝质-弱过铝质钙碱性-高钾钙碱性I型花岗岩为主,有少量弱过铝质高钾钙碱性A型花岗岩,是宗务隆洋俯冲消减以及碰撞后伸展过程的产物。 相似文献
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7.
Harangi Szabolcs; Downes Hilary; Thirlwall Matthew; Gmeling Katalin 《Journal of Petrology》2007,48(12):2261-2287
We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension 相似文献
8.
J.B. Smith 《Chemical Geology》2003,194(4):275-295
Four felsic igneous rock suites in the Archaean West Pilbara have been identified based on geochemistry and geochronology. A voluminous TTG suite formed at ca. 3260 Ma, which appears to be from melting of a mafic-subducted oceanic slab and thus represents generation of new continental crust. A tholeiitic to calc-alkaline volcanic assemblage and coeval granitoids formed at ca. 3120 Ma in an extensional environment. Further TTG magmatism occurred at ca. 3000 Ma, generating both large granitoid complexes and small plutons, again adding new continental crust to the West Pilbara. At 2930-Ma crustal reworking, most likely of the 3000-Ma rocks, generated small plutons that are coeval with layered ultramafic-mafic intrusions in the region. The changes from new crustal material to crustal reworking infer changing tectonic regimes, which is important for models of Archaean continental crust generation. The data presented here indicate that crustal generation mechanisms varied and were episodic in the West Pilbara, implying that early crustal evolution was a result of periodic changes in tectonic regime, which is reflected in the geochemistry of the rocks. 相似文献
9.
Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks. 相似文献
10.
The Wild Bight Group (WBG) and South Lake Igneous Complex (SLIC) together comprise one of the Ordovician accreted oceanic
terranes of the central mobile belt of the Newfoundland Appalachians. Combined detailed mapping, geochemistry, Sm-Nd isotopic
studies and U-Pb geochronology have shown that sheeted dykes and hornblende diorite and tonalite plutons of the SLIC are genetically
related to a discrete package of volcanic rocks in the WBG. These igneous rocks are geochemically, isotopically and temporally
distinct from volcanic rocks in the rest of the WBG. Plutonic rocks of the SLIC range in age from 486 ± 3 Ma to 489 ± 3 Ma,
and a cross-cutting gabbro dyke gives a minimum age of 486 ± 4 Ma for the related volcanic sequence. Volcanic rocks in the
rest of the WBG sequence are predominantly younger than 472 ± 3 Ma. The older volcanic sequence of the WBG and the SLIC occur
as fault-bounded packages interleaved within the younger WBG sequence. A conformable stratigraphic relationship between the
older and younger sequences of the WBG has not been demonstrated. The mafic rocks of the older package include boninites and
low-Ti, high-Mg tholeiitic island arc basalts which are interpreted to be genetically related, and normal island arc tholeiites
(IAT). The high-Mg mafic rocks are interpreted to have formed in an extensional setting during subduction zone initiation,
and the normal IAT are thought to represent stabilisation of the volcanic front. The associated high-Si, low-K rhyolite and
tonalite are interpreted to be the products of secondary melting at the base of thickened early arc crust. Sm-Nd isotopic
compositions indicate that the characteristic trace element signature of the boninites developed at or near their time of
generation and was not a long lived characteristic of the source region. The boninites and low-Ti tholeiites are interpreted
to have originated from a similar source, which was metasomatized by different subduction-related components. Apparent decoupling
of Sm-Nd geochemical and isotopic compositions suggests that these very depleted rocks may be recording the effect of subduction
zone processes not yet fully understood.
Received: 31 October 1997 / Accepted: 6 May 1998 相似文献
11.
The polyphasal magmatic evolution of the Caledonian Karmøy Ophiolite Complex includes: (1) formation of an axis sequence from island-arc tholeiitic (IAT) and more MORB-like magmas (493+7/-4 Ma); (2) intrusion of magmas of boninitic affinity (485±2 Ma); (3) intrusion of MORB- and IAT-like magmas; (4) intrusion and extrusion of calc-alkaline magmas (470+9/-5 Ma); (5) intrusion and extrusion of basalts with alkaline trace-element affinity. Repeated intrusion of MORB and IAT-like magmas may be explained by intermittent magmatism involving magma-chamber solidification and remelting of a source characterized by initial Nd of approximately +6.5. The boninitic rocks may have formed from two LREE-depleted sources: the primary source of the axis-sequence magmas and the residual source left after extraction of these magmas. These sources have been enriched in LREE, Th and Zr from subducted material exhibiting a continental Nd-isotope signature with initial Nd less than-8. Covariation between Nd and Th, Zr, Nd, Y and Yb may be explained by metasomatic enrichment of a LREE-depleted mantle source by a LREE-enriched subduction component, followed by partial melting during which the degree of melting of the metasomatized mantle source increased linearly with the amount of subduction component added to the mantle source. The calc-alkaline magmas may have formed by remelting of a highly depleted source, which became enriched in some trace elements derived from the source of the subsequent alkaline magmatism. The geology and geochemistry of the Karmøy Ophiolite Complex suggest growth of an island-arc upon newly-formed oceanic crust, followed by arc-splitting and the development of a new basin. 相似文献
12.
《Gondwana Research》2000,3(1):7-19
The eastern side of the Dom Feliciano Belt consists of supracrustal rocks and granitic batholiths whose emplacements were controlled by the tectonic evolution of the belt. The evolution of this belt had both a tangential tectonic regime and a transcurrent one. The tangential regime, defined by low angle planar and linear structures with W-NW tectonic transport, was responsible for crustal thickening and tectonic imbrication and controlled the syn-kinematic injections of high-K calc-alkaline granitoid plutons of the Arroio Solidão Intrusive Suite about 800 Ma. The transcurrent regime is defined by high angle deformation sets of planar and linear structures, parallel to the elongation of the belt, and indicative of a N-NE tectonic transport. Its associated magmatism began about 672 Ma with the intrusion of the calc-alkaline granitoids of the Arroio Moinho Intrusive Suite, followed by basic dykes and the 630–617 Ma crustal melt granites of the Cordilheira Intrusive Suite. The transcurrent tectonics generated shear zones hundreds of meters in width, which probably extend deep into the mantle. At the end of this process, an extensional regime was installed and during this transition, late- to post-kinematic calc-alkaline granitoids of the Campinas Intrusive Suite were intruded. This magmatism developed during the extensive regime and is represented by the 585 Ma post-transcurrent calc-alkaline granitoids of the Canguçu Intrusive Suite. Alkaline-metaluminous granitoids of the Encruzilhada Intrusive Suite and a few small bodies of peralkaline granitoids represent the final episodes of the granitic magmatism in this region. The syn-tangential granitoids are high-K calc-alkaline and have similarities to those of a continental collision regime. The syn- to post-transcurrent high-K calc-alkaline granitoids are similar to the late- to post-orogenic injections of other orogenic belts. Peraluminous leucogranites of the Cordilheira Intrusive Suite occur also in this transcurrent tectonic setting. The main characteristics of the calc-alkaline magmatism can be related to the participation of a thick continental crust during its generation similar to the granitic systems of a continental collision. Their negative ɛNd values may reflect a magma source closely connected to the continental crust, with their igneous protoliths of relatively homogeneous composition related to a continental collision phase. This continental collision occurred concurrent with the closing of a passive platform-type basin, after the end of sedimentation and tholeiitic volcanic activities. 相似文献
13.
The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt.Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt.Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels).The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone. 相似文献
14.
The Araçuaí orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Araçuaí–West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the São Francisco–Congo Craton, this portion of the Adamastor was only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma. The Rio Doce magmatic arc terminates in the northern portion of the Araçuaí orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to study magmatism at arc terminations, and to understand the interplay between calc-alkaline magma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb–Ta, Sr and Ti anomalies, typically associated with subduction-related magmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. Zircon U–Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590 Ma, experienced a migmatization event at ca. 585 Ma. The younger granitoids, emplaced between 570 and 590 Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydration-melting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done. 相似文献
15.
Binyam W. Woldemichael Jun-Ichi Kimura Daniel J. Dunkley Kenichiro Tani Hiroto Ohira 《International Journal of Earth Sciences》2010,99(8):1773-1790
The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three
N–S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST)
and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary
complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the
CVST and in adjacent areas. New zircon SHRIMP U–Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the
WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860–850 and 795–785 Ma, are documented with the former
for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon
ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura
hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable
at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive εNd (4.5–7.0) and low initial 87Sr/86Sr (0.7029 ± 0.0002) and a mean T
DM model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T
DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle
sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with
oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages
of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism
at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic
and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination
of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~830–690 Ma). 相似文献
16.
V. P. Kovach V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovskyi A. B. Kotov L. B. Terent’eva 《Petrology》2011,19(4):399-425
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes
of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological,
geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental
crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic
crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic
island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction
zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents,
which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites
and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source
contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating
with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient
crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks
of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks
of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were
accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted
mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate
role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly
from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic
mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early
Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile
continental crust and allow us to distinguish a corresponding stage of juvenile crust formation. 相似文献
17.
The Araçuaí orogen of southeastern Brazil, together with its counterpart located in Africa, the West Congo belt, formed through closure of a gulf connected to the Adamastor Ocean by the end of the Ediacaran and beginning of the Cambrian. Convergence of the margins of the gulf led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma on a continental basement mostly composed of Rhyacian orthogneisses. The Rio Doce arc mainly consists of tonalite-granodiorite batholiths, generally crowded with mafic to dioritic enclaves, and minor gabbronorite-enderbite-charnockite plutons, suggesting mixing processes involving crustal and mantle sources. We investigate the basement, magma sources and emplacement ages of the Rio Doce arc. Our data suggest the arc comprises three main granitic rock groups: i) Opx-bearing rocks mostly of enderbite to charnockite composition; ii) enclave-rich tonalite-granodiorite (ETG); and iii) enclave-poor granite-granodiorite with minor tonalite (GT). The Opx-bearing rocks are magnesian, calc-alkalic to alkali-calcic and metaluminous. Together, the ETG and GT rock groups range in composition from tonalite to granite, are metaluminous to slightly peraluminous, show a predominantly medium- to high-K, expanded calc-alkaline signature, and other geochemical and isotopic attributes typical of a pre-collisional volcanic arc formed on a continental margin setting. Mineralogical, chemical, and geochronological data suggest the involvement of HT-melting of granulitic (H2O-depleted) sources of Rhyacian age for the generation of Opx-bearing granitic rocks, additionally to magma mixing and fractional crystallization processes. In conclusion, the studied rock groups of the Rio Doce arc were likely formed by interactions of mantle and crustal processes, in an active continental margin setting. These processes involved ascent of mantle magmas that induced partial melting on the continental basement represented by the Rhyacian gneisses. 相似文献
18.
Richard A. Stern Eric C. Syme Alan H. Bailes Stephen B. Lucas 《Contributions to Mineralogy and Petrology》1995,119(2-3):117-141
Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO2), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial Nd=–0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial Nd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation. 相似文献
19.
Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains 总被引：6，自引：0，他引：6
C. G. Barnes Scott W. Petersen Ronald W. Kistler Robert Murray M. Allan Kays 《Contributions to Mineralogy and Petrology》1996,123(1):40-60
In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about
144 to 136 Ma (Early Cretaceous). It occurred about 5 to 10 m.y. after the ∼165 to 159 Ma Josephine ophiolite was thrust beneath
older parts of the province during the Nevadan orogeny (thrusting from ∼155 to 148 Ma). The magmatism also corresponds to
a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but
one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor
gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist
almost entirely of tonalite and trondhjemite. Poorly-defined concentric zoning is common. Tonalitic rocks are typically of
the low-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite.
The suite is characterized by low abundances of K2O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate (∼450 ppm) by comparison with Sr-rich arc lavas
interpreted to be slab melts (up to 2000 ppm). Initial 87Sr/86Sr, δ
18O, and ɛ
Nd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within
plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean
ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual
assemblage of clinopyroxene+garnet±plagioclase±amphibole; residual plagioclase suggests a deep crustal origin rather than
melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source.
Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat
was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite
suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low-
and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary.
Received: 1 September 1994 / Accepted: 28 August 1995 相似文献
20.
壳幔作用与花岗岩成因——以中国东南沿海为例 总被引：31，自引：3，他引：28
笔者在近年来的中国东南沿海花岗岩成因研究中,注意到下地壳之下的岩石圈地幔与下地壳之间必然有着十分密切的关系,认识到壳幔作用的重要形式是发生于壳－幔接口的玄武岩浆的底侵作用,它涉及地幔对地壳在“成分”和“热”两方面的贡献。研究表明,本区底侵作用十分发育,是中国东南大陆边缘陆壳演化的重要 过程。中国东南部中生代早期形成的花岗岩多为Ｓ－型花岗岩,它们主要是板块强烈挤压和导致地壳增厚,陆壳重熔形成的岩石。而且这期大规模花岗岩浆活动是与弧后拉张、岩石圈减薄 软流圈上涌作用直接有美．早期太平洋板块向欧亚大陆板块俯冲对大陆裂解起了诱导作用中国东南部晚中生代缝合带的年龄为100～11OMa,可能代表了晚中生代构造－岩浆作用由挤压,地壳增厚,陆壳重熔向扩张岩石圈减薄一双蜂式岩浆作用机制的转变年龄。 相似文献