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Volume 39 Issue 3
Jun.  2021
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LI Hong, LI Fei, GONG QiaoLin, ZENG Kai, DENG JiaTing, WANG HaoZheng, SU ChengPeng. Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan[J]. Acta Sedimentologica Sinica, 2021, 39(3): 525-539. doi: 10.14027/j.issn.1000-0550.2020.073
Citation: LI Hong, LI Fei, GONG QiaoLin, ZENG Kai, DENG JiaTing, WANG HaoZheng, SU ChengPeng. Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan[J]. Acta Sedimentologica Sinica, 2021, 39(3): 525-539. doi: 10.14027/j.issn.1000-0550.2020.073

Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan

doi: 10.14027/j.issn.1000-0550.2020.073
Funds:

National Natural Science Foundation of China 41872119, 41502115

Science and Technology Plan Project of Sichuan Province 20YYJC1185

  • Received Date: 2020-07-02
  • Rev Recd Date: 2020-08-23
  • Publish Date: 2021-06-10
  • The mixed siliciclastic-carbonate rocks can provide clues for understanding both the carbonate sedimentation and the sources of terrigenous particles. Accordingly, these rocks have received increasingly attention in the fields of studies on sedimentary dynamics, paleogeography, paleoclimatology, and provenance analysis. It should be noted that, affected by the nature of the particles, as well as the hydrodynamic, weathering, and diagenetic conditions, the content of the terrigenous fractions in the mixed siliciclastic-carbonate rocks is changeable and the particle size is generally small. Metastable-unstable heavy minerals may be lost during the weathering and diagenetic processes in varying degrees. Therefore, some traditional provenance analysis methods to indicate the source of original terrestrial compositions may not be applicable. In this work, systemic petrological and morphological studies were conducted on silt-sized heavy minerals in the mixed siliciclastic-carbonate systems of Lower Cambrian (Stage 3) in the Hannan-Micangshan area. The composition, morphological (grain size, elongation, and roundness), weathering and diagenetic characteristics of heavy minerals in different sedimentary environments have been preliminarily explored. The results show that the percentage of terrigenous particles in the coastal environments was much higher than those of the shelf margins, and the proportion of easily weathered heavy minerals also decreases in the rimmed shelf. On the other hand, the particle size and elongation of detrital zircon in the stable heavy minerals on the platform margin are markedly smaller than those in the coastal environment, and the roundness of zircon becomes better under the same level of elongation. In addition, this study also found that there were relatively continuous shallow-water, high-energy ooid shoals and archaeocyath-microbial mounds developed along the Hannan-Micangshan area. These evidence indicates that the Hannan Massif had being developed during the Cambrian Age 3 and could provide large amounts of terrigenous material to the northern Upper Yangtze area. The Lower Cambrian of the northern Sichuan Basin is a potential replacement for oil and gas exploration in the Sichuan Basin. It is of great value to clarify the provenance of the Canglangpuian for understanding the Early Cambrian paleogeography of the northern Upper Yangtze area of the South China Block.
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Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan

doi: 10.14027/j.issn.1000-0550.2020.073
Funds:

National Natural Science Foundation of China 41872119, 41502115

Science and Technology Plan Project of Sichuan Province 20YYJC1185

Abstract: The mixed siliciclastic-carbonate rocks can provide clues for understanding both the carbonate sedimentation and the sources of terrigenous particles. Accordingly, these rocks have received increasingly attention in the fields of studies on sedimentary dynamics, paleogeography, paleoclimatology, and provenance analysis. It should be noted that, affected by the nature of the particles, as well as the hydrodynamic, weathering, and diagenetic conditions, the content of the terrigenous fractions in the mixed siliciclastic-carbonate rocks is changeable and the particle size is generally small. Metastable-unstable heavy minerals may be lost during the weathering and diagenetic processes in varying degrees. Therefore, some traditional provenance analysis methods to indicate the source of original terrestrial compositions may not be applicable. In this work, systemic petrological and morphological studies were conducted on silt-sized heavy minerals in the mixed siliciclastic-carbonate systems of Lower Cambrian (Stage 3) in the Hannan-Micangshan area. The composition, morphological (grain size, elongation, and roundness), weathering and diagenetic characteristics of heavy minerals in different sedimentary environments have been preliminarily explored. The results show that the percentage of terrigenous particles in the coastal environments was much higher than those of the shelf margins, and the proportion of easily weathered heavy minerals also decreases in the rimmed shelf. On the other hand, the particle size and elongation of detrital zircon in the stable heavy minerals on the platform margin are markedly smaller than those in the coastal environment, and the roundness of zircon becomes better under the same level of elongation. In addition, this study also found that there were relatively continuous shallow-water, high-energy ooid shoals and archaeocyath-microbial mounds developed along the Hannan-Micangshan area. These evidence indicates that the Hannan Massif had being developed during the Cambrian Age 3 and could provide large amounts of terrigenous material to the northern Upper Yangtze area. The Lower Cambrian of the northern Sichuan Basin is a potential replacement for oil and gas exploration in the Sichuan Basin. It is of great value to clarify the provenance of the Canglangpuian for understanding the Early Cambrian paleogeography of the northern Upper Yangtze area of the South China Block.

LI Hong, LI Fei, GONG QiaoLin, ZENG Kai, DENG JiaTing, WANG HaoZheng, SU ChengPeng. Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan[J]. Acta Sedimentologica Sinica, 2021, 39(3): 525-539. doi: 10.14027/j.issn.1000-0550.2020.073
Citation: LI Hong, LI Fei, GONG QiaoLin, ZENG Kai, DENG JiaTing, WANG HaoZheng, SU ChengPeng. Morphological Characteristics and Provenance Significance of Heavy Minerals in the Mixed Siliciclastic-carbonate Sedimentation: A case study from the Xiannüdong Formation, Cambrian (Series 2), northern Sichuan[J]. Acta Sedimentologica Sinica, 2021, 39(3): 525-539. doi: 10.14027/j.issn.1000-0550.2020.073
  • 物源分析在判断沉积物来源、确定源区母岩类别,进而重建古气候特征与古地理环境、恢复源区地质构造背景和构造演化史等方面具有重要意义[1-6]。随着研究的不断深入以及测试技术的不断进步,物源分析方法已经从传统的矿物类型、粒度和组合特征,扩展到单矿物元素与同位素分析、矿物年代学、矿物拉曼光谱学等方面[7-8]。近年来,研究者们对现代河流沉积物以及“深时”碎屑岩的物源分析方法研究取得了一系列重要进展。例如,利用重矿物颗粒形貌学特征,包括延长系数、磨圆度、裂隙等颗粒表面形态的变化推断沉积物搬运和埋藏过程及物源情况[9-13];利用单矿物年代学(如U-Pb和(U-Th)/He)、元素和同位素(如Hf和O同位素等)地球化学方法分析源区性质、构造背景和演化过程[5,14-21];利用矿物化学风化程度分析古陆剥蚀过程与气候变化[22]等。长期以来,物源分析都是以碎屑岩为研究对象,对于混积岩等较为特殊情况下的物源分析仍极具挑战性,有待深入探讨。

    陆源碎屑与碳酸盐组分在层内的原地混合沉积,经成岩作用后保存下来的岩石类型称为混积岩[23-24]。混积岩不仅能反映碳酸盐沉积过程中的海水性质,提供生物与环境演化方面的线索[25],同时在指示陆源碎屑物质来源,以及化学风化程度等方面也具有潜力[25-28]。但是,根据机械分异作用原理和现代滨海混合沉积实例[29-31],陆源碎屑颗粒在碳酸盐沉积主导的环境中粒径普遍较小,以粉砂级为主。此外,滨海环境下沉积物分布多数受水动力条件影响(如波浪、风暴、潮流等),颗粒分选性较强[32-35],造成反映原始物质组成的一些物源分析方法可能不太适用,例如Dickinson三元判别图解[36-37]、全岩地球化学分析[31]、重矿物特征指数(ZTR、ATi和MTi)等[38]。不同矿物对水动力条件的响应会受其物理性质(粒度、形状和密度等)的影响,例如密度相对较小的片状云母类矿物易迁移,在斜坡和盆地环境常见,而独居石、锆石和金红石等密度较大的重矿物多在近岸环境[39-41]。此外,古代混积岩与现代混合沉积中的重矿物组合存在明显差异,表现为易溶重矿物(例如辉石、角闪石和绿帘石等)比例明显降低[42-43]。这是由于在深埋藏条件下,成岩作用会优先溶解闪石类和辉石类等不稳定重矿物(图1),因而在对古老沉积岩中的重矿物组合进行物源分析时,需要保持谨慎[44-45]

    Figure 1.  Relative stability of detrital heavy minerals in burial diagenetic conditions (after reference [44])

    单矿物分析是一种较为有效的物源分析方法。这是由于在长期风化、剥蚀和搬运过程中,锆石、电气石等稳定重矿物受输送距离、机械侵蚀、埋藏成岩作用等影响有限,经河流入海后大量卸载,虽然波浪、潮汐和洋流等会对陆源沉积物进行改造,但其分布和沉积过程仍然受到颗粒性质和水动力分选的影响,排除再旋回因素后,总体表现为从滨岸向浅海粒度变细的特点,同时台地或陆架边缘高能相带受波浪颠选影响亦会出现重矿物相对富集的现象[32,34-35,46]。因此,在缺乏充足陆源碎屑供给的情况下,从水体能量相对较高的混积岩中分选出陆源碎屑组分并对其中特定的重矿物进行岩石学和形貌学分析,可为认识当地物源情况提供线索。

    四川盆地北缘寒武纪沧浪铺期古地理格局尚不明确,其中关于川北地区浅海陆源碎屑组分的来源情况存在较大争议:一类观点认为川北米仓山地区(汉南古陆)与川西北地区(碧口地体)各存在一个古陆,为上扬子地区北部提供物源[24,47-50];另一类观点认为米仓山地区由南向北水体逐渐加深,不存在古陆,主要物源供给方向为川西北碧口地体和川西南康滇古陆[51-52]。由于米仓山地区寒武纪沧浪铺期主要为混合沉积,碎屑组分含量不稳定且多为粉砂级颗粒,因此寻找可靠的物源证据对厘清上扬子北部古地理格局具有实际意义。基于此,本文选取川北地区寒武系第二统仙女洞组发育典型的三个剖面,对同时期不同沉积环境下混积岩中的细粒碎屑组分进行系统的岩石学和重矿物形貌学工作,包括岩性、重矿物类型及组合、重矿物的形态和风化侵蚀特征等,对该地区仙女洞组的物源情况进行了初步探讨,以期为川北地区古地理格局恢复、混积岩体系下细粒碎屑组分的物源分析方法提供参考。

  • 研究区位于上扬子地区北缘米仓山一带,北临秦岭造山带,南接四川盆地,东西两侧分别为大巴山构造带和松潘—甘孜造山带[53-54]。新元古代末期至寒武纪筇竹寺期,强烈的构造隆升和裂陷运动造成四川盆地形成“隆坳相间”的沉积格局,例如绵阳—长宁拉张槽,川中水下古隆起等[55-57]。筇竹寺期,整个上扬子地区发生了寒武纪规模最大的一次海侵,沉积了一套巨厚的陆棚相泥岩和粉砂岩[57-60]。沧浪铺期,随着海水的逐渐退却,研究区逐渐转为碎屑组分—碳酸盐混合沉积体系[24,61-62]

    研究区寒武系自下而上依次出露宽川铺组(麦地坪组)、郭家坝组、仙女洞组、阎王碥组和孔明洞组,苗岭统和芙蓉统普遍缺失[50,63]图2)。本次研究主要关注仙女洞组物源情况,选取的三个野外剖面露头出露情况良好,分别为陕西勉县大河坝,四川旺苍县唐家河,以及四川南江县田垭(图3)。三个剖面的郭家坝组均由薄层钙质粉(细)砂岩或泥岩组成。大河坝剖面仙女洞组发育的混积岩以碎屑组分为主,上部可见砂质鲕粒岩和砂质古杯—凝块岩等。而唐家河和田垭剖面仙女洞组发育的混积岩则以碳酸盐组分占主导,岩性为钙质细砂岩、(含)砂质鲕粒灰岩、含砂凝块岩等。各个剖面上覆阎王碥组底部均以偏红色细粒碎屑岩发育为特征[24,60]图4)。按照最新的国内寒武系划分方案,本次研究所关注的仙女洞组属于第二统第三阶岩石地层单元(图2)。

    Figure 2.  Diagram exhibiting the lithostratigraphic units of the Lower Cambrian Strata (Series 2) in the Sichuan Basin and adjacent areas (after references [49,64⁃65])

    Figure 3.  Location and paleogeography during Cambrian Age 3 (Canglangpuian) in northern Sichuan (modified from references [24,66])

    Figure 4.  Lithological columns of the Xiannüdong Formation (Cambrian Stage 3) in the study area (modified from reference [24])

  • 样品采样位置如图3所示(TY-1、TJH-1和DHB-1)。对选取的三个剖面仙女洞组混积岩样品,笔者分别进行了薄片观察(Leica DM4P显微镜)、重砂分选以及重矿物形貌学分析。其中重砂分选按细粒单矿物分选方法,将采集的样品经过粉碎、筛选、酸洗、淘选、磁选和重液分选等步骤[67],人工挑选出纯净的锆石、磷灰石、石榴石等颗粒,然后进行样品制靶、抛光等处理,以便后期进行形貌学分析。用于单矿物识别和形貌学特征分析的电子探针(JEOL JXA-8230)实验在西南石油大学地球科学与技术学院完成;重矿物分选在廊坊岩拓地质服务有限公司完成;重矿物制靶、阴极发光及透反射照相等在北京中科矿研检测技术有限公司完成。

    挑选出来的锆石颗粒采用ImageJ软件测量其圆度值,测量出的颗粒长度和宽度用于计算晶粒的延长系数(长宽比) [11,13,68]。本次研究共挑选597颗碎屑锆石进行形貌学统计,选择了186颗具有相对完整晶型的碎屑锆石来计算其延长系数。虽然前人对碎屑锆石形貌学研究已有一定基础,但对于不同形态和磨圆度的区分标准仍存在分歧[10]。根据现代河流沉积物中的碎屑矿物磨圆分类方案[42],将磨圆度划分为四类:棱角状(0~0.4)、次圆状(0.4~0.6)、圆状(0.6~0.8)和破裂改造圆状(0.8~1.0)(图5)。其中,比较特殊的破裂改造颗粒主要呈圆状或次圆状,其中一侧因机械断裂而呈现出较锐利的边缘。将统计的矿物延长系数划分为小(<1.5)、中(1.5~2.0)、大(>2.0)三个组。需要注意的是,复杂物源背景下磨圆度良好的锆石形貌主要受控于早期的变质或者岩浆成因,反映再旋回特征;强水动力条件或长距离搬运并不会显著改造锆石磨圆程度[69]

    Figure 5.  Reference roundness scheme for detrital zircons (criterion sourced from reference [42])

  • 大河坝、唐家河和田垭剖面的样品岩性分别为砂质微生物岩(DHB-1)、含砂泥质鲕粒微生物岩(TJH-1)和砂质鲕粒灰岩(TY-1)。碳酸盐主导的混积岩颜色主要为浅青灰色,碳酸盐颗粒类型包括古杯、鲕粒,和簇状钙化蓝细菌等,颗粒之间以钙质胶结为主;碎屑物质主要以粉砂—泥级组分占主导(图6a~c)。镜下观察发现,细粒碎屑组分主要分布于碳酸盐颗粒或胶结物之间的孔隙,呈分散状,类型主要为石英、长石、岩屑和黏土矿物,粒级在粉砂—泥级,其中陆源组分含量据估计不超过15%,而重矿物含量不足1%(图6d~f)。进一步的电子探针分析显示这些重矿物粒径在10~100 μm,常见锆石、黄铁矿、磷灰石和金红石等(图6g~l)。大部分的重矿物呈棱角—次圆状,小部分重矿物常作为石英或长石包裹体产出(图6h~j)。

    Figure 6.  Photographs of detrital light and heavy minerals in the mixed siliciclastic⁃carbonate rocks of the Xiannüdong Formation (Micangshan area)

  • 样品中重矿物类型及含量详见表1,重矿物类型主要包括磷灰石、锆石、电气石、黄铁矿和褐铁矿等。田垭和唐家河剖面样品中,黄铁矿在重矿物中的占比最高(分别为50.2%、88.5%),以单体形式产出,晶型较好(图6h),亦常作为长石包裹体(图6j)。大河坝剖面样品中重矿物种类比田垭和唐家河剖面丰富,包括含量较高的金红石、白钛石、褐铁矿以及特有的石榴石,但几乎不含黄铁矿(表1)。虽然黄铁矿在研究区是一种主要的重矿物类型,但是由于其本身成因和来源复杂,本次研究暂不考虑其物源意义。

    样品编号 岩性 锆石 磷灰石 金红石 电气石 黄铁矿 锐钛矿 褐铁矿 白钛石 石榴石 其他
    TY-1 TJH-1 DHB-1 砂质鲕粒灰岩 1.53 0.30 0.70 88.41 9.06
    含砂泥质鲕粒微生物岩 0.60 16.50 0.60 50.20 32.10
    砂质微生物岩 12.40 3.85 4.20 3.82 0.50 38.10 2.20 6.52 28.40

    Table 1.  Heavy⁃mineral compositions (wt.%), Cambrian Xiannüdong Formation in three study sections, northern Sichuan

    大河坝剖面重矿物特征:锆石透明度高,90%为浅粉色,次圆状,少数为半自形柱状,偶见自形晶,晶体表面较粗糙,可见凹坑;玫瑰色占10%,次圆状为主,少量半自形柱状,晶体表面凹坑常见。阴极发光(CL)图像下,大多数锆石内部具有清晰的振荡环带且环带较窄,少数具有比较特殊的扇形分带结构(图7a),这是环境变化造成锆石在结晶时各晶面生长速率不一致而形成的[70];部分锆石有重结晶、增生边等现象,个别边部变质重结晶锆石已经切割了原岩岩浆锆石的环带;锆石大多为暗灰色至亮灰色,少数颗粒颜色明亮(图7a),初步判断大部分锆石为岩浆成因,少数为变质成因。岩浆锆石中,锆石粒径范围主要在30~150 μm,延长系数1.0~3.1。三组延长系数中,大延长组比例为21.7%,中延长组和小延长组分别为46.7%和31.7%(图8a)。磨圆度方面,破裂改造圆状颗粒占比为13.6%,完整晶型的棱角状、次圆状和圆状锆石平均比例分别为2.8%、39.5%和44.2%(图8d)。磷灰石主要呈次圆状—圆状,棱角状偶见,大部分颗粒粒径范围为50~150 μm,延长系数1.2~1.5。电气石主要为次圆状,棱角状次之,透明,晶粒大小主要为50~120 μm,少数为130~200 μm,长宽比为1.2~2.0。

    Figure 7.  Cathodoluminescence images of detrital zircon grains in three study sections

    Figure 8.  Elongation and roundness of detrital zircon grains of Xiannüdong Formation in the study area

    唐家河剖面重矿物特征:锆石透明度高,其中60%为浅粉色,圆状—次圆状为主,表面较粗糙,可见凹坑。玫瑰色锆石约占40%左右,多数为次圆—半自形柱状。CL图像显示,大多数锆石内部具振荡环带,颜色以暗到稍亮为主。少数锆石内部呈均匀的灰色或亮灰色的弱分带情况,边缘含增生边或重结晶特征(图7b),初步推测大部分为岩浆成因,少数为变质成因。锆石粒径主要在50~140 μm,延长系数为1.1~3.3,其中约有40.6%的碎屑锆石颗粒在中显示为1.5~2.0的延长系数(图8b)。唐家河样品中锆石的圆状颗粒约占一半,次圆状约25.9%,破裂改造圆状占21.3%,棱角状颗粒只有3.0%(图8e)。磷灰石以次圆状为主,棱角状偶见,其颗粒大小主要在40~100 μm,少数为110~250 μm。

    田垭剖面重矿物特征:锆石透明,其中浅粉色约占锆石总量的65%左右,主要为圆状—次圆状,少量为半自形柱状,晶体表面可见凹坑;玫瑰色锆石约占35%左右,次圆状为主,晶体表面粗糙,凹坑较多。锆石的阴极发光图像灰度特征为灰色—灰白,与大河坝和唐家河剖面类似,也存在指示岩浆成因的振荡环带结构(图7c)。锆石粒径范围为30~100 μm,延长系数(长宽比)为1.0~2.6。碎屑锆石的小延长组最多,约占45.2%(图8c),其次为中延长组(1.5~2.0)和大延长组(>2.0),分别占38.7%和16.1%。田垭样品中锆石的磨圆情况比唐家河剖面更好,次圆状、圆状和破裂改造圆状晶体的平均比例分别为30.8%、42.7%、24.9%,棱角状仅占1.62%(图8f)。磷灰石粒径主要为30~100 μm,次圆状为主,棱角状偶见。

  • 受风化、搬运、沉积和成岩等过程的影响,重矿物表面形貌特征不断受到改造,其中越不稳定重矿物的溶解改造速率越快,最后甚至会出现随着埋藏深度的增加而逐渐消失的现象[44,71]。Andò et al.[42]将重矿物的风化程度分为未风化(Unweathered)、侵蚀(Corroded)、蚀刻(Etched)、深蚀(Deeply Etched)、骨架(Skeletal)五个阶段。矿物侵蚀形貌的变化主要由晶体结构和保存特征决定,大多数常见的重矿物可以识别出连续的风化阶段和侵蚀程度,例如角闪石、阳起石和普通辉石[72-73]。也有一些重矿物表面结构的差异不易识别,只能区分渐进的侵蚀程度。各向同性的石榴石没有明显的优先结晶方向,因而很少呈棱柱状,难以识别出连续的风化过程[42]。此外,锆石、电气石和金红石这类稳定的重矿物,很少出现侵蚀特征[42]

    根据Andò et al.[42]提出的矿物表面结构可视化分类方案,以重矿物类型较多的大河坝剖面为例,相对易风化的石榴石在搬运沉积过程中,容易改变自身的形貌结构,多呈不规则状。近80%的石榴石颗粒表现为高度侵蚀阶段,透、反射图中可见清晰的凹角和蚀刻痕迹;少量石榴石显示出轻度侵蚀的表面结构,未见有明显骨架特征的颗粒,整体反映出石榴石受到较强烈化学风化或成岩蚀变作用的改造(图9a,b)。大河坝剖面的磷灰石粒径主要在50~150 μm,多数磨圆较好,表面有撞击痕迹和小的凹坑,少数较为平整光滑,偶见棱角状颗粒。磷灰石多表现为轻微的侵蚀(约90%),颗粒边缘侵蚀现象明显;少数未风化(约5%),也未见高级侵蚀或带骨架特征的颗粒(图9c,d)。电气石中棱角状—次圆状颗粒比圆状颗粒多,在透、反射图片中很少见深度蚀刻坑。电气石侵蚀特征以未风化—初期侵蚀阶段为主,颗粒表面较为光滑,凹坑小、撞击特征较磷灰石少;个别棱角状颗粒的表面具凹坑,边缘有机械断裂痕迹(图9e,f)。

    Figure 9.  Corrosion features of detrital garnet, apatite, and tourmaline minerals at Daheba section

  • 陆源碎屑组分中重矿物的组成受源区性质、沉积过程以及成岩作用等多方面因素的共同影响[45],而水动力条件是影响滨海环境颗粒分选的重要因素之一。受水动力条件的影响,重矿物粒度、形状和密度的差别,会显著影响其沉积环境。在沉积物的搬运过程中,矿物的密度差别越大,分选效果越好[74]表2);化学风化和埋藏成岩时间越久,不稳定—亚稳定重矿物表面结构受侵蚀越严重,并且重矿物的多样性会随埋藏深度的增加而减少,很多不稳定重矿物会在埋藏成岩过程中消失[44,71](图1)。锆石、电气石、磷灰石、石榴石以及铁质矿物(褐铁矿、钛铁矿等)的化学性质稳定,抗磨蚀、成岩改造能力强;不稳定的闪石类、辉石类和片状云母类矿物,受风化蚀变和成岩作用的影响较大[44]。研究区混积滨岸—台缘背景下的重矿物组合以稳定—极稳定的锆石、磷灰石、电气石和金红石等为主,大部分石榴石显示出较强的风化侵蚀结构,磷灰石和电气石的侵蚀相对较弱,未见不稳定的闪石类、帘石类以及片状矿物,且重矿物种类明显偏少。这指示了后期成岩作用对不稳定重矿物的改造较为强烈。

    黏土类(1.8~3.1) 轻矿物(δ ≤2.9) 重矿物(δ >2.9)
    蒙脱石 2.5 石英 2.65 黑云母 3.0 电气石 3.1 角闪石 3.2 磷灰石 3.2
    高岭石 2.65 长石 2.5-2.8 绿帘石 3.4 石榴石 3.5-4.2 锐钛矿 3.9 褐铁矿 3.9
    伊利石 2.8 方解石 2.7 金红石 4.2 锆石 4.3 黄铁矿 5.0 独居石 5.2

    Table 2.  Densities of different minerals in siliciclastic components (modified from references[74⁃76])

    由于河流入海口离陆地更近,碎屑物质向盆内输送时受水动力分选和矿物密度差的影响,使得细粒物质和轻矿物更易被搬运带走,而相对较粗的颗粒和重矿物多沉积在近岸区域。因此,在滨岸环境中,近源重矿物的输送使得所含重矿物种类多,颗粒粒径也相对较大;而混积台地边缘离陆地较远,缺乏陆源碎屑物质的及时供应,因而重矿物粒径与滨岸环境相比要偏小。

  • 寒武纪初期四川盆地古地理格局具有“隆坳相间”的特点,其中康滇古陆与川中水下高地之间为绵阳—长宁裂陷槽,川中高地与川北米仓山地区之间亦为坳陷的陆棚环境[57]图3)。目前国内对上扬子地区沧浪铺期古地理展布的研究还不够深入,特别是对其北部米仓山地区的研究还相对匮乏。一般认为,米仓山地区继承了灯影组沉积期隆起的水下高地特征,沧浪铺早期海退之后汉南—米仓山地区古陆开始出露,古地理格局整体为北高南低,沉积相类型由北至南依次发育三角洲—混积滨岸、混积台地(陆架)和斜坡,与川中水下高地之间为水体较深的陆棚环境[77-78]图3)。而张英利等[5152]最新研究认为米仓山地区由南向北水体逐渐加深,在汉南—米仓山地区形成深水斜坡,认为沧浪铺期汉南古陆不发育,并提出当地主要物源供给方向为川西北碧口地体和川西南康滇古陆。

    研究区重矿物形貌学特征支持沧浪铺期汉南古陆已经开始发育的认识。作为性质非常稳定的重矿物,锆石颗粒受成岩改造和沉积搬运过程中磨蚀作用的影响非常小[11]。虽然三个剖面的锆石颗粒延长系数均主要在1.0~3.0,但是大河坝剖面的锆石粒径最大且延长系数在1.5~2.0区间的比例最高。与唐家河剖面相比,田垭剖面的碎屑锆石粒径和中—大延长组的比例最小,但两个剖面的锆石粒径均小于大河坝剖面,延长系数也主要为小延长组(<1.5)(图8a~c、图10)。不同锆石延长组内,大河坝、唐家河、田垭三个剖面的锆石粒径整体均呈逐渐减小的趋势(图11)。这表明台内—滨岸沉积环境下的颗粒粒径差异主要受水动力条件影响[79-80],与现代混合沉积发育的澳大利亚大堡礁地区具有可对比性[81]。同时,北部大河坝剖面混积岩中所含重矿物种类多、磨圆度整体较差(表1、图810),暗示了勉县大河坝当时更靠近古陆。

    Figure 10.  Crossplot of length vs. width for detrital zircons in the study area

    Figure 11.  Cathodoluminescence characteristics of zircons with different elongation in the study area

    通过对研究区仙女洞组的岩性调查发现,在混积台地上呈环带状发育鲕粒滩和微生物礁(丘)建造[24,61-62,82-84]。如果物源来自西部碧口地体,那么川北沧浪铺期浅水混积台地应该主要围绕碧口地体,而不是沿汉南—米仓山地区。其次,从重矿物种类、锆石粒径和磨圆度等特征来看,在地理位置上更靠近碧口地体的唐家河剖面未显示出近源沉积的特点。由于碧口地体和汉南—米仓山地区之间存在一个深水陆棚,西面碧口地体的物源可能主要填充该凹陷槽,难以向川北其他地区供给[65,78,85]图3)。此外,由于康滇古陆来源的碎屑物质在沧浪铺期主要填充临近的绵阳—长宁拉张槽,且川中水下高地存在明显遮挡[78],从四川盆地西南部向北部米仓山地区远距离输送物源较难实现。

    因此,本次研究认为米仓山地区仙女洞组混积岩中陆源碎屑物质主要来自于东北部古陆,这进一步确认了沧浪铺期汉南古陆的存在,表明重矿物的形貌学特征在滨海环境物源分析中具有一定参考价值。

  • 以碳酸盐为主的混积体系下,陆源碎屑组分含量低,成岩过程中不稳定重矿物的溶解又丢失了部分有价值的物源信息,给“源—汇”分析工作带来了一定难度。本次研究对川北米仓山地区寒武系仙女洞组三个剖面的混积岩物源情况进行了探索。从重矿物组合、化学风化侵蚀程度、延长系数等方面的特征来看,研究区碎屑物质主要来源于邻近汉南古陆。同时,沿米仓山周缘浅水高能(含砂)砂质鲕粒滩和(含砂)砂质古杯—凝块石丘(礁)的发育也暗示了陆源碎屑近源供给的特点。此外,通过本次研究我们认为混积体系下滨岸至台地环境的细粒重矿物形貌学特征在“深时”物源判别上具有一定价值,更系统的工作有待下一步深入探讨。

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