榴辉岩型钛矿化的地球化学制约

根据对国内外已发表的榴辉岩地球化学资料的统计分析,表明榴辉岩中TiO2含量与全铁(TFeO)具正相关性,而与MgO, 特别是MgO/(MgO TFeO)比值具负相关性.按TiO2含量的高低,可将榴辉岩区分为高钛型(TiO2＞2.0%)、中钛型(TiO2=0.60%～2.00%)和低钛型(TiO2＜0.6%)三类,文章重点讨论了高钛与低钛榴辉岩地球化学特征的差异.与低钛型榴辉岩相比,高钛型榴辉岩相对偏基性,富铁,贫镁,并具有较高的高场强元素(如Nb、Ta、Zr、Hf等)和亲铁元素(如V、Co、Ni等)含量.原岩恢复显示低钛榴辉岩的源岩有两种类型,其一为洋壳俯冲形成的蛇绿岩残片,另一为富钙质的沉积泥灰岩.高钛型榴辉岩的源岩则具OIB特征,可能为富集地幔部分熔融或上涌的软流圈地幔物质与地壳物质混染的产物.榴辉岩中的钛矿化主要受源岩因素制约,此外,岩浆结晶分异对其也有一定影响.

GEOCHEMICAL CONSTRAINTS ON TITANIUM MINERALIZATION OF ECLOGITES

Based on statistics of the published geochemical data,it is shown that TiO_2 contents of eclogites are correlated positively with TFeO,and negatively with MgO(especially with MgO/(MgO+TFeO)).Eclogites can be tentatively divided into a high-Ti type(TiO_2>2.0%),a medium-Ti type(TiO_2=0.60%~2.00%),and a low-Ti type(TiO_2<0.6%) according to their TiO_2 concentrations.Compared with the low-Ti type eclogites,the high-Ti type eclogites are characterized by relatively low in SiO_2,MgO,and high in TFeO,and are more enriched in high field strength elements(e.g.,Nb,Ta,Zr,Hf,etc.) and siderophile elements(e.g.,V,Co,Ni,etc.).Protolith reconstruction indicates that the low-Ti eclogites have two types of protoliths,i.e.,one is the ophiolitic relics from the oceanic crust subduction,the other is the calcium-rich marl.The features of the protoliths of high-Ti eclogites are similar to that of the OIB, implying that they might be produced by partial melting of enriched mantle or by mixing of upwelling asthenosphere mantle materials and crustal materials.The titanium mineralization of eclogites is constrained mainly by the protoliths,in addition,it is also affected by fractional crystallization of magma.