松辽盆地白垩纪火山期后热液活动的岩石地球化学和年代学及其地质意义

Chronology,petrology and geochemistry of the Cretaceous crypto-explosive breccia-bearing volcanic rocks:Implications for volcanic reservoir and tectonics of the Songliao Basin,NE China

在松辽盆地东南隆起区营城组标准剖面营三段古火山口附近识别出岩浆期后热液活动的地质记录。岩石学特征表现为隐爆角砾岩,即,原有的近火山口相岩石(原岩)被高压流体炸碎形成原地角砾、之后又被灌入的富含矿物质"岩汁"胶结形成的原地角砾岩。采集隐爆角砾岩及其上覆和下伏三个层位共10个样品进行地球化学和年代学对比研究,包括两套4个对应的原岩和岩汁、下伏4个玄武粗安岩和上覆2个流纹岩。原岩高精度^(39)Ar/^(40)Ar坪年龄113.4±0.7Ma;岩汁为含铁酸性流体的隐晶质析出物,其^(39)Ar/^(40)Ar坪年龄112.9±19.6Ma;二者的年龄差反映岩浆主期与期后热液活动的时代间隔(1Ma之内)。10个样品的共性是:(1)稀土总量中-高(∑REE=81×10^(-6)～202×10^(-6))且轻重稀土分异明显((La/Yb)_N=4.91～11.45);(2)MORB标准化蛛网图上P和部分不相容元素(Cs、Th、La和Zr)正异常,而另一部分不相容元素(Sr和Pb)负异常。整体表现为双峰式裂谷火山岩特点。主要差别在于:(1)铕和钡表现为两种情况,下伏玄武粗安岩和隐爆角砾岩中的粗面岩具正异常,而隐爆角砾岩中的流纹岩和岩汁及其上覆的流纹岩具负异常;(2)只有2个岩汁样品显示K负异常,其它8个岩石样品均为K正异常;(3)下伏玄武粗安岩(4个样)显弱的Ti正异常且Rb/Sr比低(0.04～0.05),而其它6个样品为Ti负异常且Rb/Sr比高(0.62～2.83),其中的2个岩汁样品Ti负异常最强。岩汁与其下伏粗面质原岩差别显著,而与其上覆流纹岩(SHRIMP年龄110.6Ma)的地化特征相似(见正文)。该火山期后热液活动是深源热流体萃取壳源物并沿古火山通道(构造薄弱带)运移到近地表的,可能是后续流纹质火山活动的先驱。这种高压的岩浆期后热液导致围岩炸裂、发生角砾岩化、形成大量角砾间孔和裂缝。这是造成火山口-近火山口相带成为优质储层的重要因素。该类火山岩储层改善作用早于烃类运移,可构成有利于成藏的时空配置。与该期热液活动相伴生的深源天然气早于上覆圈闭的形成,因此对成藏没有贡献。

Hydrothermal breccias have been recognized within a Cretaceous volcanic center in the stratotype succession of Yingcheng Formation(K_1y) outcroped in southeast uplift of the Songliao basin.The autobreccias are composed of two counterparts,in-situ volcanic fragments cemented by hydrothermal deposits.The former is trachyte and rhyolite and the latter is very fine-grained red deposits precipitated from the Fe-rich silica hydrothermal fluids.We separated the red deposits from two kinds of in-situ volcanic breccias(4 samples) and analyzed them respectively on Ar/Ar age,major and trace elements and REE.Four underlying basaltic trachyandesites and two overlying rhyolites were simultaneously determined on all the elements above and SHRIMP for one rhyolite.The precise ^(39)Ar/^(40) Ar age of the in-situ trachyte fragment is 113.4±0.7Ma,and that of the corresponding red deposit is 112.9±19.6Ma, suggesting a time span of ca.1Ma between trachy-lava event and the subsequent hydrothermal activity.Zircon SHRIMP age of the overlying rhyolite is 110.6Ma.The common signatures for all these 10 samples are rich in LREE((La/Yb)_N = 4.91-11.45)and P as well as such incompatible elements as Cs,Th and Zr but depleted in some other incompatible elements like Sr and Pb.Main differences among them are,(1) Eu and Ba show positive anomaly for the in-situ trachyte fragment and 4 underlying basaltic trachyandesites,but negative for the five other ones;(2) only the two red deposits filled in the hydrothermal breccias show negative K-anomaly while the other 8 ones show positive K-anomaly;(3) the 4 underlying basaltic trachyandesites show positive Ti anomaly and low Rb/Sr ratios(0.04-0.05)while the 6 other samples show negative Ti anomaly and high Rb/Sr ratios(0.62-2.83).They are bimodal rift volcanics as a whole.The hydrothermal deposits show intimate geochemical prints with the following rhyolite,and are significant different from the pre-existed trychyte and basaltic trachyandesite.Therefore the crypto-explosion coupled with Fe-rich silica hydrothermal fluids are preferred to be explained as forerunner of the subsequent rhyolitic lava,and the thermal fluids gained Fe-bearing materials by leaching all the penetrated rocks while ascending.The subsequent crypto-explosion occurred preferentially within volcanic centers because of their structural weakness,and created many cracks and inter-granular space resulting in increase in porosity and permeability of the volcanic rocks.These can explain the fact that near volcano facies are favorable for the Cretaceous volcanic reservoir exploration in the basin.However,possible hydrothermal-derived natural gases of this period could not contribute to the present gas traps because they were formed before the formation of overlying seals.