燕山中段常州沟组砂岩岩石学、地球化学特征及地质意义

Petrological characteristics,geochemical characteristics,and geological significance of sandstones in Changzhougou Formation,central sector of the Yanshan Mountains

中元古代常州沟组是华北克拉通北缘燕山地区变质结晶基底上的第一套沉积盖层,不整合覆盖于新太古代各类片麻岩之上。为了解常州沟组时期的古环境、古气候及其物质来源、构造环境和盆地性质,在野外地质调查的基础上,对常州沟组砂岩进行了粒度分析和地球化学分析测试。岩石组合、沉积构造和粒度分析结果指示区内常州沟组沉积环境演化由下至上依次为冲积扇、辫状河和碎屑潮坪。常州沟组砂岩地球化学特征具有高SiO_(2)、Al_(2)O_(3)和K_(2)O,低TiO_(2)、Fe_(2)O_(3)、FeO、MgO含量的特点,成熟度较高,化学蚀变指数(CIA)介于52.00~73.23之间,化学风化作用指数(CIW)范围为93.04~98.68。由于岩石遭受钾交代作用影响,结合A-CN-K图解、Th/U-Th图解和古气候判别图综合分析,认为源区可能经历了较强的风化作用,古气候温暖、潮湿。Cr/Zr、Th/Sc、Ba/Sr、Rb/Sr值和La/Th-Hf图解指示常州沟组源区属性主要为上地壳环境,原岩成分以长英质岩石为主。构造背景判别图解显示常州沟组砂岩形成于克拉通盆地,即燕辽裂陷槽应属于克拉通内部裂解。另外,Tb/Yb、La/Sc、La/Th、Th/Sc值指示砂岩中无明显的幔源物质加入,推测燕辽裂陷槽应属于华北克拉通陆壳基底上形成的滨浅海,未达到大陆地壳裂开、洋壳出现的程度。

The Mesoproterozoic Changzhougou Formation is the first set of sedimentary cover on the metamorphic crystalline basement of the Yanshan area in the northern margin of North China Craton.It unconformably covers all kinds of Neoarchean gneiss.Based on the field geological survey,grain size analysis and geochemical analysis test were carried out on the sandstone of Changzhougou Formation,to disclose the paleoenvironment,paleoclimate,material source,tectonic environment,and basin properties during this period.The results of rock assemblages,sedimentary structure,and grain size analysis indicate that the sedimentary environment evolution of Changzhougou Formation is alluvial fans,braided rivers,and clastic tidal flats,from bottom to top.The geochemical characteristics of sandstones in Changzhougou Formation are characterized by high SiO_(2),Al_(2)O_(3),K_(2)O,and low TiO_(2),Fe_(2)O_(3),FeO,and MgO contents with high maturity.The chemical alteration index(CIA)varies from 52.00 to 73.23,and the chemical weathering index(CIW)ranges from 93.04 to 98.68.Due to the influence of potassium replacement,combined with A-CN-K diagram,Th/U-Th diagram,and paleoclimate discrimination diagram,the provenance may have experienced strong weathering,implying warm and humid paleoclimate.The ratios of Cr/Zr,Th/Sc,Ba/Sr,Rb/Sr,and La/Th-Hf diagrams indicate the provenance of Changzhougou Formation from the upper crustal environment,and the sediments were sourced predominantly from felsic rocks.The discrimination diagram of the tectonic setting shows that the sandstone of Changzhougou Formation was formed in the craton basin,that is,the Yanliao Aulacogen might be produced in the internal rift of the craton.In addition,the ratios of Tb/Yb,La/Sc,La/Th,and Th/Sc indicate a few obvious mantle materials in the sandstones.Therefore,the Yanliao Aulacogen is presumed to be the coastal⁃epicontinental sea formed on the continental crust basement of the North China Craton,not to the extent of continental crust rupture and oceanic crust occurrence.