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1.
《International Geology Review》2012,54(12):1103-1120
The Malani Igneous Suite is characterized by discontinuous, ring-shaped outcrops of peralkaline granite associated with minor exposures of volcanic rocks around Barmer town in southwestern Rajasthan, India. These granites are defined as peralkaline, within plate, and A-type based on their bulk rock compositions. The most distinctive geochemical characteristics of these A-type granites are enrichments in Na2O + K2O, Fe/Mg, Zr, Nb, Y, depletions in Al2O3, CaO, Sr, and low-absolute abundances of incompatible trace elements compared to granites from adjoining areas. The igneous activity is considered as a reflection of the ‘Pan-African Event’. The correlative mineralogy, chemical characteristics, and tectonic setting of the peralkaline granites from the study area, and comparison with data from adjoining areas, suggest their generation under a common thermal event.  相似文献   

2.
Petrochemical studies on acid plutonic (granite, microgranite) and volcanic (rhyolite, trachyte) rocks occurring in the Siner area of the Siwana Ring Complex, Malani Igneous Suite have been carried out. These rocks are characterized by high concentrations of SiO2, Na2O, K2O, Zr, Nb, Y and REE (except Eu) but low in MgO, Fe2O3(t), CaO, Cr, Ni, Sr; indicating their A-type affinity. Field studies in conjunction with the geochemical characteristic indicate that the magmatism in the Siner area is generally represented by peralkaline suite of rocks which are formed due to rift tectonics. It is also suggested that these acidic rocks could have been derived by low degree partial melting of crustal material. Characteristics of certain pathfinder elements such as Rb, Ba, Sr, K, Zr, Nb, REE and the ratios of K/Rb, Zr/Rb, Ba/Rb along with the multi elemental primitive mantle normalized spidergrams suggest that the Siner peralkaline granites and microgranites have the potential for rare metal and rare earth mineralizations.  相似文献   

3.
One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle‐derived calc‐alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S‐type) plutonism and volcanism in the late Early Silurian. We suggest that this chemical/isotopic shift required major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction‐related magmatism following burial of the pre‐existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician — Early Silurian (pre‐Benambran) deformation, associated with regional northeast‐southwest shortening. A review of ‘type’ Benambran deformation from the type area (central Lachlan Fold Belt) shows that it is constrained to a north‐northwest‐trending belt at ca 430 Ma (late Early Silurian), associated with high‐grade metamorphism and S‐type granite generation. Similar features were associated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex, and formed within a separate north‐trending belt that included the S‐type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca 430 Ma, they must have been buried already to ～20 km before the ‘type’ Benambran deformation. We suggest that this burial occurred during earlier northeast‐southwest shortening associated with regional oblique folds and thrusts, loosely referred to previously as latitudinal or east‐west structures. This event also caused the earliest Silurian uplift in the central Lachlan Fold Belt (Benambran highlands), which pre‐dated the ‘type’ Benambran deformation and is constrained as latest Ordovician — earliest Silurian (ca 450–440 Ma) in age. The south‐ to southwest‐verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early oblique structures, although similar deformation in that zone probably continued into the Devonian. We term these ‘pre’‐ and ‘type’‐Benambran events as ‘early’ and ‘late’ for historical reasons, although we do not consider that they are necessarily related. Heat‐flow modelling suggests that burial of ‘average’ Ordovician turbidites during early Benambran deformation at 450–440 Ma, to form a 30 km‐thick crustal pile, cannot provide sufficient heat to induce mid‐crustal melting at ca 430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic ca 430 Ma, Micalong Swamp Igneous Complex in the S‐type Young Batholith. Modern heat‐flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, appears to preclude older continental ‘basement terranes’ as sources for the S‐type granites. Restriction of the S‐type batholiths into two discrete, oblique, linear belts in the central and east Lachlan Fold Belt supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this ‘double convergent’ subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S‐type) magmatism. In contrast, the central Lachlan Fold Belt magmatic arc, represented by the Wagga‐Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast‐southwest shortening. The model requires that the S‐type and subsequent I‐type (Late Silurian — Devonian) granites of the Lachlan Fold Belt were associated with ongoing, subduction‐related tectonic activity.  相似文献   

4.
In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites also occur in Australia and elsewhere. Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates. The melt structure determines the trace element composition of the granite. Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more important than F.  相似文献   

5.
呼伦湖早白垩世碱性流纹岩的地球化学特征及其意义   总被引：34，自引：2，他引：34       下载免费PDF全文
大兴安岭呼伦湖一带的上库力组第3段流纹岩具有高硅(SiO2=75.41%～77.32%)、富碱(Na2O+K2O=7.98%～8.74%)、富Y、Nb、Zr及低Al、Mg、Ca、Ni、Cr、Ti和高Ga/Al比值等特点,类似于A型花岗岩,而与高度分异的I型和S型花岗岩有明显差别。该流纹岩的地球化学特征类似于A1型花岗岩,且与该区A型花岗岩同时,很可能是A型花岗岩浆喷出相的产物。流纹岩的全岩Rb-Sr等时线年龄127±5Ma,与伊列克得组玄武岩(125±2Ma)时代一致,两者构成了双峰式火山岩组合,形成于早白垩世岩石圈拉张环境。  相似文献   

6.
《International Geology Review》2012,54(14):1559-1575
The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J3) to Early Cretaceous (K1). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ～145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO2, 13.12–17.73 wt.% Al2O3, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeOT, and 4.71–7.87 wt.% total alkalis (Na2O?+?K2O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ～130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO2, 17.67–20.65 wt.% Al2O3, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeOT, and 9.24–13.92 wt.% total alkalis (Na2O?+?K2O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35.

The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.  相似文献   

7.
The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quitéria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butiá and Três Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70°E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event. The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K2O/Na2O and CaO/Na2O ratios are approximately 1 and less than 0.3, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 °C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Várzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite facies metamorphism were recorded in the Várzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 °C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic.  相似文献   

8.
The Nakora Ring Complex(NRC)(732 Ma) occurs as a part of Malani Igneous Suite(MIS) in the West-ern Rajasthan,India.This complex consists of three phases(volcanic,plutonic and dyke).Geochemically,the Na-kora granites are peralkaline,metaluminous and slightly peraluminous.They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content.The peralkaline granites show higher concentrations of SiO2,total alkalies,TiO2,MgO,Ni,Rb,Sr,Y,Zr,Th,U,La,Ce,Nd,Eu and Yb and lower concen-trations of Al2O3,total iron,Cu and Zn than metaluminous granites.AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites.Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths(16-28 km and 480-840℃) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites.The primitive mantle normalized multi-element profiles suggest that Nakora granites(peralkaline,metaluminous and peraluminous) are characterized by low La,Sr and Eu and relatively less minima of Ba,Nb and Ti which suggests the aspects related to crustal origin for Nakora magma.The Nakora granites are characterized as A-type granites(Whalen et al.,1987) and correspond to the field of "Within Plate Gran-ite"(Pearce et al.,1984).Geochemical,field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.  相似文献   

9.
Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICPMS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO_2 content(62.01–63.03 wt%) and notably high Na_2O content(7.04–7.24 wt%) and Na_2O/K_2O ratios(2.02–2.10), low MgO, Fe_2O_3 T and TiO_2, enrichment of LILEs(large-ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies(d Eu=0.53–0.56). They also display significant negative whole-rock εNd(t) values of-6.8 and zircon εHf(t) values(-9.11 to-0.27, but one is +5.30) and high initial ~(87) Sr/~(86) Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. 115.3 Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO_2 content(71.35–74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and Al_2O_3/(MgO+FeOT+TiO_2) ratios and moderate(Al_2O_3+MgO+FeOT+TiO_2) content. They show low initial ~(87) Sr/~(86) Sr(0.703408 to 0.704241) and εNd(t) values(-3.8 to-3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.  相似文献   

10.
The Xiangcheng-Luoji area is located in the conjunction of the southern part of the "Sanjiang" mineralization belt and the west margin of Yangtze craton. The geological studies were carried out to know the Indosinian large porphyry Cu polymetallic deposits. Recent studies revealed that the area existed in the superposition of Late Yanshanian acidic intrusive rock belt and developed Mo-Cu polymetallic mineralization where promising exploration results have been achieved. Through the systematic study of geochronology, formation age of the Renlin Mo-minieralization monzogranite is 81.7±1.1 Ma. Re-Os dating results concentrate on 82.34±1.2–88.27±1.23 Ma for the model ages of molbdenite of Tongchanggou Mo deposits, average age is 85 ± 2 Ma where seven data points constitute a good isochron which shows that they were the same period products of mineralization. Geochemical features shown that the rocks have a high content of SiO 2(66.59–77.36wt%), alkaline-rich(K2O=2.68–6.08wt%; Na2O=0.50–4.91wt%; K2O/Na2 O ratios are 0.71–5.56, where average ratio of 1.89) and have aluminum–rich features(Al2O3 10.38–15.15wt%) with σ values less than 3.3. Which indicate that they belong to the high-K calc-alkali to shoshonite series. Geochemistry of Yanshanian intrusions shows that rocks are enrich in LREE with obvious negative δEu anomalies, enrichment of trace elements like, LILE elements(Rb, Th, Ba) with a relative loss of Ba, and loss of high field strength elements(Nb, Ta, P, Ti) and HREE elements. The granite genetic classification diagram shows that the granites belong to A-type granite and formatted in syn-collision tectonic environment. Meanwhile, the Yanshanian granites also inherited the characteristics of island arc environment which formed in the process of crustal melting caused by upwelling of asthenospheric substances in the extensional tectonic background. The process of partial melting existed substances from the deep(lower crust or upper mantle) which have been added. In the Xiangcheng-Luoji area, monzogranite and granodiorite porphyry bodies are widely developed Mo polymetallic mineralization, the deep porphyry mineralization have great potential for geological prospecting.  相似文献   

11.
Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica...  相似文献   

12.
八宿吉利地区寒武纪变质花岗岩位于曲扎湖-提卡一带,主要由变质二长花岗岩和变质花岗闪长岩组成。这一新发现对于认识和恢复原特提斯构造历史演化具有重要意义。锆石CL图像显示变质花岗岩锆石为岩浆成因。锆石LA-ICP-MS测年得出片理化变质二长花岗岩年龄为503.7±4.7Ma、变质花岗闪长岩年龄为494.7±3.4Ma,表明该岩体形成时代属于寒武纪。通过岩石地球化学分析,变质二长花岗岩SiO2含量介于69.87%~79.89%之间;变质花岗闪长岩SiO2含量介于66.63%~70.15%之间。前者Al2O3含量变化于12.36%~14.82%,Na2O含量为2.54%~7.16%,K2O含量为0.15%~5.95%,K2O/Na2O=0.02~2.34;后者Al2O3含量变化于14.66%~15.41%,Na2O含量为3.60%~5.63%,K2O含量为0.77%~2.78%,K2O/Na2O=0.14~0.77,属于钙碱性-碱性过铝质花岗岩。在侵入岩构造环境Rb-(Y+Nb)判别图解、Rb-(Yb+Ta)判别图解中,样品均落入“火山弧花岗岩”区域中,表明其形成于大陆边缘火山弧环境。结合锆石测年结果及区域地质背景分析,认为吉利地区变质花岗岩形成于冈瓦纳大陆裂离卡穷微陆块阶段,同时表明原特提斯洋形成最早时限可追溯至寒武纪。  相似文献   

13.
ABSTRACT

We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 ± 13 Ma and 2738 ± 25 Ma, respectively. The more precise age of 2732 ± 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO2 (73.11–74.01 wt%), K2O (3.93–5.48 wt%), Na2O (3.93–4.86 wt%) and low CaO (0.30–0.69 wt%), MgO (0.17–0.30 wt%), TiO2 (0.14–0.17 wt%), P2O5 (0.01–0.06 wt%), Al2O3 (14.11–14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04–1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2–34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their εHf(t) values (?2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive εNd(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67–2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.  相似文献   

14.
Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common hornblende to actinolite and also coexist with annite-rich biotite.Whole-rock and mineral isotopic data for the different suites yield isochrons that are within error of ca. 1,260 Ma, but they have variable initial 87Sr/86Sr ratios. The initial 87Sr/86Sr of the syenites and peralkaline granites (0.7076±11) is significantly lower than the initial 87Sr/86Sr of the subsolvus granites (0.7138±22). These isotopic data provide further confirmation of the importance of a late Elsonian alkaline event in Labrador which can be correlated with Gardar igneous activity in south Greenland. The petrogenesis of the peralkaline suite is interpreted to reflect the effects of fractionation of anhydrous phases from mantle derived basic magma which was contaminated during ascent by radiogenic partial melts of crustal derivation. The non-alkaline hypersolvus and subsolvus granites are interpreted as crustal melts which formed under conditions of variable in response to the same thermal event, and which subsequently experienced feldspar fractionation during crystallization.  相似文献   

15.
Palaeozoic rapakivi granites occur in the western segment of the China Central Orogenic System. Exhibiting typical rapakivi texture, these granites contain magmatic microgranular enclaves of intermediate compositions. SHRIMP zircon U–Pb ages for the granites and enclaves are 433 ± 5 Ma and 433 ± 3 Ma, respectively. The rapakivi granites are magnesian to ferroan, calc-alkalic to alkalic, and are characterized by high FeOt/(FeOt + MgO) (0.74–0.91) and Ga/Al ratios, and SiO2, Na2O + K2O and rare earth element (apart from Eu) contents, but low CaO, Ba, and Sr contents. These are typical A-type granite geochemical features. The granites and enclaves exhibit a uniform decrease in TiO2, CaO, Na2O, K2O, FeO, and MgO with increasing SiO2, and both lithologies have similar trace element patterns. Whole-rock ?Nd(t) values vary from??9.2 to??8.7 for the granites and from??9.0 to??8.4 for the enclaves, but zircon ?Hf(t) values vary more widely from??5.8 to??0.2 and??4.6 to +5.1, respectively. Our data suggest that the granites and enclaves have crystallized from different magmas. The granites appear to have been derived from old continental crust, whereas the enclaves required a source having a juvenile component. The spherical shape and undeformed nature of the granites and their geochemical characteristics, coupled with the (ultra)-high pressure metamorphism and evolution of Palaeozoic granitoid magmatism in the North Qaidam orogen, indicate that the rapakivi granites were generated in a post-collisional setting. These rocks are therefore an example of Palaeozoic rapakivi granites emplaced in a post-collisional, extensional orogenic setting.  相似文献   

16.
The Wangrah Suite granites (Lachlan Fold Belt, Australia) reflect different stages of differentiation in the magmatic history of an A-type plutonic suite. In this study we use experimentally determined phase equilibria of four natural A-type granitic compositions of the Wangrah Suite to constrain phases and phase compositions involved in fractionation processes. Each composition represents a distinct granite intrusion in the Wangrah Suite. The intrusions are the Danswell Creek (DCG), Wangrah (WG), Eastwood (EG) and Dunskeig Granite (DG), ordered from “most mafic” to “most felsic” by increasing SiO2 and decreasing FeOtotal.

Experimental investigation show that the initial water content in melts from DCG is between 2–3 wt. % H2O. If the DCG is viewed as the parental magma for the Wangrah Suite, then (1) fractionation of magnetite, orthopyroxene and plagioclase ( 20 wt. %) of the DCG composition, leads to compositions similar to that of the EG; (2) further fractionation of plagioclase, quartz, K-feldspar and biotite ( 40 wt. %) from the EG composition, leads to the DG composition. These fractionation steps can occur nearly isobarically and are confirmed by bulk rock Ba, Sr, Rb and Zr concentrations.

In contrast, the generation of the most abundant WG composition cannot be explained by fractional crystallisation from the DCG at isobaric conditions because of the high K2O content of this granite. Magma Mixing could be the process to explain the chemical distinctiveness of the Wangrah Granite from all the other granites of the Wangrah Suite.  相似文献   


17.
Many granites have compositional features that directly reflect the composition of their source rocks. Since most granites come from the deeper parts of the Earth's crust, their study provides information about the nature of parts of that deep crust. Granites and related volcanic rocks are abundant and widely distributed in the Palaeozoic Lachlan Fold Belt of southeastern Australia. These granites show patterns of regional variation in which sharp discontinuities occur between provinces which internally are of a rather constant character. Such a discontinuity has long been recognized at the I‐S line and the extent of that line can now be defined more fully. Breaks of this type are thought to correspond to sharp changes in the composition of the deep crust that correspond to unexposed or basement terranes. Nine such basement terranes can be recognized in the Lachlan Fold Belt. The character of these basement terranes appears to be different from that of the terranes recognized in the Mesozoic‐Cainozoic Cordilleran fold belt, in which the plates accreted during the period of tectonism reflected in the exposed surface rocks. In the Lachlan Fold Belt, it is postulated that fragments of continental crust, or microplates, were assembled in the Late Proterozoic or Early Palaeozoic to form the substrate of the presently exposed Palaeozoic sedimentary rocks; the compositional features of these fragments were later redistributed vertically by magmatic processes. The identification of basement terranes of this type shows that models which involve the lateral growth of the Lachlan Fold Belt during the Palaeozoic, in a manner analogous to the accretion of younger belts, are untenable. These basement terranes have implications for mineral exploration because the content of heavy metals can vary from one to another and this would ultimately affect the probability of concentrating these metals to form a mineral deposit.  相似文献   

18.
The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO2 contents ranging from 65.18 to 72.72 wt.%, K2O from 4.68 to 6.62 wt.%, and Na2O from 1.35 to 4.88 wt.%. They have high Fe*[FeOt/(FeOt + MgO)] ranging from 0.84 to 0.95 wt.% and low MnO (0.03–0.09 wt.%), MgO (0.27–1.55 wt.%), CaO (0.36–2.04 wt.%), TiO2 (0.4–1.12 wt.%), and P2O5 (0.04–0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K2O + Na2O, with K2O/Na2O > 1), Rb, Y, Nb, and REEs (except for Eu); high FeOt/MgO and Ga/Al ratios; and low CaO, Al2O3, and Sr contents. New secondary ion mass spectroscopy (SIMS) zircon U–Pb ages reveal that the Luoling granites were emplaced at 1786 ± 7 Ma and thus were approximately coeval with Xiong'er volcanic rocks in the area. Their negative bulk-rock initial Nd and zircon initial Hf isotopic ratios suggest that they have affinities to EM-I-type mantle and both are the products of Xiong'er magmatism during the Palaeoproterozoic. We regard them as produced under a continental rift setting during the Palaeoproterozoic, genetically related to the break-up of the Columbia supercontinent.  相似文献   

19.
澜沧江南段临沧花岗岩的锆石U-Pb年龄及构造意义   总被引：7，自引：5，他引：2  
王舫  刘福来  刘平华  施建荣  蔡佳 《岩石学报》2014,30(10):3034-3050
临沧花岗岩是滇西地区出露面积最大的复式岩基,它是特提斯构造域的重要组成单元,是研究古特提斯俯冲-碰撞的重要窗口。本文通过对澜沧江南段澜沧-景洪地区广泛出露的临沧花岗岩的岩石学、地球化学以及锆石年代学综合分析,系统阐述该区花岗岩的原岩性质以及其形成的构造背景。临沧花岗岩主要岩石类型为黑云母二长花岗岩和花岗闪长岩。锆石LA-ICP-MS U-Pb年代学结果表明,该区临沧花岗岩侵位时代为217~233Ma。前人在澜沧江北段花岗岩也获得相似的侵位年龄,表明临沧花岗岩的南段与北段在形成时代上具有一致性。继承锆石U-Pb年龄主要峰期集中在2494Ma、1832Ma、1382Ma、959Ma、774Ma、482Ma,指示临沧花岗岩具丰富的物质来源。全岩主微量元素分析结果显示,临沧花岗岩的Na2O/K2O比值低,铝饱和指数(A/NCK值)大于1,属高钾钙碱性系列,过铝质花岗质岩石。轻重稀土分异明显,轻稀土相对富集,具有明显的铕负异常(Eu/Eu*=0.39~0.63);相容元素Cr和Ni含量较低,富集大离子亲石元素Rb和Ba,亏损高场强元素Nb-Ta和Zr-Hf。地球化学特征显示,临沧花岗岩来源于地壳沉积物的部分熔融,属S型花岗岩,形成于古特提斯洋闭合后的构造伸展阶段。  相似文献   

20.
Inclusions in Three S-Type Granites from Southeastern Australia   总被引：11，自引：0，他引：11  
The Jillamatong Granodiorite is one of the most mafic S-typegranites in the Kosciusko regidn and is typical of widely distributed,cordierite-bearing S-type granites in the Lachlan Fold Beltof southeastern Australia. The Koetong and Granya Adamellitesbelong to the Koetong Suite of the Corryong Batholith and arerare examples in the Lachlan Fold Belt of granites that containprimary muscovite. Although subtle differences can be found,inclusions within the Jillamatong Granodiorite and the KoetongSuite are broadly similar despite the fact that the JillamatongGranodiorite belongs to a different and distinct suite (theBullenbalong Suite). Mica-rich schistose and micTogranular inclusionsdominate but other types occur, including foliated quartzofeldspathicvarieties, calcsilicates, quartzites, and pure quartz types.The total abundance of all inclusion types in each granite studiedis less than 5.1% although abundance varies from one graniteto another. All inclusions are believed to have been derived from metasedimentaryor modified metasedimentary lithologies and all inclusions,except some quartzites, were entrained at depth where the hostgranite magmas were generated by partial melting of heterogeneoussedimentary sources. The inclusions are restite but most arenot complementary to the melt component of the magma now representedby the host granite. They represent fragments from differentrefractory lithologies of a complex metasedimentary source andbecause their compositions and mineral assemblages were unsuitablefor the generation of large quantities of granite melt, theydid not melt or were melted only to small and variable extents(less than the rheological critical melt percentage of Arzi,1978). Such lithologies remained physically coherent and retainedtheir separation from the host granite magma during ascent.Lithologies that did melt extensively were physically disaggregatedand are not represented among the inclusions. Since the inclusions do not represent complementary restitecontrolling compositional variation among the host granites,their compositions cannot be used to precisely estimate thebulk compositions of the source rocks. However, the different,source-rock derived, inclusion types collectively provide informationregarding the lithologies present in the source and hence thegeneral character of the source terranes. The dominance of schistoseand microgranular inclusions in the Jillamatong Granodioriteand the Koetong Suite indicates that pelitic and quartzofeldspathiccompositions are the two dominant components in the source terranes. Inclusions of the same type from the two suites are broadlysimilar but different in detail. Inclusions reflect the mineralogicaland geochemical characteristics of their host granites and thereare textural differences between microgranular inclusions ofthe two suites examined. The differences reflect subtle butsignificant contrasts in source materials, the conditions prevailingduring partial melting and the history of emplacement and crystallizationof the host magmas.  相似文献   

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