Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region.The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km.In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results.Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm³) has been found seems to be responsible for the gravity high in that part.     

Crustal structure from deep seismic soundings along the Koyna II (Kelsi-Loni) profile in the Deccan Trap area, India

The crustal depth section obtained from deep seismic soundings along the Koyna II (Kelsi-Loni) profile, which lies near latitude 18°N roughly in the east-west direction in that part of the Deccan Trap Maharashtra State, India, shows a number of reflection segments below the Deccan Traps down to the Moho discontinuity. A deep fault below the Deccan Traps 13 km east of Mahad divides the entire cross-section including the Moho boundary into two crustal blocks. The reflection segments show updip towards the west coast in the western block. The Moho discontinuity which is at a depth of 39 km near the deep fault starts rising towards the coast, reaching a depth of 31.5 km at the west coast. The eastern block is thrown up by 1.5 km with respect to the western block along the deep fault. A structural contour map of the Moho discontinuity for the Koyna reservoir area has been prepared from the present results and the crustal information obtained along the Koyna I profile (Kaila et al., 1979a), shows that the deep fault in the Koyna area is aligned in the NNW-SSE direction.Refraction seismic data analysis by the wave front method reveals that the thickness of the Deccan Trap increases towards the west coast. The Deccan Trap is 600–700 m thick in the eastern region between Nira (SP 130) and Loni (SP 200) and attains a thickness of 1500 m at 10 km east of the west coast. The longitudinal wave velocity in the Deccan Traps along the profile varies from 4.8 to 5.0 km/sec and in the crystalline basement from 6.0 to 6.15 km/sec. A tentative isopach contour map of the Deccan Traps and a tentative structural contour map of the Pre-Deccan Trap contact have been prepared for the Koyna reservoir area from the results along the Koyna II and Koyna I profiles. A flexure aligned in a NNW-SSE direction, in the Pre-Deccan Trap contact, which is an expression of the deep fault into the basement, has been clearly brought out. The flexure coincides in general with the orientation of the Deccan volcanic scarp in this area.     

Crustal studies of the Koyna region using explosion data from deep seismic soundings

This study is based on the seismic data collected as a result of explosions carried out during the 1976 and 1978 Deep Seismic Sounding (DSS) field operations in the Koyna region. These shots were exploded from twelve shot points by the National Geophysical Research Institute along the Guhagar-Chorochi and Kelsi-Loni profiles.Refraction studies of the records reveal a two-layered crust. The top layer consists 17 km of granite and the second layer 19 km of basalt, giving the average depth of the Moho as 36 km in the region. The velocities of the phases P_g, P* and P_n have been computed as 5.82 ± 0.01, 6.61 ± 0.05 and 8.23 ± 0.05 km/sec respectively and those of S_g, S* and S_n as 3.41 ± 0.00, 4.09 ± 0.07 and 4.60 ± 0.08 km/sec respectively. The shear wave velocity in the basement rock has been found to be lower in comparison with other regions of the peninsular India.In some cases reflections were recorded both from the Moho as well as from the intermediate layer. These reveal a crustal thickness of 39 km with 19 km of granitic and 20 km of basaltic layers.Coda signal durations from DSS explosions recorded by microearthquake seismographs indicate a lateral heterogeneity in the crust on either side of Karad in an east-west direction.     

老挝西北部琅勃拉邦构造带放射虫硅质岩及其构造意义

琅勃拉邦构造带内放射虫硅质岩含有放射虫Entactinia vulgaris Won, Entactinosphaera palimpola Foreman和Belowea variabilis (Ormiston et Lane),时代为早石炭世,为老挝境内首次报告。硅质岩样品具有很高的SiO2质量分数(95.29%~98.17%),大部分样品表现出相类似的稀土配分模式,部分样品配分模式图表现为上凸状,具有中稀土富集,均具有明显的Ce负异常,Ce/Ce*值为0.64~0.74,其中部分样品具有Eu的负异常,为0.58~0.68。Y/Ho比值为31.05~40.96,类似日本Sasayama中-晚二叠世的远洋硅质岩。地球化学显示其为含酸性火山碎屑非热液成因的远洋硅质岩。这些研究证实了在思茅板块和印支板块之间存在一个开阔的石炭纪时期弧后盆地。     

韩国太白山盆地古生界砂岩碎屑锆石U-Pb年代及其区域构造含义

韩国中东部的太白山盆地位于京畿、岭南两个构造单元之间。采用LA-ICP-MS方法,从太白山盆地下寒武统、中-上石炭统（下二叠统？）砂岩碎屑锆石中分别获得27个和47个U-Pb有效年龄,前者记录了1820~1945Ma、2172~2195Ma、2473~2593Ma等3期可信的构造热事件,相对概率峰值分别为1897Ma、2177Ma以及2528Ma;后者记录了288~340Ma、461~474Ma、1780~1892Ma、1941~1959Ma、2012~2050Ma等5期可信的构造热事件,其中相对概率峰值为305Ma和1867Ma的两组年龄比较集中。研究认为,太白山盆地与京畿、岭南两个地块均发育大约1850Ma的构造热事件,缺乏华南比较特征的新元古代构造热事件;进一步依据太白山盆地相应时段沉积古流向和古地理分布,说明太白山盆地至少与岭南地块之间存在密切的碎屑物源和构造属性关系,其构造热事件记录可与华北克拉通对比。另一方面,尽管中-上石炭统（下二叠统？）碎屑锆石记录了峰值为305Ma和468Ma可能发生在华北-朝鲜联合块体南缘的俯冲或构造热事件,但上述两个样品的碎屑锆石均缺乏新元古代构造热事件的年龄记录,所以亲华南的陆块至少在晚石炭世（早二叠世？）以前并未向太白山盆地提供碎屑物源。     

Paleomagnetic study of Cenozoic sediments from the Zaisan basin (SE Kazakhstan) and the Chuya depression (Siberian Altai): tectonic implications for central Asia

This paper presents new paleomagnetic results on Cenozoic rocks from northern central Asia. Eighteen sites were sampled in Pliocene to Miocene clays and sandy clays of the Zaisan basin (southeastern Kazakhstan) and 12 sites in the upper Oligocene to Pleistocene clays and sandstones of the Chuya depression (Siberian Altai).Thermal demagnetization of isothermal remanent magnetization (IRM) showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the Zaisan basin. Stepwise thermal demagnetization up to 640–660 °C isolated a characteristic (ChRM) component of either normal or reverse polarity at nine sites. At two other sites, the great circles convergence method yielded a definite direction. Measurements of the anisotropy of magnetic susceptibility showed that the hematite-bearing sediments preserved their depositional fabric. These results suggest a primary origin of the ChRM and were substantiated by positive fold and reversal tests. The mean paleomagnetic direction for the Zaisan basin (D=9°, I=59°, k=19, α₉₅=11°) is close to the expected direction derived from the APW path of Eurasia [J. Geophys. Res. 96 (1991) 4029] and shows that the basin did not rotated relative to stable Asia during the Tertiary.In the upper Pliocene–Pleistocene sandstones of the Chuya depression, a very stable ChRM carried by hematite was found. Its mean direction (D=9°, I=46°, k=25, α₉₅=7°) is characterized by declination close to the one excepted for early Quaternary, whereas inclination is lower. In the middle Miocene to lower Pliocene clays and sandstones, a stable ChRM of both normal and reverse polarities carried by magnetite was isolated. Its mean direction (D=332°, I=63°, k=31, α₉₅=4°) is deviated with respect to the reference direction and implies a Neogene, 39±8° counterclockwise rotation of the Chuya depression relative to stable Asia. These results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style, predominantly compressive in the Zaisan basin and transpressive in the Siberian Altai. At a larger scale, the pattern of vertical axis rotations deduced from paleomagnetic data in northern central Asia is consistent with the hypothesis of a large left-lateral shear zone running from the Pamirs to the Baikal. Heterogeneous rotations, however, indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations.     

华北盆地地震剖面地质解释及其构造演化

从地震剖面显示华北盆地发育为手风琴式演化史,大体上形成有三层“断-坳”结构：（l）中新元古界的“断”古生界—三叠系的“坳”;（2）侏罗系的“断”上白垩统的“坳”;（3）古近系的“断”新近系的“坳”。断陷与坳陷分别由拉张与挤压应力场所致,这种应力场的变化是由相邻洋壳板块俯冲倾角由小到大的变化所引起的。由于地壳的多旋回运动,多次发生构造沉积演变,构成了多套生储盖组合,多领域、多种圈闭类型的油气藏,从中新元古界—古生界—中生界—新生界,各断陷-坳陷结构的盆地都有可能形成油气藏的地质条件。     

Petrographic characteristics of the Proterozoic Vempalle carbonates,Cuddapah Basin,India and their implications

The Vempalle Formation of the Proterozoic Cuddapah basin has a well developed sequence of carbonate rocks, which are interbedded with shales, siltstones and chert. The stromatolitic carbonates are conspicuous at many places but the oolitic carbonates are less prominent and are present only in some areas. All the carbonates are pervasively dolomitized. Petrographic examination of these carbonates revealed that they are predominantly made up of fine grained micrite with patchy development of sparite and chert/quartz. The stromatolitic carbonates show distinct banding of alternate carbonate and cherty layers. The latter are rich in organic matter indicating prevalence of profuse biogenic activity. The oolitic carbonates comprise of ooids showing both concentric and radial patterns and made up of carbonate/chert and cemented by micro/mega quartz or carbonate itself. Diagenetic and post depositional features are reflected in cementation, recrystallization, compaction, stylolite formation and silicification processes. Various stages of cementing material are observed. Secondary vein fillings of carbonate or quartz traverse the carbonate/cherty groundmass. Intraclasts present suggest occasional erosional destruction of associated sediments, short lived transport and local redeposition. Accessory silicate minerals represent terrigenous influx during deposition. Dolomitization of the carbonates was fabric retentive and early diagenetic. The environmental conditions were characterised by low energy, within a shallow water zone, in occasional higher energy events and turbulence. The carbonates appear to have been deposited on a shallow water ramp within a tidal regime.     

柴北缘达达肯乌拉山地区闪长岩锆石LA-ICP-MSU-Pb年龄、岩石地球化学特征及其构造意义

达达肯乌拉山位于柴达木盆地北缘中东部,区内发育闪长岩体,以小岩株形式产出。通过对达达肯乌拉山闪长岩岩石LA-ICP-MS锆石U-Pb定年和地球化学特征的研究,获得闪长岩体的年龄为(240.5±1.7) Ma,属早印支期。闪长岩体的w(K_2O)较高,为2.03%～2.30%,w(Na2O)含量较低,为3.2%～3.44%,为一套准铝质及弱过铝质高K、低Na的高钾钙碱性型I型花岗岩。岩石稀土元素总量为158.35×10~(-6)～200.41×10~(-6),球粒陨石标准化曲线向右倾斜,重稀土元素曲线呈水平状,轻稀土元素富集,但负铕异常不明显,岩浆分异程度不高。岩石的La/Ta(54.32～68.51)、Sm/Nd(0.18～0.19)及Rb/Sr(0.11～0.19)比值特征反映该岩体岩浆具有壳幔混合的特点。通过构造判别,反映达达肯乌拉山岩体可能形成于俯冲陆壳断离、幔源岩浆底侵的地球动力学背景,在中央造山带早中生代统一的板块碰撞与挤压构造体制下,以滩间山蛇绿岩带为基础,发生陆壳俯冲和断离作用,并诱发幔源岩浆的底侵和下地壳物质的部分熔融,沿火山机构侵位形成达达肯乌拉山岩体。     

青海共和盆地周缘印支期花岗岩类的成因及其构造意义

本文对青海共和盆地周缘印支期黑马河岩体、温泉岩体、大河坝岩体和同仁岩体花岗闪长岩进行了主量元素、微量元素和Pb-Sr-Nd同位素地球化学研究,并对黑马河岩体和温泉岩体进行锆石U-Pb LA-ICP-MS年代学研究.结果表明,黑马河岩体的岩浆结晶年龄为235±2 Ma,属印支早期,而温泉岩体的岩浆结晶年龄为218±2Ma,属印支晚期.这些印支期花岗闪长岩的SiO2=63.34～68.06%,K2O/Na2O=0.82～1.36,岩石均为准铝质(A/CNK=0.9～1.0),并属中钾到高钾钙碱性岩系.它们总体上具有相似的微量元素组成特征,并有着极为相似的稀土元素组成模式,(La/Yb)N值主要介于10～15之间,存在微弱到中等程度的负Eu异常(Eu/Eu*=0.5～0.8).岩石初始Sr同位素比值ISr=0.70701-0.70952,εNd(t)=-3.8到-8.4,指示它们的岩浆物质主要来自于地壳物质的部分熔融.这些岩石以高放射成因Pb同位素组成为特征,其全岩初始Pb同位素比值为(206 Pb/204 Pb)t=18.068～18.748、(207 Pb/204 Pb)t=15.591～15.649、(208 Pb/204 Pb)t=38.167～38.554.地球化学特征指示共和盆地周缘印支期花岗岩类的原岩为下地壳变玄武岩类,并且这类原岩可能派生于元古宙富集地幔,但在不同区段,下地壳变玄武岩类存在着一定程度化学组成的不均一性.根据花岗岩类对深部地壳物质的地球化学示踪及其区域对比,共和盆地周缘的西秦岭、柴达木(包括东昆仑)和欧龙布鲁克块体具有统一的地壳基底组成,并具有扬子型块体的构造属性.结合区域地质背景的分析,共和盆地周缘印支早期花岗岩类(以黑马河岩体为代表)可能形成于俯冲陆壳断离的地球动力学背景,而印支晚期花岗岩类(以温泉岩体为代表)形成于中央造山带在地壳加厚作用后岩石圈拆沉作用的地球动力学背景.     

云岗盆地晚中生代沉积-火山充填序列及其构造意义

利用凝灰岩夹层及安山岩的锆石U-Pb同位素测年,对云岗盆地晚中生代地层充填序列的关键时限进行了限定。结合地层接触关系、安山岩的地球化学特征,分析了重点地质界面的构造意义。取得了以下主要认识:(1)早中侏罗世,盆地为一套湖进序列的连续沉积,以大同组含煤地层为代表,暗示着区域拉张的构造背景;(2)盆地整体缺失了晚侏罗世(160.4±1.1)Ma至早白垩世(130.1±0.7)Ma的沉积地层,其发生的动力学背景可能与古太平洋、西伯利亚、特提斯同时向东亚大陆汇聚产生的远程效应有关;(3)旧高山安山岩形成于早白垩世(130.1±0.7)Ma大陆板内裂谷的构造背景,与华北克拉通破坏的动力学背景紧密相关。研究成果对旧高山安山岩时代归属提出了新的认识,并不归属于上侏罗统髫髻山组,而相当于下白垩统张家口组。     

皖南屯溪盆地火山岩的地球化学、年代学及其构造意义

安徽南部侏罗至白垩纪屯溪盆地位于NE向中下扬子火山岩带和NNE向中国东南部火山岩带的交接部位,其火山岩的年代学和成因研究可以提供华南晚中生代构造演化的信息。4个火山岩样品的LA-ICP-MS锆石U-Pb年龄及其岩石地球化学数据指示屯溪盆地发育两期火山岩:早期火山岩形成于156~152 Ma,为中、高钾钙碱性系列,稀土元素分布型式为轻稀土元素富集的右倾型,铕负异常不明显,富集轻稀土元素和大离子亲石元素,亏损高场强元素Nb、Ti、P;晚期火山岩形成于136~130 Ma,具有双峰式火山岩特征,其酸性组分属中钾钙碱性系列,稀土元素分布型式为右倾斜的"V"型,铕负异常明显,同样具有富集轻稀土元素、大离子亲石元素及亏损高场强元素Nb、Ti、P的特征。皖南屯溪盆地晚侏罗世-早白垩世火山活动类似于中国东南部火山岩带。结合前人资料,认为屯溪盆地晚侏罗世火山岩形成于挤压构造环境,而早白垩世火山岩形成于伸展环境,其成因与晚侏罗至早白垩世古太平洋板块俯冲角度变化和俯冲板片后撤作用有关。     

松辽盆地火山岩的同位素年代、地球化学特征及意义

沉积盆地内部的火山岩是研究盆地形成的构造环境以及探讨盆地发展演化的动力学特征的重要线索。松辽盆地具有二元结构特征，断陷构造层发育大量的火山岩。但是，前人尚未对比进行深入系统的研究。根据采自钻井岩芯的火山岩样品，通过同位素测年（K／Ar)和地球化学分析等结果，讨论了松辽盆地晚中生代火山岩的岩石学、岩石化学和地球化学特征，探讨了盆地发育早期的构造环境及其地球动力学背景。指出松辽盆地在其形成早期（即断陷期）是一个发育陆缘岩浆弧构造背景上的走滑拉张盆地，火山岩岩浆源区具有多成分、复合性的演化特点。     

Contrasting sapphirine parageneses from Wilson Lake, Labrador and their tectonic implications

Abstract Regionally distributed pelitic granulites in the Wilson Lake region contain the assemblage sapphirine + hypersthene + sillimanite + quartz. Geochronology and geobarometry suggest it developed in early Proterozoic rocks at temperatures approaching 900°C and pressures above 10 kbar. Vein-like metasomatized rocks around a suite of mafic to ultramafic intrusions, emplaced near the peak of metamorphism about 1700 Ma ago, contain sapphirine, but these assemblages developed at temperatures near 750°C and pressures of 4.5 kbar. Both types of assemblage occur as relics in amphibolite-grade (biotite–sillimanite) migmatites.
P–T determinations indicate rapid isothermal uplift of 20 km accompanied by mafic intrusion and hydration. The metamorphic history and tectonic setting suggest exposure of deep continental crust by thrusting during continental collision, followed by essentially isothermal decompression.     

塔里木二叠纪石英正长斑岩岩墙的发现及其构造意义

新疆塔里木盆地内部发育了大量二叠纪(260～292 Ma)玄武岩类为主的岩浆岩,主要由玄武岩、辉绿岩、玄武安山岩、超镁铁质岩石和正长岩等组成。作者野外地质调查过程中在巴楚水工团附近发现石英正长斑岩岩墙的出现。这些石英正长斑岩属准铝质(A/CNK＜1),其中SiO_2含量为66～67%,并以富K_2O+Na_2O(10～11%)、K_2O/Na_2O(0．8～0．9)和较低的Mg/(Mg+Fe)比值为特征,且具有富集大离子亲石元素(Ba、Rb)和高场强元素含量(Zr、Nb、Y)、Ga/Al比值、高的稀土总量(631～734×10~(-6))、高度富集的LREE/HREE比值以及具Eu的负异常。这些特征并结合构造判别图解显示其具有典型的A型花岗岩的特征。低的Y/Nb(0．4)＜1．2以及微量元素蜘蛛网图中呈现大离子亲石元素富集以及Nb平坦或稍凸的图谱指示其源区来自于地幔,且在典型板内环境下形成。与小海子水库出露的正长岩的成分特征比较接近并很可能具有相同的源区成分。笔者对小海子正长岩体中粗粒正长岩采用精确的SHRIMP锆石U-Pb同位素定年,获得可靠的一致性年龄值为277±4 Ma,暗示小海子正长岩体侵位时代为277 Ma。根据野外观察、地球化学特征和构造环境的初步研究和对比,石英正长斑岩岩墙与小海子正长岩体一起很可能都是在早二叠世晚期形成,约在277Ma左右,它们代表了二叠纪陆内稳定环境下形成的产物,并可能作为塔里木地区最后一次大的岩浆热事件结束的标志。     

渤海湾新生代盆地的两种构造系统及其成因解释

渤海湾古近纪盆地可以划分为3个裂陷带和1个裂陷区，都分布在上地幔隆起部位。盆地构造变形可以分为伸展构造和走滑构造两个相对独立、相互关联的新生代构造系统。伸展卡句造由不同尺度的伸展断层和与伸展断层垂直或斜交的变换断层构成连锁断层系统，在盆地区具有分散的透人性特点，并控制着古近纪断陷的分布和演化。在伸展构造变形基础上叠加了3条北北东向—北东向右旋走滑断裂(带)，后者及其伴生构造组成盆地中的呈带状展布的新生代走滑构造系统。伸展构造是一种“水平层状的”薄皮构造。正断层向深部收敛或终止于中地壳内的拆离断层面上。走滑构造是一种“垂直带状的”厚皮构造。浅层的走滑断层以多种方式并入到深断裂带中。这两种构造系统是盆地区新生代时期主动裂陷和被动裂陷两种作用机制的具体表现。     

七宝山早白垩世火山岩地球化学特征及其构造意义

七宝山晚中生代火山岩位于郯庐断裂带山东段管帅盆地东缘,出露于青山群八亩地组中.本文通过对火山岩样品TYS48和TYS52中锆石LA-ICP MS U-Pb测年,获得其年龄分别为120.1±0.6 Ma和118.5±1.1 Ma,显示该区火山岩为早白垩世岩浆活动的产物.研究区火山岩SiO2含量为56.02％～66.45％...     

七宝山早白垩世火山岩地球化学特征及其构造意义

七宝山晚中生代火山岩位于郯庐断裂带山东段管帅盆地东缘,出露于青山群八亩地组中。本文通过对火山岩样品TYS48和TYS52中锆石LA-ICP MS U-Pb测年,获得其年龄分别为120.1±0.6Ma和118.5±1.1 Ma,显示该区火山岩为早白垩世岩浆活动的产物。研究区火山岩SiO2含量为56.02％～66.45％,富钠、富碱、偏碱性（σ平均为4.01）,为一套高钾钙碱性系列的火山岩。与区内埃达克质岩相比,除TYS49样品外,其余样品均具有较低的MgO含量为（0.52％～2.02％）和Mg#（16～42）。地球化学特征显示,火山岩富集Rb、Ba、Sr等大离子亲石元素（LILE）和轻稀土元素（LREE）,相对亏损Nb、Ta、Ti等高场强元素（HFSE）和重稀土元素,具有弱的Eu负异常（δEu平均为0.85）; 具有富集的Sr-Nd-Pb同位素组成：87Sr/86Sr(t)=0.7084～0.7098,εNd(t)=-20.4～-16.2,206Pb/204Pb(t)=16.44～17.19,207Pb/204Pb(t)=15.39～15.47, 208Pb/204Pb(t)=36.82～37.65。研究表明,在郯庐断裂带伸展活动和岩石圈减薄背景之下,软流圈物质上涌使得地温升高而造成下地壳部分熔融,熔融的下地壳与岩石圈地幔相互作用形成了七宝山火山岩。断裂带的存在与活动,对华北克拉通可能具有破坏作用,使得岩石圈的稳定性和整体性遭到破坏,从而成为华北岩石圈减薄中的有利位置。     

碧口群火山旋回及其地质构造意义

通过详细的野外剖面观察和室内岩石地球化学研究,将碧口碧火山岩划分为3个火山旋回,每个旋回下部为基性火山岩（细碧岩及细碧质凝灰岩）,上部为酸性火山岩（石英角斑岩或石英角斑质凝灰岩）。基性火山岩富集LREE及LIL。第一旋回基性火山岩属碱性系列,第二、三旋回基性火山岩属拉斑玄武岩系列。基性火山岩浆形成于与洋岛玄武岩类似的地幔柱源,其中第三旋回基性火山岩浆晚期受岩石圈地幔混染;酸性火山岩为地壳重熔作用的产物。碧口群火山岩属大陆裂谷双峰式火山岩系,是中一新元古代扬子地块北缘大陆拉张作用的产物。这种拉张作用是秦岭造山带从震旦纪开始的大规模扩张作用的先兆。