岩浆作用过程的矿物记录:以邯邢地区中生代侵入岩为例

The mineralogical records of magmatic process:cases from Mesozoic intrusive rocks in the Handan-Xingtai region

矿物环带结构是岩浆过程的良好记录,而矿物温压计可以定量计算岩浆的物理化学条件。该次研究对邯邢地区中生代侵入岩中两个浅成侵入体中典型的斜长石斑晶进行了电子探针和LA-ICP-MS元素面扫描,同时对其中不同成因的角闪石晶体进行了电子探针分析和物理化学条件计算。通过研究发现这两个样品中的斜长石斑晶属于循环晶。辉石闪长玢岩中的斜长石斑晶主要成分为更长石,而斑状石英二长岩中斜长石斑晶主要成分为中长石和拉长石。两者具有明显不同的环带结构,分别记录了不同的岩浆混合过程:前者记录了基性岩浆多次注入酸性岩浆房,并最终获得体积优势;后者记录了少量基性岩浆注入酸性岩浆房内,两者不断混合,形成均质化岩浆的过程。Fe元素从斜长石中心到边部有富集的趋势,表明岩浆最终向富铁的方向演化。角闪石温压计的计算结果表明,辉石闪长玢岩的岩浆房深度在5.5km左右,温度大约为850℃,氧逸度大约为△NNO+1,熔体中的含水量大约为4.0%,其最终侵位于2.1km的地壳深度。而斑状石英二长岩最终定位于2.0km的地壳深度,角闪石结晶的平均温度为737℃,相对氧逸度为△NNO+1.2,熔体含水量为3.9%。

The mineral zoning texture is a good recording of the magmatic process,and the physicochemical conditions of magma could be quantitatively calculated by mineral thermobarometer.In this study,we carried out electron microprobe and LA-ICP-MS element mapping of typical plagioclase phenocrysts of two Mesozoic hypabyssal intrusions in the Handan-Xingtai region,along with electron microprobe analysis and physicochemical condition calculation of amphibole crystals of different genesis.We found that plagioclase phenocrysts in these two samples belong to antecrysts.They are mainly composed of oligoclase in pyroxene diorite porphyrite and andesine and labradorite in porphyritic quartz monzonite.The two intrusions have obviously different zoning textures and record different magma mixing processes.The former records multiple injections of basic magma into acidic magma chamber to obtain eventual volume advantage;the latter records continuous mixing of small amounts of basic magma with large volume of acidic magma to form homogenized magma.There is a tendency of iron enrichment from the core to the rim of plagioclase phenocryst,indicating iron is gradually enriched with magma evolution.The amphibole thermobarometric calculation results show that the depth of magma chamber containing pyroxene diorite porphyrite was about 5.5 km,with a temperature of 850℃,oxygen fugacity of△NNO+1,and water content of 4.0 wt%.The intrusion finally emplaced at a crust depth of 2.1 km.The porphyritic quartz monzonite intrusion finally emplaced at a crust depth of 2.0 km,with an average hornblende crystallization temperature of 737℃,relative oxygen fugacity of about△NNO+1.2,and water content in melt of 3.9 wt%.