扬子北缘下高川盆地上泥盆统铁矿梁组沉积物源分析

2022年6月5日09:18:19扬子北缘下高川盆地上泥盆统铁矿梁组沉积物源分析已关闭评论
摘要

贾晓彤+张英利+王宗起+王坤明+王刚 摘要:下高川盆地為扬子地块和秦岭造山带结合处的特殊构造单元,其构造演化与相邻的构造单元存在明显差异,晚泥盆世尤为突出。为全面理解下高川盆地的构造演化,重点研究了下高川盆地上泥盆统铁矿梁组砂岩的沉积物源。以石英砂岩为研究对象,分析重矿物组成,进而进行电气石电子探针

贾晓彤+张英利+王宗起+王坤明+王刚



摘要:下高川盆地為扬子地块和秦岭造山带结合处的特殊构造单元,其构造演化与相邻的构造单元存在明显差异,晚泥盆世尤为突出。为全面理解下高川盆地的构造演化,重点研究了下高川盆地上泥盆统铁矿梁组砂岩的沉积物源。以石英砂岩为研究对象,分析重矿物组成,进而进行电气石电子探针分析和LAMCICPMS锆石UPb定年。砂岩样品的闪锌矿、白钛石、钛铁矿重矿物组合说明其物源为岩浆岩。电气石电子探针分析结果表明,铁矿梁组石英砂岩主要来自于变质板岩和变质砂岩,少量为贫锂花岗岩类、伟晶岩和细晶岩。砂岩锆石UPb年龄谱分析表明,铁矿梁组砂岩的主要物源来自425~530 Ma的天水和丹凤花岗岩、佛坪片麻岩、清水英安岩,以及578~982 Ma的柞水(黑沟)碱性花岗岩、天水和西峡的花岗质片麻岩、丹凤和卢氏的花岗岩。另外,1 015~1 551 Ma的佛坪黑云母斜长片麻岩,1 658~1 957 Ma的太白和宝鸡花岗岩、佛坪片麻岩,2 329~2 502 Ma的丹凤石榴黑云斜长片麻岩和2 625~3 147 Ma的商南太古宙结晶基底岩石均经历多次搬运,为扬子北缘下高川盆地提供次要物源。综合区域资料,晚泥盆世的物源大致来自北部,同时古地理格局表现为天水、丹凤、清水、西峡等地区岩体隆升剥蚀,成为物源区,而在华北古陆与扬子北缘下高川盆地之间可能存在多个较小古陆,如佛坪也属于物源区。
关键词:沉积物源;铁矿梁组;晚泥盆世;重矿物;电气石电子探针;锆石UPb年龄;下高川盆地;扬子北缘
中图分类号:P512.2文献标志码:A
Abstract: Xiagaochuan Basin is a special tectonic unit between Yangtze Block and Qinling orogen. The tectonic evolution of Late Devonian is obviously different from that of the adjacent tectonic units. In order to understand the tectonic evolution of Xiagaochuan Basin, the sediment provenance of sandstone of Upper Devonian Tiekuangliang Formation was studied. Taking quartz sandstone as the research object, heavy mineral assemblages were analyzed, and then the tourmaline electron microprobe analysis and LAMCICPMS zircon UPb dating were determined, respectively. The heavy mineral assemblages of sphalerite, leucite and ilmenite indicate that they can be provided by magmatic rocks. The tourmaline electron microprobe analysis shows that the sandstone from Tiekuangliang Formation is mainly from metapelites and metapsammites, and a small amount of Lipoor granitoids and associated pegmatites and aplites. The zircon UPb age spectra of sandstone suggest that the provenance of sandstone from 〖JP+2〗Tiekuangliang Formation mainly comes from the rocks with the ages of 425530 Ma and 578982 Ma. The 425530 Ma rocks are Tianshui and Danfeng granites, Foping gneiss and Qingshui dacite; the 578982 Ma rocks are Zhashui (Heigou) alkaline granite, Tianshui and Xixia gneisses, and Danfeng and Lushi granites. Furthermore, the rocks including 1 0151 551 Ma Foping biotite plagioclase gneiss, 1 6581 957 Ma Taibai and Baoji granites, and Foping gneiss, 2 3292 502 Ma Danfeng garnet gneiss, and 2 6253 147 Ma Shangnan Archean crystalline basement, have undergone several transportation and provided secondary provenance from Xiagaochuan Basin, the northern margin of Yangtze Plate. Based on the regional data, the Late Devonian provenance is roughly from the north. Meantime, the paleogeographic pattern shows that the rocks in Tianshui, Danfeng, Qingshui and Xixia areas are uplifted and eroded, becoming the provenance areas. Moreover, there are many small oldlands between North China paleoland and Xiagaochuan Basin, the northern margin of Yangtze Plate, such as Foping, which is also the provenance area.
Key words: sediment provenance; Tiekuangliang Formation; Late Devonian; heavy mineral; tourmaline electron microprobe; zircon UPb age; Xiagaochuan Basin; the northern margin of Yangtze Plate
0引言
下高川盆地位于扬子地块和秦岭造山带结合处,地层分区上属于褚河地层小区。其独特的构造位置成为研究秦岭造山带和扬子地块演化的理想场所。桑宝樑等对下高川地区进行了大量研究,建立了研究区地层格架[15]。与周边的构造单元相比较,下高川盆地有着独特的沉积序列,发育震旦系、寒武系、上泥盆统—下三叠统地层,缺失奥陶系、志留系和中—下泥盆统。李瑞保等认为其早期的构造演化和勉略带大致相似,是晚古生代勉略裂谷的一部分,从勉略带上挤入到大巴山冲断推覆构造带与大巴山前陆构造带之间,并被挟持在大巴山构造带浅部[68]。梅志超等认为泥盆纪时期南秦岭向北俯冲的同时,地壳首先与南侧的勉县—巴山断裂带沿线发生扩张,演化成有限洋盆,与扬子地块分离,逐渐演化成裂陷盆地与断块隆起相间的格局[9]。沉积学研究表明,南秦岭泥盆系是被古陆分隔的几个裂陷盆地的沉积产物,有南北成带、东西分隔的趋势[1011]。下高川盆地作为晚古生代秦岭南缘裂谷[12]或新生坳拉槽的一部分[13],只保留了沉积序列,晚泥盆世总体表现为缓坡型碳酸盐台地体系[12]。因此,对于下高川盆地的演化仍然存在着争议。
沉积物源分析(包括碎屑锆石UPb定年和电子探针分析等)可以确定沉积物源区岩石年龄、性质等[1418],反演物源区的剥露过程[13,19],为构造演化提供进一步证据。本文以下高川盆地上泥盆统铁矿梁组(D3t)砂岩沉积物源为研究对象,分析重矿物,进而对电气石、碎屑锆石进行电子探针和LAICPMS测年分析;根据野外数据和获取的测试结果,综合分析下高川盆地晚泥盆世沉积物的源区母岩性质,结合区域地质特征及前人研究成果,确定其源区特征,为下高川盆地的构造演化提供依据。
1区域地质背景
下高川盆地位于扬子地块和秦岭造山带结合处,发育两条深大断裂,即大巴山断裂和红椿坝断裂(图1)。大巴山断裂主要由星子山—断头崖断裂、大市川—断头崖断裂和大市川—麻柳坝断裂组成,大巴山断裂所夹持的地区即为下高川盆地,星子山—断头崖断裂以西为南大巴山地区,大巴山断裂以东为北大巴山地区,而北大巴山地区又被红椿坝断裂所分隔[2224]。研究区在地层分区上属于褚河小区,发育地层包括震旦系、寒武系、上泥盆统、石炭系、二叠系和中—下三叠统,其中震旦系以灰岩和含碳质泥质页岩为主。寒武系主要为灰岩及白云岩,寒武系与上泥盆统呈平行不整合接触,上泥盆统与上覆石炭系为整合接触。上泥盆统铁矿梁组主要为灰褐色石英砂岩夹灰黑色薄层钙质泥岩;蟠龙山组(D3p)为深灰及灰色的薄、厚层状生物碎屑灰岩。石炭系灰岩最发育,二叠系为灰黑色粉砂岩、泥岩,中—下三叠统发育灰黄色及浅灰色白云质灰岩、灰岩夹泥岩,与下伏二叠系为断层或平行不整合接触。北大巴山地区发育早古生代地层以及基性侵入岩;南大巴山地区出露二叠纪—早三叠世灰岩。
2沉积学特征
本次工作对下高川盆地镇巴地区火焰溪剖面上泥盆统铁矿梁组进行详细观测(图2)。结果表明:铁矿梁组与上覆蟠龙山组灰岩呈整合接触,与下伏寒武纪白云岩呈平行不整合接触;铁矿梁组主要为灰褐色石英砂岩夹灰黑色薄层钙质泥岩;泥岩中发育黄铁矿结核,砂岩中可见泥岩团块和植物碎屑。显微镜下,铁矿梁组砂岩主要由石英组成,长石和岩屑含量较少,分选较好,磨圆一般,为次棱角—次圆状,砂岩为颗粒支撑结构,杂基含量较低,说明沉积物经过了较长距离的搬运和筛选。黄铁矿结核的出现表明其处于还原环境。综合以上特征认为,铁矿梁组沉积环境为滨岸相。
3分析方法
样品12HYX11采自位于陕西省镇巴县兴隆场东的火焰溪剖面上泥盆统铁矿梁组,采样坐标为(32°35′16″N,108°03′20″E)(图1),剖面内采样位置见图2。样品12HYX11为石英砂岩,质量约8 kg。
3.1重矿物分析
重砂鉴定分析在河北省区域地质矿产调查研究所实验室完成。首先取样品10 kg进行碎样,筛分之后分别进行淘洗、磁选、电磁选等,计算各种重矿物含量(质量分数,下同)。挑选重矿物中的电气石和锆石,以便进行电子探针分析和LAMCICPMS锆石UPb定年测试分析。
3.2电气石电子探针分析
电子探针分析在中国地质大学(北京)电子探针实验室完成,仪器型号为日本岛津公司生产的EPMA1600。测試条件包括:加速电压为15 kV,激发电流为10 nA,电子束直径为1 μm。测试结果采用ZAF法修正。分析标样采用磁铁矿(Fe)、钠长石(Si、Na、Al)、磷灰石(Ca、P)、金红石(Ti)、蔷薇辉石(Mn)、透长石(K)、橄榄石(Mg)、萤石(F)等。主元素(含量高于20%)允许的相对误差不高于5%;含量在3%~20%之间的元素允许相对误差不高于10%;含量在1%~3%之间的元素允许的相对误差不高于30%;而含量在05%~10%之间的元素允许的相对误差低于50%。基于31个氧原子[2526],采用Excel经验公式[27]对电子探针分析数据进行处理。
3.3碎屑锆石UPb定年
样品碎样和锆石的挑选工作在河北省区域地质矿产调查研究所实验室完成。锆石样品的制靶工作由中国地质科学院地质研究所大陆构造与动力学国家重点实验室完成。锆石的阴极发光(CL)图像在中国地质科学院地质研究所HITACHI S3000N型扫描电子显微镜及GATAN公司Chroma阴极发光探头分析仪器上完成的。锆石UPb年龄测定之前,依据透射光图像、反射光图像和阴极发光图像,对碎屑锆石样品随机圈定裂隙和包裹体不发育的颗粒。LAMCICPMS锆石UPb定年在中国地质调查局天津地质调查中心同位素实验室完成,详细试验过程参见文献[28]和[29]。采用GJ1作为外部锆石年龄标准进行U、Pb同位素分馏校正[30]。利用NIST612玻璃标样作为外标计算锆石样品的Pb、U、Th含量。数据处理采用ICPMSDataCal程序[31],普通Pb校正采用Anderson方法[32],锆石年龄谐和图由Isoplot 3.0程序完成[33]。
对于锆石年龄大于1 000 Ma的数据,采用N(207Pb)/N(206Pb)年龄,而对于年龄小于1 000 Ma的数据,采用n(206Pb)/n(238U)年龄[3435]。以n(206Pb)/n(238U)年龄和N(207Pb)/N(206Pb)年龄的比值作为标准遴选UPb年龄数据[14,34,3638],不谐和度绝对值不高于10%的数据为有效数据。
4结果分析
4.1重矿物组分
铁矿梁组砂岩识别的重矿物主要有锆石、金红石、黄铁矿、电气石、闪锌矿、白钛石、钛铁矿等(表1)。主要重矿物为黄铁矿和锆石,含量分别为5672%和2615%,金红石和电气石次之。显微镜下,粉黄色锆石较少,约占锆石总量的35%,呈半自形,搬运痕迹不太明显,推测距母岩区较近;另外〖CM(22〗一种锆石是褐玫瑰色,约占锆石总量的65%,磨圆度较高,分选性较好,推测锆石经一定距离搬运。重矿物分析中,由于在还原作用下可形成自生黄铁矿,所以黄铁矿体积分数对于物源分析没有指示意义。金红石和电气石有多种来源,故矿物本身不能确定母岩类型。其余矿物组合(如闪锌矿、白钛石、钛铁矿)可指示物源来自岩浆岩。
4.2电气石电子探针分析
样品12HYX11电气石背散射图像中无明显的〖LL〗核边结构,成分比较均一(图3),而且电子探针数据也表明,电气石的核部成分与其他部位无明显差异(图3中6、7、8),很少受外部侵蚀。电气石颗粒呈棱角—次圆状,磨圆由较差到较好,但次棱—次圆居多,说明物源未经搬运或者经过短距离搬运。
电子探针分析结果显示(表2),电气石成分中除SiO2之外,Al2O3[KG-30x]占的比例较大,MgO和FeO依〖CM(22〗次减小,而且FeO、Al2O3[KG-30x]含量与SiO2含量呈负相关关系,而MgO含量与SiO2[KG-20x]含量呈正相关关系(相关系数为0.6)。在AlFeMg三元图解[图4(a)]中,电气石物源来自变质板岩和变质砂岩,少量来自贫锂花岗岩类及其伴生的伟晶岩和细晶岩,以及富铁电气石石英岩、钙质硅酸盐岩和变质板岩;在CaFeMg三元图解[图4(b)]中,电气物源石主要源自贫钙变质板岩、变质砂岩和电气石石英岩,少量来自贫锂花岗岩类及其伴生的伟晶岩和细晶岩。因此,综合AlFeMg、CaFeMg三元图解(图4),笔者认为铁矿梁组石英砂岩主要来自于变质板岩和变质砂岩,少量为贫锂花岗岩类、伟晶岩和细晶岩。
4.3碎屑锆石UPb年龄
〖HJ51x〗对砂岩样品12HYX11进行LAMCICPMS锆石UPb定年,共分析碎屑锆石颗粒109个,获得有效年龄108个(表3)。其UPb谐和曲线及UPb年龄谱图见图5。锆石年龄分布于425~3 142 Ma,集中分布于425~530、578~982 Ma,峰值较明显,而1 658~1 957、2 329~2 502、1 015~1 551、2 625~3 147 Ma区间相对分散,没有明显的峰值。主要年龄峰值与区域上主要构造事件大致对应:425~530 Ma记录了早古生代晚加里东期—早海西期北秦岭发生较大规模的造山运动[9,4143];而578~982 Ma记录了新元古代早期秦岭造山带发生洋壳俯冲、陆陆碰撞事件造山作用,以及之后秦岭造山带中秦岭与扬子克拉通北缘发生了大规模裂解[4448]。
大部分锆石磨圆较好,形态不完整,是典型的碎屑锆石(图6),经历了多次搬运,直接来自于沉积岩源区。而部分磨圆较差、较完整的岩浆锆石和变质锆石可能直接来自于岩浆岩和变质岩源区。另外,大多数碎屑鋯石具有规律的振荡环带,w(Th)/w(U)值大于04,而且Th、U含量高(图6中25、82、88、91、92等),指示其属于岩浆成因;还有部分碎屑锆石明显具有变质特征(如出现明显的白色变质边)(图6中53、70、75),属于变质成因。
5讨论
〖BT2#〗5.1沉积物源
扬子北缘下高川盆地所处的秦巴地区由于早古生代晚期的南北地块逐渐拼合,晚古生代时结束了活动的地质发展阶段,进入相对稳定期,从早泥盆世开始,缓慢海侵,直到晚泥盆世,海水逐渐变通畅,水〖CM(22〗域扩大[4849]。岩相古地理资料表明,巴山弧形断裂对其到商丹缝合带之间的南秦岭陆缘弧后裂谷盆地的发育影响明显,导致靠近断裂一侧为盆地中心,古地形上北高南低,并使盆地呈南倾斜的不对称箕状[50]。而且,秦岭总体呈北高南低的古地形特征,海侵总体呈SW—NE向[51]。高川地区仅发育上泥盆统地层,上部蟠龙山组地层厚度大于下部的铁矿梁组滨岸相地层。因此,下高川盆地虽然形成较晚,但是充填序列仍显示向上变细变深[9],甚至铁矿梁组的沉积厚度自北而南逐渐变薄[5]。
在泥盆纪时期,秦巴地区的沉积中心不断迁移,晚泥盆世海侵范围达到最大,北达天水、太白、周至、商县的南侧,南部东段已到西乡—镇巴—旬阳,佛坪地区成为被海水包围的佛坪岛,西峡位于秦巴地区的东侧,属华北古陆。由于南部的扬子古陆以及若尔盖古陆地形高差不大,碎屑物供给极少,主要陆屑供给仍是北部的华北古陆[52]。因此,综合区域资料,笔者认为晚泥盆世的物源大致来自北部。
5.2锆石UPb年龄谱
源自425~530 Ma的锆石颗粒约占总数的15%。大部分锆石颗粒自形程度高,具有明显的振荡环带(图6中56、82、88、91、92),w(Th)/w(U)值大于04,而且Th、U含量高,指示其属于岩浆成因。已有的研究资料显示:北大巴山地区基性岩墙的锆石年龄为400~450 Ma[5358];天水二长花岗岩的LAICPMS锆石UPb年龄为(438±3)Ma[59];清水新城英安岩的SHRIMP锆石UPb年龄为(447±8)Ma[60];丹凤奥长花岗岩的单颗粒锆石PbPb年龄为(490±10)Ma[61];秦岭佛坪变质结晶岩系龙草坪黑云斜长片麻岩SHRIMP锆石UPb年龄为430~510 Ma[62]。上述岩石年龄与本文中处于425~530 Ma的碎屑锆石年龄接近,而且与新元古代北秦岭向华北陆块聚合时间(423~470 Ma)、沿商丹缝合带发育的俯冲作用时间(422~514 Ma)一致[63],因此,该期间产出的岩体为铁矿梁组提供物源。重矿物分析中缺少基性岩的指相矿物(如辉石或者尖晶石等),因此,北大巴山的基性岩墙为铁矿梁组提供物源的可能性极小。电气石电子探针分析结果显示,铁矿梁组主要来自于变质板岩和变质砂岩,少量物源为贫锂花岗岩类、伟晶岩和细晶岩。另外,锆石基本呈棱角状,说明搬运距离较短。因此,铁矿梁组的沉积物可能直接来自天水和丹凤的花岗岩、佛坪的片麻岩和清水的英安岩。
源自578~982 Ma的碎屑锆石颗粒为48个,占总碎屑颗粒的4486%。大部分锆石呈自形;锆石阴极发光图像(图6中37、75、90)显示,岩浆环带明显,Th、U含量较高,w(Th)/w(U)值大于04,指示其为岩浆成因。少部分锆石(图6中53、70、75)为变质成因,可见白色变质边。结合区域岩体的年龄分布,本次研究的锆石年龄多与卢氏牛角山岩体的S型花岗岩体和花岗岩脉(SHRIMP锆石UPb年龄为(955±13)、(929±25)Ma[64])、丹凤石槽沟花岗岩(LAICPMS锆石UPb年龄为(925±11)Ma[65])、天水元龙花岗质片麻岩(SHRIMP锆石UPb年龄为(924.2±2.7)、(914.7±7.6)Ma[66])、天水新阳花岗质片麻岩(SHRIMP锆石UPb年龄为(9785±4.8)Ma[67])、西峡德河黑云母二长花岗片麻岩(SHRIMP锆石UPb年龄为(943±18)Ma[68])一致。对应的锆石形态表明其多为圆—次圆状(图6中37、84、95),说明源岩经历多次搬运到达研究区。而少量棱角状(图6中45)锆石颗粒经过较短距离到达铁矿梁组,与来自柞水(黑沟)的碱性花岗岩年龄(SHRIMP锆石UPb年龄为(686±10)Ma[69])一致。由岩相古地理资料可知,晚泥盆世的柞水地区由于海水进一步入侵成为沉降区[52],所以柞水(黑沟)碱性花岗岩成为物源区的可能性较小。综合电气石电子探针分析结果、区域岩体的年龄分布以及锆石形态,天水和西峡的花岗片麻岩以及丹凤和卢氏的花岗岩经历多次搬运,为铁矿梁组提供物源。
源自1 015~1 551 Ma的锆石约占碎屑锆石总数的14%,具有典型的环带结构,部分锆石属于面状结构,w(Th)/w(U)值为014~146,指示其为典型的岩浆锆石。新元古代同造山期的花岗岩侵入体集中分布于秦岭造山带的“北秦岭变质体”之中,几乎都经历了变质作用,并形成片麻岩[46]。1 015~1 551 Ma期间产出的火山沉积浅变质岩系属于过渡性基底,出露广泛[70]。区域岩石年龄数据显示,秦岭佛坪变质结晶岩系龙草坪黑云斜长片麻岩SHRIMP锆石UPb年龄为1 033~1 483 Ma[62],而年龄为(1 356±30)Ma的锆石颗粒(图6中26)呈次圆状,与佛坪黑云母角闪斜长片麻岩(原岩为中酸性岩浆岩)的锆石年龄(SHRIMP锆石UPb年龄为1 371 Ma[62])及特征一致,因此,推测源岩先期变质形成佛坪黑云母片麻岩,之后经历风化剥蚀和多次搬运,沉积于铁矿梁组。
1 658~1 957 Ma的锆石约占碎屑锆石总数的11%,多为长柱状,具有韵律环带,部分环带较弱且多数锆石w(Th)/w(U)值大于04,指示其为岩浆成因。而1 658~1 957 Ma的锆石对应的岩浆侵入作用恰好与吕梁运动及Columbia超大陆的形成时代[44,46,71]相当。已有的年龄数据显示:北秦岭太白的二长花岗岩LAICPMS锆石UPb年龄为(1 741±41)Ma[72];宝鸡的二长花岗岩LAICPMS锆石UPb年龄为(1 770±41)Ma[73];秦岭佛坪变质结晶岩片麻岩锆石年龄为1 853~1 944 Ma[62,74]。锆石形态多呈圆—次圆狀(图6中30),说明源岩经历了多次搬运。综合电气石电子探针分析结果,太白和宝鸡的花岗岩以及佛坪片麻岩多次搬运沉积在铁矿梁组,因此,铁矿梁组的沉积物可间接来源于太白、宝鸡的花岗岩和佛坪片麻岩。
2 329~2 502 Ma的锆石约占碎屑锆石总数的10%,但年龄比较分散,峰值不明显。锆石多磨圆较好,不完整;大部分锆石有规律的环带,w(Th)/w(U)值绝大部分大于04,指示其属于岩浆成因。已有的研究区及周边地区岩体的年龄很少,仅有丹凤石榴黑云斜长片麻岩(原岩为碎屑岩)的部分锆石年龄(LAICPMS锆石UPb年龄为951~2 472 Ma[75])与本次研究获得的锆石年龄(图6中48)一致。而电子探针分析得到铁矿梁组的原岩主要为变质板岩和变质砂岩,因此,推测丹凤地区陆源碎屑岩先期变质形成片麻岩,之后经过搬运沉积在铁矿梁组砂岩中。
2 625~3 147 Ma的锆石颗粒有5个,仅占碎屑锆石总数的6%。锆石岩浆环带明显,磨圆极好,是典型的岩浆锆石。研究区及周边地区岩体关于此时期的年龄报道很少。张宗清等测定陕西省商南县秦岭造山带出露的太古宙结晶基底中部的斜长角闪岩浅粒岩(由玄武岩、英安岩至流纹质火山岩构成的变质火山岩套)和黑云二长石英片岩(变质泥砂质岩石)的SHRIMP锆石UPb年龄均大于(2 488±8)Ma[76]。因此,商南的秦岭造山带太古宙结晶基底的岩石在出露之后遭受剥蚀,并经过多次搬运,最终沉积在铁矿梁组。
锆石UPb年龄谱分析和电子探针分析表明,铁矿梁组砂岩的物源主要来自425~530 Ma的天水和丹凤花岗岩、佛坪片麻岩、清水英安岩,578~982 Ma的柞水(黑沟)碱性花岗岩、天水和西峡的花岗质片麻岩、丹凤和卢氏的花岗岩。另外,1 015~1 551 Ma的佛坪黑云母斜长片麻岩,1 658~1 957 Ma的太白和宝鸡花岗岩、佛坪片麻岩,2 329~2 502 Ma的丹凤石榴黑云斜长片麻岩和2 625~3 147 Ma的商南太古宙结晶基底岩石均经历多次搬运,为扬子北缘下高川盆地提供次要物源。
综上所述,晚泥盆世古地理格局为天水、丹凤、佛坪、清水、西峡等地区岩体隆升剥蚀,成为物源区,而在华北古陆与扬子北缘下高川盆地之间可能存在多个较小古陆,如佛坪也属于物源区。
6结语
(1)扬子北缘下高川盆地铁矿梁组砂岩的重矿物主要有锆石、金红石、黄铁矿、电气石、闪锌矿、白钛石、钛铁矿等。其中,闪锌矿、白钛石、钛铁矿重矿物组合指示其物源可来自于岩浆岩。
(2)电气石电子探针分析结果表明,铁矿梁组石英砂岩主要来自于变质板岩和变质砂岩,少量来自贫锂花岗岩类、伟晶岩和细晶岩。
(3)砂岩碎屑LAICPMS锆石UPb年龄谱分析和电子探针分析表明,铁矿梁组砂岩的主要物源来自425~530 Ma的天水和丹凤花岗岩、佛坪片麻岩、清水英安岩,578~982 Ma的柞水(黑沟)碱性花岗岩、天水和西峡的花岗质片麻岩、丹凤和卢氏的花岗岩。另外,1 015~1 551 Ma的佛坪黑云母斜长片麻岩,1 658~1 957 Ma的太白和宝鸡花岗岩、佛坪片麻岩,2 329~2 502 Ma的丹凤石榴黑云斜长片麻岩和2 625~3 147 Ma的商南太古宙结晶基底岩石均经历多次搬运,为扬子北缘下高川盆地提供次要物源。
(4)晚泥盆世的物源大致来自北部,同时古地理格局表现为天水、丹凤、清水、西峡等地区岩体隆升剥蚀,成为物源区,而在华北古陆与扬子北缘下高川盆地之间可能存在多个较小古陆,如佛坪也属于物源区。
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收稿日期:20160918
基金项目:中国地质调查局地质调查项目(DD20160176,12120114009401);
中国地质科学院矿产资源研究所基本科研业务费专项资金项目(K1613(2016))
作者简介:贾晓彤(1992),女,山东枣庄人,中国地质大学(北京)理学硕士研究生,Email:jiaxiaotong1925@163.com。