Paleoproterozoic basaltoids in the North Baikal volcanoplutonic belt of the Siberian craton: age and petrogenesis

The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO₂ (an average of 0.89 vs. 1.94 wt.%), Fe₂O₃¹ (9.54 vs. 14.71 wt.%), and P₂O₅ (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, ε_Nd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.     

Early Proterozoic granitoids of the Olenek complex (northern Siberian craton): petrogenesis and geodynamic setting

The paper deals with geological and geochemical studies of granitoids of the Olenek complex in the Olenek uplift of the basement of the northern Siberian craton. The age of these granitoids was earlier estimated at 2036 ± 11 Ma. The granitoids of the Olenek complex correspond in composition to high-alumina quartz diorites, granites, and leucogranites of the normal petrochemical series. According to geochemical and mineralogical characteristics, the quartz diorites can be assigned to granites of the transitional I-S type, and the granites and leucogranites, to S-type granites. The 8_Nd(T values in the granites of the Olenek complex vary from -0.2 to + 1.4, and the Nd model age is 2.4-2.5 Ga. The quartz diorite is characterized by 8_Nd(T) = + 3.0 and a Nd model age T(DM) = 2.2 Ga. The geochemical characteristics of the granites and leucogranites indicate their formation through the melting of a source of graywacke composition, whereas the quartz diorites resulted, most likely, from the mixing of granitic and basaltic melts. The fact that the granitoids of the Olenek complex intruded the folded rocks of the Eekit Formation but stay virtually undeformed massive bodies suggests that they formed at the postdeformation stage of the regional evolution after the completion of the Paleoproterozoic orogenic events. The intrusion of granitoids marks the completion of the formation of the Early Proterozoic Eekit fold belt on the western (in the recent coordinates) margin of the Birekta terrane of the Olenek superterraine and the final formation of the superterrane structure. At the next stage of magmatism (1.98-1.96 Ga), best pronounced in the uplifts of the basement of the northern Siberian craton, all terranes forming the Anabar and Olenek superterranes assembled into a single structure.     

Phanerozoic mafic magmatism in the southern Siberian craton: geodynamic implications

The Phanerozoic history of mafic magmatism in the southern Siberian craton included three major events. The earliest event (～500 Ma) recorded in dolerite dikes occurred during accretion and collision at the early stage of the Central Asian orogen. Injection of mafic melts into the upper crust was possible in zones of diffuse extension within the southern Siberian craton which acted as an indenter. The Late Paleozoic event (～275 Ma) produced dikes that intruded in a setting of subduction-related extension at the back of the active continental margin of Siberia during closure of the Mongolia–Okhotsk ocean, as well as slightly older volcanics (290 Ma) in the Transbaikalian segment of the Central Asian orogen. Early Mesozoic magmatism in the southern Siberian craton resulted in numerous 240–250 Ma mafic intrusions in the Angara–Taseeva basin. The intrusions (Siberian traps) appeared as the subducting slab of the Mongolia–Okhotsk ocean interacted with a lower mantle plume. The post-Late Paleozoic ages of flood basalts (290–275 Ma) correspond to progressive northwestward (in present coordinates) motion of the slab beneath the southern craton margin which likely ceased after the slab had reached the zone of the Siberian superplume. Since its consolidation after the Early Mesozoic activity, the crust in the area has no longer experienced extension favorable for intrusion of basaltic magma.     

New carbonatite complex in the western Baikal area,southern Siberian craton: Mineralogy,age, geochemistry,and petrogenesis

A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable ⁴⁰Ar–³⁹Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO₂ concentrations and is rich in K₂O (K₂O/Na₂O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO₂, P₂O₅ (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)_CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K₂O/Na₂O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.     

Age and sources of the Paleoproterozoic premetamorphic granitoids of the Goloustnaya block of the Siberian craton: Geodynamic applications

This paper reports the results of geological, geochronological, and isotope geochemical investigations of two premetamorphic granite massifs of the Goloustnaya block of the Baikal salient of the basement of the Siberian craton and granite gneisses from the migmatite–gneiss sequence of this block. The U–Pb zircon age of the granites of the Khomut massif is 2153 ± 11 Ma. The age of the Elovka massif was previously determined by us as 2018 ± 28 Ma. The Khomut and Elovka granites underwent structural and metamorphic transformations accompanied by migmatization. An age of 1.98–1.97 Ga was obtained for the structural and metamorphic processes in the Goloustnaya block from the analysis of margins of zircon grains from the Khomut granites and zircon from the granite gneisses. The biotite granites of the Khomut massif show transitional I–S-type geochemical characteristics, which allowed us to suggest that they were derived by melting of a crustal source of intermediate–acid composition. The Khomut granites show positive ε_Nd(T) values from +2.0 to +2.2 and a Nd model age of 2.4 Ga, which may indicate their formation owing to the reworking of the Paleoproterozoic juvenile continental crust. The combined isotope geochemical data are consistent with collision of island arcs as a possible environment for the formation of the Khomut granites. The formation of these granites was not related to the development of the structure of the Siberian craton, similar to a few other anorogenic magmatic complexes of the margin of the Chara–Olekma terrane of the Aldan shield with ages of ~2.2–2.1 Ga, including the granites of the Katugin complex. The biotite–amphibole granites of the Elovka massif with an age of ~2.02 Ga are geochemically similar to I-type granites. The geochemical characteristics of these granites, including elevated Sr and Ba and low Nb and Ta contents, were inherited from a subduction-related source. Negative ε_Nd(T) values from–0.9 to–1.8 and rather high contents of K₂O and Th allow us to suppose a metamagmatic crustal source for the granites of the Elovka massif. The combined isotope geochemical characteristics of the Elovka granites suggest that a mature island arc or an active continental margin is the most probable environment of their formation. The estimates of the age of structural and metamorphic processes affecting the Goloustnaya block (1.98–1.97 Ga) coinciding with the time of similar transformations in the central part of the Aldan shield and eastern Anabar shield (1.99–1.96 Ga) indicate wide occurrence of collisional events of similar age in the Siberian craton and allow us to consider this age interval as an early large-scale stage of the formaiton of the structure of the Siberian craton.     

Age and tectonic setting of the Kengurak-Sergachi gabbro-anorthosite massif (the Selenga-Stanovoi superterrane,southern frame of the Siberian craton)

The results of geochemical and geochronological study of the Kengurak-Sergachi gabbroanorthosite massif in the Selenga-Stanovoi superterrane, southern frame of the Siberian craton, are presented. According to geochemical peculiarities, the massif rocks are close to the autonomous “massif-type anorthosite.” The massif age corresponds to 1866 ± 6 Ma based on the results of U-Pb zircon dating. The Kengurak-Sergachi massif was intruded most likely in post-collision epoch concurrently to formation of the South Siberian giant post-collision magmatic belt (1.87–1.84 Ga) extending along the southwestern flank of the Siberian craton.     

Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value T_Hf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).     

Neoarchean and Paleoproterozoic granitoids marginal to the Jeceaba-Bom Sucesso lineament (SE border of the southern São Francisco craton): Genesis and tectonic evolution

The sialic crust of the southern São Francisco craton along the Jeceaba-Bom Sucesso lineament, central-southern part of Minas Gerais (Brazil), encompasses, among other rock types, Neoarchean and Paleoproterozoic granitoids. These granitoids, according to their petrographic, lithogeochemical and geochronologic characteristics, were grouped into two Neoarchean suites (Samambaia-Bom Sucesso and Salto Paraopeba-Babilônia) and three Paleoproterozoic suites (Cassiterita-Tabuões, Ritápolis and São Tiago). Varied processes and tectonic environments were involved in the genesis of these suites. In particular, the lithogeochemistry of the (Archean and Paleoproterozoic) TTG-type granitoids indicates an origin by partial melting of hydrated basaltic crust in a subduction environment. In the Neoarchean, between 2780 and 2703 Ma, a dominant TTG granitoid genesis related to an active continental margin was followed by another granite genesis related to crustal anatexis processes at 2612–2550 Ma. In the Paleoproterozoic, the generation of TTG and granites s.s. occurred at three distinct times: 2162, 2127 and 1887 Ma. This fact, plus the rock-type diversity produced by this granite genesis, indicates that the continental margin of the southern portion of the São Francisco craton was affected by more than one consumption episode of oceanic crust, involving different island arc segments, and the late Neoarchean consolidate continent. A Paleoproterozoic tectonic evolution in three stages is proposed in this work.     

Early Proterozoic postcollisional granitoids of the Biryusa block of the Siberian craton

Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberian craton basement. U-Pb zircon dating of the granites yielded an age of 1874 ± 14 Ma. The rocks of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-type granites. The low CaO/Na₂O ratios (< 0.3), K₂O - 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7-1.9) and Rb/Sr (3.9-6.8) ratios indicate that the two-mica granites resulted from the melting of a metapelitic source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in £_Nd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at 740-800 °C (zircon saturation temperature). The age of the S-type two-mica granites agrees with the estimated ages of I- and A-type granitoids present in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik-Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik-Iya terrane, and Archean Tunguska superterrane into the Siberian craton.     

Late-Neoproterozoic ferroan granitoids of the Transversal subprovince,Borborema Province,NE Brazil: petrogenesis and geodynamic implications

ABSTRACT

Ferroan granites (585–530 Ma) have been described in the Transversal subprovince of the Borborema Province (BP) and in Pan-African counterparts. They comprise two groups: Group 1 – slightly peraluminous to metaluminous, alkali-calcic rocks, with low Fe# mica and crystallized under intermediate fO₂ (Aroeiras Complex and Serra Branca – Coxixola dike swarms); Group 2 metaluminous to slightly peraluminous, alkalic to alkali-calcic rocks, with high Fe# mica and crystallized under low fO₂ (Queimadas and Prata intrusions). Group 1 marks transition from collision to transcurrence (ca. 585 Ma), or from transcurrence to uplift and transtension (ca. 545 Ma). Group 2 – represents granitoids intruded during extensional tectonics in transcurrent setting (ca. 550 Ma), or coeval with deposition of transtensional intracratonic basins (ca. 530 Ma). Hf and Nd model ages are older than 2.0 Ga, suggesting that the ferroan granitoids involved partial melting of Paleoproterozoic rocks. The data presented in this paper show that the ferroan magmatism was widespread in the BP and its counterparts in Africa in pre-drift reconstructions.     

Using the isotope dating of endocontact hybrid rocks for the age determination of mafic rocks (southern Siberian craton)

Geological observations and petrological and geochemical criteria are used to detect hybrid rocks at the endocontact of a dolerite dike. The hybrid rocks were produced when the material of a mafic intrusion mixed with a felsic melt. The latter was produced by the melting of the metamorphic rocks making up the Goloustnaya basement inlier of the Siberian craton, under the thermal effect of the intruded dike. Two age groups of zircon have been identified in the hybrid rock by SHRIMP analysis. The Paleoproterozoic age of inherited zircon (1902, 1864, 1859, and 1855 Ma) reflects the contribution of ancient sources to the hybrid-rock composition. The young, primary-magmatic, zircon grains, produced by melting at the endocontact of the mafic intrusion (494 ± 5 Ma), are coeval with the hybrid rocks, and their age indicates when the mafic rocks intruded the metamorphic framework. Dikes of close age, with similar geochemical characteristics, are present on the vast southern margin of the Siberian craton—from Goloustnaya to Biryusa salients.     

湘东北金井地区花岗岩成因及地球动力学暗示：岩石学、地球化学和Sr-Nd同位素制约

出露于湘东北地区的燕山期花岗质岩岩体,呈岩基或岩株状广泛侵位于中元古界冷家溪群浅变质碎屑沉积岩中, 部分岩体为白垩纪红层覆盖。文章详细研究了金井岩体的岩石学、地球化学及Sr-Nd同位素特征,并与其围岩冷家溪群进行了对比。该岩体具有较高的SiO2(71．44-74．31wt．％)、K2O(3．86-4．98wt．％)和Na2O(3．24-3．84wt．％),以及较低的 FeO Fe2O3 MgO TiO2(均小于3％),结合高含量的Al2O3(13．25-14．89wt．％)及高ASI值(普遍大于1．1),为典型的强过铝质(SP)高钾钙碱性花岗岩。金井花岗岩富硅、碱和高Sr、Ba丰度、高LILE／HSFE和LREE／HREE比值,低镁铁质及REE 含量,表明金井岩体为高度分异的花岗岩,其源区的残留物中黑云母含量高。REE配分图和原始地幔多元素标准化图谱上 Rb、Th、U、K和LREE相对富集,而Ba、Sr和高强场元素(Nb、Ta、P和Ti)相对亏损,Eu可变的负异常(δEu:0．53-0．78),表明存在角闪石、黑云母、堇青石、磷灰石和钛铁矿等分异演化的影响,类似于同碰撞型花岗岩地球化学特征。高Rb／Sr(3-6)、低 Sr／Ba(0．2-0．7)、高P2O5含量(0．09-0．15wt．％)、低Al2O3／TiO2比值(40-700．4和<0．2两组)以及Nd同位素的不同(高Nd和低Nd两组),类似于围岩冷家溪群砂质和泥质沉积岩特征,以及高Sr[(87Sr／86Sr)i=0．72204-0．72540]、低Nd[εNd(145 Ma)=-10--12]等均表明花岗质岩浆主要起源于中元古代冷家溪群变质砂岩(高Ca)和变质泥岩(低Ca)的部分熔融或中下地壳物质的熔融,但有少量地幔物质的加入。结合印支期以来华南及湘东北地区地球动力学演化史,作者认为金井燕山期花岗岩形成于陆内碰撞造山晚期,是160-140Ma 陆内俯冲(A型俯冲)碰撞造山作用增温减压体制达到鼎盛阶段的产物,标志湘东北地区由此全面转入陆内伸展阶段。     

The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.     

Paleoproterozoic accretion in the Northeast Siberian craton: Isotopic dating of the Anabar collision system

Geochronological database considered in the work and characterizing the Anabar collision system in the Northeast Siberian craton includes coordinated results of Sm-Nd and Rb-Sr dating of samples from crustal xenoliths in kimberlites, deep drill holes, and bedrock outcrops. As is inferred, collision developed in three stages dated at 2200–2100, 1940–1760, and 1710–1630 Ma. The age of 2000–1960 Ma is established for substratum of mafic rocks, which probably originated during the lower crust interaction with asthenosphere due to the local collapse of the collision prism. Comparison of Sm-Nd and Rb-Sr isochron dates shows that the system cooling from ≈700 to ≈300°C lasted approximately 300 m.y. with a substantial lag relative to collision metamorphism and granite formation. It is assumed that accretion of the Siberian craton resulted in formation of a giant collision mountainous structure of the Himalayan type that was eroded by 1.65 Ga ago, when accumulation of gently dipping Meso-to Neoproterozoic (Riphean) platform cover commenced.     

Late Proterozoic older granitoids from the North Eastern desert of Egypt: petrogenesis and geodynamic implications

Major, trace, and REE data for three localities of calc?Calkaline older granitoid rocks exposed in the north Eastern Desert of Egypt are presented. These rocks were selected to cover wide compositional spectrum of the Egyptian older granitoid varieties. They are petrographically represented by granodiorite, tonalite, quartz?Cdiorite, and quartz?Cmonzodiorite. The rocks are comparable with the peraluminous, unfractionated calc?Calkaline suites and fall within the volcanic arc and I-type granite fields. So, they can be regarded as belonging to the volcanic arc collision stage (665?C614?Ma). The granitoids are geochemically similar to other rocks recorded from continental margin arc-systems being exhibit light-REE enriched patterns with variable but chiefly positive Eu anomaly. The latter has reverse relationship with the ??REE, which was attributed to the fractionation of hornblende during partial melting. These patterns are comparable with models involving partial melting of amphibolitic source. This source must represent basalts, gabbros, or volcanics of an island arc system that were transformed to the level of the island arc crust during continental growth where the P?CT conditions are suitable for partial melting. Thus, it is plausible that the studied rocks were derived by partial melting of LREE-enriched, garnet-free, and amphibole-bearing (i.e., hydrated) mafic source. Wadi Milaha granitoids are consistent with the derivation by a high degree of partial melting (30?C40%) of amphibolite protolith in the deep crust. However, the two other localities (Wadi Umm Anab and West Gharib) are matching with 20?C30% partial melting. Within each locality, variation in rock types from granodiorite to tonalite is said to be dominated by variable degree of restite separation during magma ascent. The high water and volatile contents in Wadi Milaha granitoids allowed higher degree of partial melting (30?C40%). Moreover, the lower volatile contents in the other two localities (Wadi Umm Anab and West Gharib) gave lower degrees of partial melting (20?C30%). These processes may resemble important geodynamic features of the Arabo-Nubian Shield evolution in the north Eastern Desert of Egypt.     

Signature of Precambrian extension events in the southern Siberian craton

We investigate extension events in the southern Siberian craton between 1.8 and 0.7 Ga. Signature of Late Paleoproterozoic within-plate extension in the Northern Baikal region is found in 167 4± 29 Ma dike swarms. A Mesoproterozoic extension event was associated with intrusion of the 1535 ± 14 Ma Chernaya Zima granitoids into the Urik-Iya graben deposits. Neoproterozoic extension recorded in the Sayan-Baikal dike belt (740-780 Ma dike complexes) was concurrent with the breakup of the Rodinia supercontinent and the initiation of the Paleoasian passive margin along the southern edge of the Siberian craton. The scale of rifting-related magmatism and the features of the coeval sedimentary complexes in the southern Siberian craton indicate that Late Paleoproterozoic and Early Mesoproterozoic extension did not cause ocean opening, and the Paleoasian Ocean opened as a result of Neoproterozoic rifting.     

吉林南部通化地区古元古代辽吉花岗岩的侵位年代与形成构造背景

辽吉南部古元古代花岗岩极为发育,其中最著名的是具有条痕状构造的正长花岗岩,俗称辽吉花岗岩。因该花岗岩具有条带状、条痕状和似层状构造特征,而被前人认为是由太古宙或元古宙沉积地层经变质变形作用形成,或是交代成因的混合岩。本文通过对分布于吉林南部通化地区辽吉花岗岩的典型代表一钱桌沟岩体的详实野外地质填图及地球化学研究,确定该岩体属岩浆成因的“A”型花岗岩,其岩浆侵位年龄为2160Ma左右。结合目前获得的该区古元古代地层的碎屑锆石年龄,本文认为辽吉花岗岩是辽吉地区古元古代地层沉积的基底岩石,是地层沉积之前地壳拉张作用的结果,属于一种非造山型花岗岩。