大兴安岭诺敏河石榴石橄榄岩捕虏体的发现及其地质意义

在重力梯度带北端的大兴安岭诺敏河第四纪火山岩中发现石榴石相和尖晶石相地幔橄榄岩捕虏体,其矿物组成分别为Ol_40~55Opx_20~35Cpx_0~10Grt_5~25和Ol_45~65Opx_30~40 Cpx_0~15Sp_0~10。两种地幔相橄榄岩均以方辉橄榄岩为主,说明研究区上地幔具有难熔的特点。其高Mg^#橄榄石Fo_91~92和高Cr^#_31~47尖晶石特征与南区哈河的地幔橄榄岩特征一致。在橄榄石含量与Fo图解上,敏河橄榄岩包体落在太古代和元古代地幔区域,揭示大兴安岭地区岩石圈地幔可能保留了较多古老的残余地幔。根据石榴石橄榄岩的平衡温压条件(1114~1168℃和2.14~2.33GPa),推测来源深度为70~75km。这些数据暗示研究区具有较高的地温梯度,与中国东部新生代火山岩区的地温梯度相似。

Discovery of peridotite xenoliths from the Nuomin river Quaternary volcanic field,the Great Xing’an Range,and its geological significance

The Nuomin river Quarternary volcanos are located in northern Great Xing’an Range,at the northern part of the North-South Gravity Lineament,Mantle xenoliths from these volcanos include abundant Sp-peridotite(Ol40～55Opx20～35Cpx0～10Grt5～25) and subordinate Gt-peridotite(Ol45～65Opx30～40 Cpx0～15Sp0～10),The low Cpx mode(usually less than 3%) in these peridotites reveals that the mantle rocks are mainly harzburgites,Similar to mantle xenoliths recovered in Hahe area,most of the Nuomin river Sp-harzburgites have high-Mg# olivine(Fo91～92) and high Cr#(31～47) chromite,indicating a refractory nature of the lithospheric mantle beneath the Great Xing’an Range,The plot of olivine mode again Fo contents in olivine further indicates the preservation of Proterozoic-Late Archeacn mantle in this area,Application of geothermobarometers to the mantle xenoliths suggests that the Gt-peridotites are equilibrated at a pressure of 2.14～2.33GPa(corresponding to 70～75km) and a temperature of 1114～1168℃,These data define a relatively high thermal gradient for the upper mantle beneath the Great Xing’an Range,similar to those for many Cenozoic volcanic fields in eastern China.