成都理工大学学报（自然科学版）
成都理工大學學報（自然科學版）
성도리공대학학보（자연과학판）
Journal of Chengdu University of Technology (Science & Technology Edition)
2015年
5期
565-578
,共14页
黄可可%李小宁%胡作维%钟怡江%黄思静
黃可可%李小寧%鬍作維%鐘怡江%黃思靜
황가가%리소저%호작유%종이강%황사정
北碚地区%嘉陵江组%碳酸盐岩%碳同位素%成岩蚀变评估
北碚地區%嘉陵江組%碳痠鹽巖%碳同位素%成巖蝕變評估
북배지구%가릉강조%탄산염암%탄동위소%성암식변평고
Beibei area%Jialingjiang Formation%carbonate rock%carbon isotope%evaluation for diagenetic alteration
早三叠世海水的碳同位素组成是近年来地学研究的热点之一，但全球早三叠世碳同位素组成并没有获得统一的认识。上扬子地区早三叠世晚期普遍发育蒸发岩，地表剖面则因表生成岩作用而形成一套夹盐溶角砾岩及次生灰岩的碳酸盐地层，其地球化学信息不同程度地偏离海水，因而样品成岩蚀变性对古海水碳同位素组成的代表性的评估显得十分重要。本文以中、下三叠统界线清楚的重庆北碚剖面为对象，研究了下三叠统嘉陵江组第三、第四段和中三叠统雷口坡组底部样品的元素组成，碳、氧同位素组成及彼此之间的相互关系，选择 Mn 含量和氧同位素作为条件评价碳酸盐岩的成岩蚀变性及其对海水碳同位素组成的代表性，在此基础上建立了早三叠世晚期海水的碳同位素演化曲线。研究表明：（1）北碚剖面嘉陵江组第三、第四段（嘉三、嘉四段）及雷口坡组底部碳酸盐岩的 Mn、Fe 含量与碳同位素组成之间具有良好的负相关性，氧同位素组成和碳同位素组成之间则具有良好的正相关性，大量样品具有低δ18 O 值、低δ13 C 值以及较高 Mn、Fe 含量的特征，说明其碳同位素组成偏离海水。（2）以δ18 O＞－7．5‰、方解石的 Mn 质量分数＜100×10－6、白云石的 Mn 质量分数＜120×10－6为评价条件，北碚剖面嘉三、嘉四段（含雷口坡组底部）的73个样品中有38个需要在建立海水碳同位素曲线时剔除，占全部样品的52％。其中嘉四段需要剔除的样品占嘉四段样品的78％，而嘉三段的该数值为11％，说明作为原始沉积的灰岩段对海水地球化学信息保存较好，而分布盐溶角砾岩及次生灰岩的层位对海水地球化学信息保存很差。（3）氧同位素作为成岩蚀变评价标准比 Mn 含量更为苛刻，并可基本覆盖 Mn 含量评价条件，说明对于嘉三—嘉四段这样的石灰岩—白云岩—蒸发岩地层的地表剖面的样品来说，选择氧同位素作为评价条件可较好地对碳同位素的原生性进行控制。（4）白云岩对海水地球化学信息总体上保存较好，与相邻石灰岩相比，白云岩具有更高的δ18 O 和δ13 C 值，但其原因仍然需要进一步研究，可能与同位素分馏系数的差别以及白云岩形成于盐度更高的流体有关，因而白云岩所代表的海水地球化学信息可能不完全与伴生石灰岩相同。（5）除绿豆岩附近出现了短时间的变化超过4‰的大幅度波动以外，早三叠世晚期海水的碳同位素演化相对平稳，大致在－0．3‰～－1．8‰的范围内波动，呈现一个低幅的“V”字形，显示早三叠世晚期地球生态的恢复与碳循环系统的重建。
早三疊世海水的碳同位素組成是近年來地學研究的熱點之一，但全毬早三疊世碳同位素組成併沒有穫得統一的認識。上颺子地區早三疊世晚期普遍髮育蒸髮巖，地錶剖麵則因錶生成巖作用而形成一套夾鹽溶角礫巖及次生灰巖的碳痠鹽地層，其地毬化學信息不同程度地偏離海水，因而樣品成巖蝕變性對古海水碳同位素組成的代錶性的評估顯得十分重要。本文以中、下三疊統界線清楚的重慶北碚剖麵為對象，研究瞭下三疊統嘉陵江組第三、第四段和中三疊統雷口坡組底部樣品的元素組成，碳、氧同位素組成及彼此之間的相互關繫，選擇 Mn 含量和氧同位素作為條件評價碳痠鹽巖的成巖蝕變性及其對海水碳同位素組成的代錶性，在此基礎上建立瞭早三疊世晚期海水的碳同位素縯化麯線。研究錶明：（1）北碚剖麵嘉陵江組第三、第四段（嘉三、嘉四段）及雷口坡組底部碳痠鹽巖的 Mn、Fe 含量與碳同位素組成之間具有良好的負相關性，氧同位素組成和碳同位素組成之間則具有良好的正相關性，大量樣品具有低δ18 O 值、低δ13 C 值以及較高 Mn、Fe 含量的特徵，說明其碳同位素組成偏離海水。（2）以δ18 O＞－7．5‰、方解石的 Mn 質量分數＜100×10－6、白雲石的 Mn 質量分數＜120×10－6為評價條件，北碚剖麵嘉三、嘉四段（含雷口坡組底部）的73箇樣品中有38箇需要在建立海水碳同位素麯線時剔除，佔全部樣品的52％。其中嘉四段需要剔除的樣品佔嘉四段樣品的78％，而嘉三段的該數值為11％，說明作為原始沉積的灰巖段對海水地毬化學信息保存較好，而分佈鹽溶角礫巖及次生灰巖的層位對海水地毬化學信息保存很差。（3）氧同位素作為成巖蝕變評價標準比 Mn 含量更為苛刻，併可基本覆蓋 Mn 含量評價條件，說明對于嘉三—嘉四段這樣的石灰巖—白雲巖—蒸髮巖地層的地錶剖麵的樣品來說，選擇氧同位素作為評價條件可較好地對碳同位素的原生性進行控製。（4）白雲巖對海水地毬化學信息總體上保存較好，與相鄰石灰巖相比，白雲巖具有更高的δ18 O 和δ13 C 值，但其原因仍然需要進一步研究，可能與同位素分餾繫數的差彆以及白雲巖形成于鹽度更高的流體有關，因而白雲巖所代錶的海水地毬化學信息可能不完全與伴生石灰巖相同。（5）除綠豆巖附近齣現瞭短時間的變化超過4‰的大幅度波動以外，早三疊世晚期海水的碳同位素縯化相對平穩，大緻在－0．3‰～－1．8‰的範圍內波動，呈現一箇低幅的“V”字形，顯示早三疊世晚期地毬生態的恢複與碳循環繫統的重建。
조삼첩세해수적탄동위소조성시근년래지학연구적열점지일，단전구조삼첩세탄동위소조성병몰유획득통일적인식。상양자지구조삼첩세만기보편발육증발암，지표부면칙인표생성암작용이형성일투협염용각력암급차생회암적탄산염지층，기지구화학신식불동정도지편리해수，인이양품성암식변성대고해수탄동위소조성적대표성적평고현득십분중요。본문이중、하삼첩통계선청초적중경북배부면위대상，연구료하삼첩통가릉강조제삼、제사단화중삼첩통뢰구파조저부양품적원소조성，탄、양동위소조성급피차지간적상호관계，선택 Mn 함량화양동위소작위조건평개탄산염암적성암식변성급기대해수탄동위소조성적대표성，재차기출상건립료조삼첩세만기해수적탄동위소연화곡선。연구표명：（1）북배부면가릉강조제삼、제사단（가삼、가사단）급뢰구파조저부탄산염암적 Mn、Fe 함량여탄동위소조성지간구유량호적부상관성，양동위소조성화탄동위소조성지간칙구유량호적정상관성，대량양품구유저δ18 O 치、저δ13 C 치이급교고 Mn、Fe 함량적특정，설명기탄동위소조성편리해수。（2）이δ18 O＞－7．5‰、방해석적 Mn 질량분수＜100×10－6、백운석적 Mn 질량분수＜120×10－6위평개조건，북배부면가삼、가사단（함뢰구파조저부）적73개양품중유38개수요재건립해수탄동위소곡선시척제，점전부양품적52％。기중가사단수요척제적양품점가사단양품적78％，이가삼단적해수치위11％，설명작위원시침적적회암단대해수지구화학신식보존교호，이분포염용각력암급차생회암적층위대해수지구화학신식보존흔차。（3）양동위소작위성암식변평개표준비 Mn 함량경위가각，병가기본복개 Mn 함량평개조건，설명대우가삼—가사단저양적석회암—백운암—증발암지층적지표부면적양품래설，선택양동위소작위평개조건가교호지대탄동위소적원생성진행공제。（4）백운암대해수지구화학신식총체상보존교호，여상린석회암상비，백운암구유경고적δ18 O 화δ13 C 치，단기원인잉연수요진일보연구，가능여동위소분류계수적차별이급백운암형성우염도경고적류체유관，인이백운암소대표적해수지구화학신식가능불완전여반생석회암상동。（5）제록두암부근출현료단시간적변화초과4‰적대폭도파동이외，조삼첩세만기해수적탄동위소연화상대평은，대치재－0．3‰～－1．8‰적범위내파동，정현일개저폭적“V”자형，현시조삼첩세만기지구생태적회복여탄순배계통적중건。
The carbon isotopic composition of seawater during Early Triassic is recently one of the hot topics in the geological research field,but the recognition on the Early Triassic C isotopic composition in the world is not consistent.In the Upper Yangtze region,the upper part of the Lower Triassic sequence is dominated by evaporates.There forms a suit of carbonate rocks strata mixed with secondary limestone and evaporite-solution breccia in the outcrop section during epidiagenesis.The geochemical signatures in the altered rocks deviate from the coeval seawater. Therefore, the evaluation of diagenetic alteration for the representativeness as the C isotopic composition of paleo-seawater is very important.Based on the study of the constitution and the interrelationship of the elements,the carbon and oxygen isotopic compositions of carbonate rocks from the Beibei section of Chongqing,the Mn content and oxygen isotopic composition are selected as the factors to evaluate the diagenetic alteration of the carbonate rocks and the representativeness as primary carbon isotope in order to establish the C isotopic profile of the late stage of Early Triassic.The results are as follow. (1 )There is negative correlation between Mn vs.δ13 C and Fe vs.δ13 C,and positive correlation betweenδ18 O vs.δ13 C in the carbonate rocks in Member 3 & Member 4 of Jialingjiang Formation and the bottom of Leikoupo Formation.This suggests that a number of samples with high content of Mn and Fe and lowδ18 O also have a lowδ13 C value which are obviously deviates from the coeval seawater. (2)Taking the mass fraction of Mn <100 ×10 -6 ~120 ×10 -6 andδ18 O>-7.5‰ as the evaluation factors,there are 38 samples must be winkled out from the total 73 samples with a percent of 52%. Among them,32 samples are winkled out from Member 4 of Jialingjiang Formation with a percent of 78% and 3 samples are winkled out from Member 3 of Jialingjiang Formation with a percent of 11%. This shows that the original seawater geochemical information is preserved better in the primary depositional limestone but worse in the secondary limestone and the evaporite-solution breccia.(3 ) Oxygen isotope as a factor to evaluate the diagenetic alteration is stricter than Mn content, consequently,and the data winkled by oxygen isotope overlap that by Mn content basically.This indicates that taking oxygen isotope as a factor can more rigorously control the primary nature of carbon isotope for the samples from the outcrop section of the limestone-dolomite-evaporate sequence. (4)The preservation of the original seawater geochemical information in dolomite is normally better than that in limestone,since theδ18 O andδ13 C of dolomite are higher than those of limestone.The possible reasons for that include the difference of the isotopic fractionation efficients between dolomite-fluid and calcite-fluid,and/or the difference of seawater salinity between dolomite and limestone formed,which suggests that the dolomite may originate from the seawater with higher salinity than the limestone.(5 )δ13 C trend of the seawater at the end of Early Triassic shows a relatively stable and gradual V-shaped variation with a relatively narrow fluctuation range of -0.3‰~ -1.8‰,except a large δ13 C fluctuation with an amplitude over 4‰ in the samples nearby the mung bean rocks.This indicate an ecosystem recovery and reconstruction.
