吉林蛟河上地幔岩碎块内熔融微区矿物学及其地质意义

吉林蛟河地幔岩碎块是被碱性橄榄玄武岩岩浆喷发携带至地壳浅部或地表的。碱性橄榄玄武岩中地幔岩碎块含量40%~55%,局部达60%以上;碎块大小不等,一般直径以5~10 cm居多,大者达20~35 cm,故定名为地幔岩集块熔岩(岩流)。地幔岩碎块以尖晶石二辉橄榄岩和尖晶石斜辉橄榄岩碎块为主,纯橄榄岩次之,未发现石榴石橄榄岩;胶结物为碱性橄榄玄武岩岩浆。本次研究发现地幔岩内存在丰富的、不同成分和形态的熔融微区。熔融微区类型以其形状可分为滴状、扇状、球状、不规则状、短脉状和环边状,以其特征新生矿物分为OL型、K型、Na+Chl型、PL型、OL+SP型、C+SP型和SP+Chl+Ser型。熔融微区结构为玻基间隐结构或放射状结构;矿物呈骸晶状、中空为玻璃质;残余玻璃脱玻化,产生少量针状和不透明黑色雏晶。熔融微区的形状、结构、物质组成及矿物结晶等特征具有标型性,表征这些熔融体是在上地幔深度保存的幔源岩熔融交代的产物,幔源结晶岩是固相残留。该幔源岩经历强火山喷发使其发生爆炸的地质事件,导致K、Na、Al、Ca易熔组分和H₂O、CO₂等挥发分开始熔融和气体释放,营造快速固化结晶和淬火的环境。这些少量的熔融物择优占据矿物间隙、裂隙、位错或晶体缺陷处汇聚并熔融交代相邻矿物,不断扩展空间,遂形成滴状等特征形状的“微区”。由于熔融程度不同,产生的熔融物的化学成分和结晶程度也有差异,所代表的初始岩浆性质也不一样,可以是超基性或碱性橄榄玄武质,抑或碧玄岩质岩浆。从检测出的这些信息证实,蛟河地幔岩是被不一致熔融抽取后的地幔残留,即岩石圈地幔。

Mineralogy and geological significance of the melting zone in the Jiaohe upper mantle fragment,Jilin

The Jiaohe mantle rock fragment was brought to the surface by magmatic alkali olivine basalt, of which, 40-55% (in some parts 60%) are mantle fragments of different tile sizes, generally from 4×5×8 cm to 2×3×4 cm, with the largest measuring 20×22×35 cm and so named mantle rock agglomerate lava. The main mantle rock fragments are spinel iherzolite and clinopyroxene. The cement is alkali olivine basalt. Abundant melt microzones with different compositions and shapes have been found. These microzones can be divided according to shape into drop, fan, ball, irregular short vein and ringed edge. The new minerals can be divided into OL, K, Na+Chl, PL, Chl and SP+Chl+Ser types. The structure of the melt zone is the hidden or radial structure between glass bases, with minerals in skeletal crystal or hollow glass forms. The residual glass is devitrified, forming a small number of acicular and black opaque primary crystallites. The morphology, structure and material composition and mineral crystallization of the melt microzones have the characteristics of residual mantle crystalline rocks. The mantle rock underwent an explosion caused by strong volcanic eruption, resulting in the melting of fusible components such as K, Na and Ca and releasing of volatile components such as H₂O and CO₂, creating a rapid crystallization and quenching environment. These melts preferentially occupy mineral gaps, fissures, dislocation or crystal defect sites and gather and fuse nearby metasomatic minerals, continuously expanding space and forming microzone blobs. The melts have different chemical compositions and crystallization characteristics due to different degrees of melting. They represent different initial magmas, including ultrabasic, alkaline basaltic or lamprophyre magmas. Our data indicate that the Jiaohe mantle xenoliths are residue mantle from extracted melts, i.e., lithospheric mantle.