页岩气储层浅表样品垂向风化特征——以贵州习水龙马溪组页岩为例

页岩风化作用较为复杂,风化页岩由于有机质和矿物组成等发生改变,在页岩气资源评估过程中不能直接利用地表样品。前人主要利用露头样品关注其对环境的影响,少有从影响页岩气资源评估的角度进行研究。本文以页岩气重要储层下志留龙马溪组黑色页岩为研究对象,采集了贵州省习水骑龙村龙马溪组竖直钻孔样品共5 m,运用主微量元素分析、X射线衍射、总有机碳含量测试、热分析以及扫描电镜等方法,系统研究了该储层浅表样品的垂向风化特征。结果表明,研究区地表0 m处页岩严重风化,0.5~1.5 m范围内中等风化,2~5 m范围内可视为未风化的母岩。因此在页岩气资源初期评估时需要尽可能采集2 m以下新鲜样品。本研究查明了贵州习水龙马溪组页岩垂向风化作用规律,为页岩气早期资源评估野外采样提供了科研依据。     

扬子地台西缘下寒武统黑色页岩土壤元素地球化学特征北大核心CSCD

本文分析扬子地台西缘黑色页岩土壤(BSS)及其成土新鲜黑色页岩(FB)的主、微量元素的地球化学特征,探讨黑色页岩土壤成土地球化学作用机制。结果表明,黑色页岩土壤明显亏损MgO、CaO、Na2O、P2O5和LOI(有机质),而富集Al2O3和Fe2O3等主量组分;且明显亏损Ba、V、Ni、Cu、Zn和Sr等微量元素,但其他微量元素则不然。与成土母岩相比,黑色页岩土壤的化学组成相对均一。主量组分对黑色页岩与黑色页岩土壤之间化学组成有不同的影响,这种影响与CaO、Na2O和MgO等碱质组分、LOI所代表的有机质等的淋滤释出,及Fe2O3和Al2O3等氧化物的次生富集有关,其影响程度顺序为:CaO>Na2O>LOI>MgO>Al2O3>Fe2O3>MnO>K2O>SiO2>P2O5。推断黑色页岩风化-成土过程中有机质、黄铁矿、方解石、钠长石、斜绿泥石等不稳定矿物的化学蚀变分解和长石等硅酸盐矿物的水解导致CaO、LOI、Na2O、MgO、K2O和SiO2等主量组分,以及V、Cr、Ni、Cu、Zn、Ge、Ba、Sr等微量元素的淋滤释出;而黏土矿物、铁-锰氧化物等次生矿物的次生富集,导致Fe2O3、MnO和Al2O3等主量组分,以及Pb、Cs、Co、Th、Sc、Ga、Rb、REE等微量元素在土壤中形成次生富集。故而黑色页岩可为环境重金属污染源,而黑色页岩土壤本身也可存在重金属污染。     

扬子地台西缘下寒武统黑色页岩土壤元素地球化学特征

本文分析扬子地台西缘黑色页岩土壤(BSS)及其成土新鲜黑色页岩(FB)的主、微量元素的地球化学特征,探讨黑色页岩土壤成土地球化学作用机制。结果表明,黑色页岩土壤明显亏损MgO、CaO、Na_2O、P_2O_5和LOI(有机质),而富集Al_2O_3和Fe_2O_3等主量组分;且明显亏损Ba、V、Ni、Cu、Zn和Sr等微量元素,但其他微量元素则不然。与成土母岩相比,黑色页岩土壤的化学组成相对均一。主量组分对黑色页岩与黑色页岩土壤之间化学组成有不同的影响,这种影响与CaO、Na_2O和MgO等碱质组分、LOI所代表的有机质等的淋滤释出,及Fe_2O_3和Al_2O_3等氧化物的次生富集有关,其影响程度顺序为:CaONa_2OLOIMgOAl_2O_3Fe_2O_3MnOK_2O SiO_2P_2O_5。推断黑色页岩风化-成土过程中有机质、黄铁矿、方解石、钠长石、斜绿泥石等不稳定矿物的化学蚀变分解和长石等硅酸盐矿物的水解导致CaO、LOI、Na_2O、MgO、K_2O和SiO_2等主量组分,以及V、Cr、Ni、Cu、Zn、Ge、Ba、Sr等微量元素的淋滤释出;而黏土矿物、铁-锰氧化物等次生矿物的次生富集,导致Fe_2O_3、MnO和Al_2O_3等主量组分,以及Pb、Cs、Co、Th、Sc、Ga、Rb、REE等微量元素在土壤中形成次生富集。故而黑色页岩可为环境重金属污染源,而黑色页岩土壤本身也可存在重金属污染。     

黔北下寒武统黑色岩系的沉积环境与地球化学响应

下寒武统黑色岩系在我国华北、华南及塔里木盆地等广泛发育,但富有机质泥页岩的分布特征及其控制因素未有定论。本文对黔北地区下寒武统牛蹄塘组(及同时期地层)不同沉积相带典型剖面中的富有机质泥页岩及其上下层位进行了地球化学分析,识别出寒武纪早期不同沉积相带氧化还原环境的明显差异。台内凹陷相主要岩性为黑色页岩,底部为不等厚的薄层磷块岩和硅质岩或硅磷质结核,沉积的黑色页岩厚度大,有机质丰度高,V、Ni、Mo、U等微量元素显著富集,反映了贫氧-缺氧环境,以及短暂动态的硫化环境,Mo/TOC反映了其为局限盆地特征;上斜坡相为贫氧到氧化的沉积环境,但更多地是表现为氧化的沉积环境,其中牛蹄塘期存在局部的硫化环境,沉积的黑色页岩厚度小,但有机质丰度最高,V、Ni、Mo、U等微量元素的富集程度较低;盆地相为黑色硅质岩与富有机质的黑色硅质页岩间互沉积,同期沉积厚度较小,有机质丰度相对较低,V和U的富集程度大于台内凹陷相,而Ni和Mo的富集程度则低于台内凹陷相,指示了缺氧环境,期间伴随有贫氧和短暂的氧化环境存在,且可能存在局部的硫化环境,Mo/TOC反映了中等的局限程度。总的来说,黔北下寒武统黑色岩系的有机质丰度、厚度以及微量元素富集程度等特征主要受控于不同的氧化还原环境。     

黑色页岩化学风化程度指标研究

黑色页岩是一套化学组成变化大、成份分布极不均一的岩石。建立在化学组成相对均一的岩浆岩风化基础之上的化学风化指数如CIA(MIA)、CIW、CIX、PIA、STI、R、WIP、V、W等,因不能较好地把母岩化学组成变化与风化反应引起的化学变化区分开来,用于厘定黑色页岩的风化程度时,存在灵敏度低、与实际风化程度不相符等问题。故需要建立新的风化指数来厘定黑色页岩的风化程度。本文以湘中地区下寒武统新鲜和风化黑色页岩的主量元素分析为基础,利用单因素方差分析和多变量判别分析等方法,确定引起新鲜和风化黑色页岩化学组成差别的关键化学组分,并甄别其影响程度。建立影响黑色页岩风化程度的主量元素判别函数,进而构建厘定黑色页岩风化程度的化学风化指数。研究表明,引起新鲜和风化黑色页岩化学组成差别的主量元素,其影响程度依TiO_2Al_2O_3Fe_2O_3LOIMnOCaOSiO_2MgOK_2ONa_2OP_2O_5顺序而降低。以此为基础,构建厘定黑色页岩风化程度的化学风化指数为:WB=28.8×ln(SiO_2)+16.67×ln(TiO_2)+10.52×ln(Al_2O_3)+5.62×ln(Fe_2O_3)-2.01×ln(MgO)+4.10×ln(CaO)-4.24×ln(K_2O)-5.06×ln(Na_2O)+5.07×ln(LOI)-158.13。该风化指数(WB)能克服现有各风化指数的不足,适合用于厘定黑色页岩风化程度。     

黔北地区下寒武统底部黑色页岩沉积环境条件与源区构造背景分析

为了深入探讨黔北地区下寒武统底部黑色页岩的沉积环境条件和源区构造背景,对黔北地区下寒武统牛蹄塘组黑色页岩进行系统采样,对其微量元素和稀土元素地球化学特征进行了系统分析。研究表明,微量元素中高的WV/(WNi+WV)和U/Th比值等反映该区黑色页岩形成于海域开阔的还原环境。区内黑色页岩样品中Mo、Sb、U、Cd、 V、Ba、Tl、Ni、W、 Cr、Cs、Cu、Zn、Bi等元素的高富集以及正Eu异常和较低的Co/Zn比值均反映了黔北下寒武统底部黑色页岩受到深部热液活动的影响。根据主、微量和稀土元素组合及比值特征可以看出黔北地区下寒武统底部黑色页岩的源岩具有花岗岩、沉积岩和玄武岩等多成因性质。源区构造背景以被动大陆边缘为主,由于受深部热液活动的影响,也显示大陆岛弧构造背景的特征。     

铂族元素的表生地球化学研究进展

基性-超基性岩及黑色页岩等风化过程中铂族元素发生了活化和迁移。铂族元素在岩体附近土壤、植物和水体等表生介质中的迁移富集特征可以用来寻找岩浆型铂族元素矿床。黑色页岩风化过程中，Os的表生迁移对海洋^187Os／^188Os值上升有重要意义。此外，作为重金属元素，机动车铂族元素催化剂在环境中的释放，也会带来潜在的生态环境和人体健康问题。     

黑色页岩风化过程中元素的活动性及参照系的选取初探--以湖南省麻田、桃花江剖面为例

系统考察了湖南省麻田和桃花江两个不同发育程度的下寒武统黑色页岩风化剖面,并提出了元素在理想剖面中的分布模式(概念模型);通过对比分析这两个剖面中元素的分布特征与理想分布模式的关系,并结合元素相关性研究、等质量图解的判别分析、微量元素的赋存状态及地球化学性质的探讨,认为在黑色页岩风化过程中,Ti为不活动元素,可作为参照系来计算其它元素的相对迁移性,Nb、Ta的活动性较小,Zr与Hf之间存在着共迁移现象;研究表明化学风化程度是影响元素活动性的重要因素.     

湘中桃江锰矿废矿堆环境地球化学分析

对湘中桃江锰矿废矿堆的废石进行了主量元素、稀土元素、微量元素和重金属元素的地球化学分析。野外观察及分析结果表明:组成废矿堆的岩石主要是赋矿围岩中奥陶统黑色页岩和原生碳酸锰矿石。这些废石富含Cu、Pb、Zn、Cr、Tl、Sb、U等重金属元素。废石暴露地表而遭受风化分解,导致Sc、V、Cr、U、Cd、Th等重金属元素淋滤释出,在废石样品中均表现出不同程度的迁移特征,且以V、Cd、U的迁移性最为强烈。此外,黑色页岩中Ni、Cu、Zn、Pb、Tl、Sb也明显发生淋失。这些重金属元素如Cd、Tl等毒性极强,进入矿区周围不断积聚,便可能对环境造成严重的影响。故对区内分布的废矿堆作为重金属污染源应高度重视。     

湘西下寒武统黑色页岩风化元素活动规律

为探讨黑色页岩风化过程中的元素活动规律,采集了湘西东坪、柑子坪下寒武统黑色页岩风化-成土剖面的样品进行了主、微量元素地球化学分析。结果表明,两剖面风化层有不同的主量元素组成,但其土壤层的主量元素组成相对均一。主量元素Si、K、Na、P等在风化过程中活泼而被淋滤释出;而Al、Fe、Mg等元素既可被淋滤释出,又可产生次生富集。而成土过程中,所有主量元素都表现被淋出的特征。元素Mn的次生富集明显;重金属V、Ni、Cu、Cr、Co、Zn、Pb等在风化-成土过程中的化学活性与Al、Fe等主量元素相似;亲石元素Sc、Rb、Sr、Cs、Ba、Th、U、Ga、Ge等在风化-成土过程中的活性较强,趋于被淋出,且其淋滤释出率随风化程度增强而升高;高强场元素Zr、Hf、Nb、Ta等的化学活性相对稳定。     

Geochemistry of organic matter and elements of black shale during weathering in Northern Guizhou,Southwestern China: Their mobilization and inter-connection

Different from previous studies on effect of weathering upon geochemical variation along a single weathered profile, this paper provides a new methodology validated by comparing a weathered outcrop samples and their stratigraphic counterpart un-weathered core samples in a nearby shallow borehole. This outcrop and borehole penetrated the Ordovician-Silurian Wufeng–Longmaxi shales, located in the same anticline structure in the northern part of Guizhou Province, Southern China. The mineral composition, major, trace and rare earth elements (REEs) composition and Rock-Eval parameters of outcrop and core samples were analyzed and compared. Organic matter (OM) was observed in the microscope and extracted for elements analysis. The results show that short-term weathering still has significant influence on OM, mineral and elemental composition of black shales. The elements composition shows the outcrop profile was moderately weathered. The REEs compositions do not alter much during weathering process and the REEs composition and their relative ratios still are valid for rock origin determination. The OM, mainly composed by graptolite and bitumen, even entering the highly-over thermal maturity, is still sensitive to the weathering with a systematic loss 30–50% of TOC along the outcrop profile, which suggests that the OM consumption is predominantly controlled by weathering duration and the distance from the weathering surface. In turn, OM has significant influence on the trace elements transportation behavior during weathering. Some trace elements associated with the OM such as V, Cr, Th, U, Ni and Co, change significantly in their absolute concentration during weathering, but their relative ratios do not necessarily change too much and might be still reliable proxies for paleo-environmental determination. The mobility of shale minerals during weathering is in the following order: plagioclase?>?potassium feldspar and dolomite >pyrite and OM. Short-term weathering can also result in considerable transportation of elements and significant variation of minerals content in black shale, which may pose potentially high environmental and engineering risk in the regions rich in black shale.     

Release of heavy metals during weathering of the Lower Cambrian Black Shales in western Hunan,China

Weathering of heavy metal enriched black shales may be one of the most important sources of environmental contamination in areas where black shales are distributed. Heavy metal release during weathering of the Lower Cambrian Black Shales (LCBS) in western Hunan, China, was investigated using traditional geochemical methods and the ICP-MS analytical technique. Concentrations of 16 heavy metals, 8 trace elements and P were measured for samples from selected weathering profiles at the Taiping vanadium ore mine (TP), the Matian phosphorous ore mine (MT), and Taojiang stone-coal mine (TJ). The results show that the bedrock at these three profiles is enriched with Sc, V, Cr, Co, Ni, Cu, Zn, Pb, Th, U, Mo, Cd, Sb, Tl, and P. Based on mass-balance calculation, the percentages of heavy metals released (in % loss) relative to immobile element Nb were estimated. The results show significant rates of release during weathering of: V, Cr, Co, Ni, Cu, Zn, U, Mo, Cd, Sn, Sb, and Tl for the TP profile; Sc, Cr, Mn, Co, Ni, Cu, Zn, Pb, Th, Cd, and Sn for the MT profile; and Sc, Mn, Co, Ni, Zn, Th, Cd, Sn, and Tl for the TJ profile. Among these heavy metals, Co, Ni, Zn, Cd, and Sn show very similar features of release from each of the three weathering profiles. The heavy metals released during weathering may affect the environment (especially topsoil and surface waters) and are possibly related to an observed high incidence of endemic diseases in the area.     

浙江中部典型黑色岩系分布区土壤-作物富硒特征与重金属风险评价

富硒土地是生产富硒农产品的宝贵资源,黑色岩系形成的土壤通常富含硒和重金属元素。为了指导这类富硒土地资源的安全开发利用,选择浙江中部典型黑色岩系分布区,通过调查土壤-作物中硒和重金属的浓度及生物有效性,评价农产品富硒率,识别作物重金属超标风险。结果显示:研究区富硒土壤占38%,足硒土壤占62%,水稻和莲子富硒率分别为85.71%和100%;受黑色岩系风化成土影响,研究区土壤镉含量介于污染筛选值和风险管控值之间的样品比例为26%,且Cd活动态和潜在活动态所占比例分别为57%和40%,水稻籽实中Cd超标率为31%,莲子Cd含量均不超标。基于土壤重金属总量和形态的风险评价表明,富硒土壤开发利用的风险区主要位于西皮山岗和腰塘边一带,建议禁止种植农产品,其他黑色岩系发育的富硒丘陵地区,虽然土壤镉含量较高,但因土壤Cd的生物有效性和水稻中Cd的超标率较低,应加以保护和合理开发利用。     

Tellurium,selenium and cobalt enrichment in Neoproterozoic black shales,Gwna Group,UK: Deep marine trace element enrichment during the Second Great Oxygenation Event

Black shales of the late Neoproterozoic Gwna Group (570–580 Ma), UK, contain enrichments of tellurium (Te), selenium (Se) and cobalt (Co) relative to average shale compositions. The Te and Co enrichments bear comparison with those of ferromanganese crusts in the modern deep ocean. Gwna Group deposition coincides with the Second Great Oxidation Event, which had a significant effect on trace element fixation globally. Selenium and Te concentrations within these black shales indicate increased continental weathering rates, high biological productivity and corresponding increases in atmospheric O₂ concentrations. Cobalt, nickel (Ni) and arsenic (As) enrichments in this succession are secondary mineralisation phases. Demand for many of the trace elements found enriched in the Gwna Group black shales make their mechanisms of accumulation, and variations through the geological record, important to understand, and suggests that new resources may be sought based on black shale protoliths from this period.     

贵州织金下寒武统黑色页岩矿物岩石成分及资源利用特征

通过野外采样、化学分析、电子探针(EPMA)和X射线衍射(XRD)分析等手段,研究了贵州织金地区黑色页岩矿物成分、化学组分、微量元素、稀土元素特征。研究区矿样化学成分以SiO_2和Al_2O_3为主,且具有高K低Na的特征。电子探针和X射线衍射分析表明,研究区黑色页岩主要矿物组成有石英、粘土矿物、白云石及黄铁矿等。织金黑色页岩中Pb、Ni、U、V、Cr等金属元素存在不同程度的富集,稀土元素总量为153.2×10~(-6)~224.89×10~(-6),属轻稀土元素富集型。同时从多金属层、页岩气、页岩提钾及近底部含磷铀矿资源等方面讨论了织金黑色页岩资源化利用。织金黑色页岩多金属层含有Mo、V、Ni、Ag及U等多金属元素,具综合利用价值;其中有机碳含量达到页岩气开发大于2%的条件,可进一步开展研究;页岩中伊利石含量较多,可提取黑色页岩中的钾制备含钾复合肥;黑色页岩底部与磷矿层接触带产出磷铀矿,主要为胶状磷铀矿,接触带可作为铀矿找矿的标志层。     

Geochemistry of soils derived from black shales in the Ganziping mine area, western Hunan, China

The geochemistry of major and trace elements (including heavy metals and rare earth elements) of the fresh and weathered black shales, and the soils derived from black shales in the Ganziping mine area in western Hunan province (China) were studied using the following techniques: X-ray fluorescence (XRF), inductively coupled plasma mass spectrometer (ICP-MS) and X-ray diffraction (XRD). The results show that the black-shale soils are significantly enriched with Al₂O₃ and Fe₂O₃, and depleted of mobile elements CaO, Na₂O and K₂O. The soils are also highly enriched with heavy metals U, V, Ni, Ba, Cu, Zn and Pb, that may cause potential heavy-metal contamination of the soils. Composition of the soils is homogeneous compared to the weathered black shales, for which the concentrations of major elements except CaO and Na₂O, and trace elements except heavy metals (U, V, Ni, Ba, Cu, Zn and Pb) as well as the mobile Sr, show lower variations than in the weathered black shales. Ratios of Zr/Hf, Ta/Nb, Y/Ho, Nd/Sm, and Ti/(Ti + Zr), of the soils are also less variable, with values constantly similar to that of the fresh and weathered black shales correspondingly. Thus, components of the soils are believed to be contributed from the parent black shales through weathering and pedogenesis. It is concluded that the soils were formed by at least two stages of geochemical processes: the early stage of chemical differentiation and the later stage of chemical homogenization. The chemical differentiation that was taken during black-shale weathering might have caused the depletion of CaO and Na₂O, and the enrichment of Al₂O₃ and Fe₂O₃; while the chemical homogenization that was taken during pedogenesis led to the depletion of SiO₂ and K₂O, and to the further enrichment of Al₂O₃ and Fe₂O₃. The heavy-metal enrichment (contamination) of the soils was then genetically related to the enrichment of Al₂O₃ and Fe₂O₃ in the soils.     

Weathering characteristics of the Lower Paleozoic black shale in northwestern Guizhou Province,south China

The northwestern Guizhou in the Yangtze Craton of south China has a tremendous potential of shale gas resource. In this paper, we present results from major and trace elements, total organic carbon, mineralogical composition analysis and petrophysical parameters to characterise shale weathering features. Further, the differences of black shale between underground and outcrops have also been presented to examine the changes of black shale after weathering. Our results show that the trace elements of shale have varying degrees of loss in the weathering leaching process, both in Niutitang shale and Longmaxi shale, the loss of B, V, Ni, Cu, Zn and Ba is obvious, but the element migration quantity in the former is greater than in the latter. Decomposition of minerals such as pyrite, feldspar and calcite result in the leaching of Na, Ca, Mg and Fe. The loss rate of total organic carbon (TOC) in black shales ranges from 18% to 70% with an average of 43%; moreover, the loss of organic carbon in samples with high TOC content is larger than in those samples with low TOC content. Results following the testing of porosity and permeability show that porosity increases significantly after weathering but permeability changes little. Furthermore, the increment of porosity is greater in the Niutitang shale (with more sulphide minerals) than in the Longmaxi shale, suggesting that the oxidation of sulphide minerals may have led to the formation of an acidic environment, causing the other minerals in the black shale to weather more quickly, thus resulting in increased porosity. The content of clay minerals in the core samples is slightly lesser than the outcrop samples, but the TOC content in the core samples is greater and has a larger specific surface area. This suggest that the TOC content played a decisive role on the specific surface area of shale. In addition, changes in the black shale caused by the weathering process mainly depend on the mineral composition and the TOC content in shale. In this study, we try to establish relations between outcrop samples and core samples, in order to better understand the underground characteristics of shale reservoir.     

Mineralogical and geochemical constraints on environmental impacts from waste rock at Taojiang Mn-ore deposit,central Hunan,China

The mineralogy and geochemistry of the waste rocks distributed at Taojiang Mn-ore deposit, central Hunan province, China, were studied using X-ray powder diffraction (XRD), electron microprobe analysis (EMPA) fitted with energy dispersive spectrometer (EDS) and inductively coupled plasma mass spectrum (atomic emission spectra) ICP-MS (AES), with the aim of predicting the environmental impacts of weathering of the waste rocks. The mineralogical results from microscope observation and XRD and EMPA studies show that the waste rock is composed of black shale and minor Mn carbonates. The oxidation of sulfide minerals such as galena, pyrite and chalcopyrite is accompanied by decomposition of Mn carbonates and K-feldspar during exposure to atmospheric O₂. The geochemical characteristics of major, rare earth elements (REE) and trace elements of the waste rocks also show that the waste rock can be divided into black shale and Mn carbonate, and both of them are currently under chemical weathering. The major alkalies and alkaline elements (Ca, Mg, Na, K, Rb, Sr and Cs) and major elements (Fe, S and P) and heavy metals (Sc, V, Cr, Th, U, Sn, Co, Ni, Cu, Zn, Pb, Mo, Cd, Sb, an Tl) are being released during weathering. The mobility of alkalis and alkaline elements Ca, Mg, Na, K, Rb, Sr and Cs is controlled by decomposition of Mn carbonates. The dispersion of Cr, Sc and Th (U) might be related to weathering of K-feldspar, and the release of the heavy metals Co, Ni, Cu, Zn, Pb, Mo, Cd Sb and Tl is dominated by the breaking of sulfide minerals. The REE of the waste rocks and surrounding soils and the spidery distribution patterns of heavy metals in the waste rocks, the surrounding soils and the surface waters show that weathering of the waste rocks and bedrock might be the sources of heavy metal contamination for the surrounding soils and surface water system for the mining area. This is predicted by the mass-balance calculation by using Zr as an immobile element. Therefore, it is urgently necessary take measures to treat the waste rocks distributed throughout the area for the local environmental protection.     

渝东北黑色页岩元素迁移特征及化学风化程度

黑色页岩是富含有机质和硫化矿物的特殊沉积岩,但人们对其风化过程的元素活动性及矿物风化机制关注较少.为探讨不同地形位置的黑色页岩化学风化过程,采集了渝东北城口某山脊 (A)、近山顶 (B) 和沟谷 (C) 的下寒武统水井沱组黑色页岩风化剖面岩样,利用XRD、XRF和化学分析手段对采集样品的矿物成分、主量元素进行测定分析.元素和矿物的质量迁移系数 (τ) 和质量迁移通量 (M_{j, flux}) 的计算结果表明,黑色页岩风化过程中Ca、Mg和Na元素具有明显的贫化现象,近地表处存在Al元素的富集现象;矿物成分方面,黄铁矿和有机质氧化后形成的酸性水环境,造成方解石、白云石、斜长石等不稳定矿物溶解,并生成含水石膏、铁质氧化物、黏土矿物等次生矿物.不同赋存位置的黑色页岩风化程度有所差异,Na/K-CIA、K/Ca*-Al/Na、A-CN-K和A-CNK-FM图解显示:A剖面处于脱Ca过程的初级风化阶段,B剖面处于脱Ca、Na初期的初等-中等风化阶段,C剖面已发生脱Ca、Na过程,并伴随脱Si作用的中等-强烈风化阶段,结合不同风化指数 (如:CIA、CIW、PIA、MWPI等),得出各剖面的化学风化强弱程度依次为C＞B＞A.      

Provenance,tectonic setting,source weathering and palaeoenvironmental implications of Middle-Upper Jurassic rocks of Ler dome,Kachchh, western India: Inferences from petrography and geochemistry

The Middle-Upper Jurassic sandstones and shales of Ler dome (Chari and Katrol Formations), Kachchh, western India, have been analyzed for modal, bulk mineralogy and geochemistry to deduce their provenance, tectonic setting, source area weathering and palaeoenvironmental conditions. The detrital modes of Ler dome sandstones indicate that they were emanated from recycled orogen (uplifted shoulders of rift) and stable cratonic source in passive margin setting. Rapid deposition of sediments from a granitic source area can be predicted from feldspar abundance. A highly mature heavy mineral assemblage characterized in the form of high Zircon-Tourmaline-Rutile (ZTR) index also endorses these findings. The X-ray diffraction patterns (XRD) and scanning electron microscope (SEM) data show the presence of clay minerals depicting moderate to extensive chemical weathering in an oxidizing environment with periodic cycles of transgression and regression. The chemical index of weathering, chemical index of alteration and plagioclase index of alteration suggest moderate to high and low to moderate weathering conditions for sandstone and shales, respectively, that took place in low to moderate relief. We postulate that Ler dome sediments are derivative of the eroded and weathered parts of the Aravalli craton located on east and northeast of the basin and the Nagarparkar Massif placed to the north and northwest.