再论金红石矿床在变质过程中的成矿机制——以湖北大阜山金红石矿床为例

Further Discussion on the Metallization of Rutile in Metamorphic Process(As an example from a super large—scale rutile ore deposit of Dafu mountain,Hubei)

金红石矿床成因与变质作用密切相关,故多分布在变质带上,不同的变质阶段,所形成的不同岩石组分,其钛矿物组合也不同。一般说来,金红石出现率与变质相由低到高成正比,而同一变质相,Al/Ca比值增高,Fe/Mg比值降低,金红石含量增多。在前进变质阶段,以形成金红石矿物相为主;在后退变质阶段,部分金红石被钛铁矿、榍石等钛矿物交代。大阜山超大型金红石矿床系基性岩在高压变质作用过程中,金红石不断富集而成矿。在前进变质阶段,原铁镁矿物含钛辉石等被冻蓝闪石等交代,出现次生石榴子石后,夺取游离钛,促使形成金红石;退变质阶段有少量金红石被钛铁矿和榍石替代。在变质高峰期温度为520~600℃,压力为6×10~8~11×10~8Pa,变质作用仅达绿帘角闪岩相。本矿床位于古海沟混杂岩带,本矿床的寄主岩是其中最大的一个基性岩块。

The genesis of rutile ore deposits,most of which are hosted in metamorphic rocks of HP-UHP metamorphic belts,are directly related to metamorphism. There occur different titanium mineral in different stages of metamorphism,or rather,rutile occur generally in the progressive metamorphism and is replaced by ilmenite,sphene etc. in the retrogressive metamorphism. Our study also show that the grade of rutile is positively related to the metamorphic grade and that in the same metamorphic grade of rocks the grade of rutile is positively related to the Al / Ca ratio or negatively related to the Fe /Mg ratio of the rocks. Dafu rutile deposit,located in northern Hubei,is formed by the rutile enrichment in metamafic rocks during high-pressure metarmorphism. There are three majorrutile-bearing rocks： 1 rutilerich garnet amphibolite;2 amphibole-rich plagioclasite,and3 amphibole plagilclasite（ Table 4） These rocks occurred in circular zonation and are resulted from the same magmatic differentiation（ Fig 4）. In prograde metamorphism,the mafic minerals such as Ti-bearing augite,brown amphibole were altered into barroisite and fine-grained uralite. Meanwhile,the garnets surrounded barroisite in a chain of circles and free Fe was captured,which make the free Ti in mafic minerals of fine-grained Ti-minerals to form abundant rutile（ plate l）. The felsic mineral-plagilclase was transformed into epidote. phengite and albite. In retrograde metamorphism,the mafrc miner-als were transformed into fine-grained amphibole and actinolite;minor rutile replaced by il-menite and sphene（ plate 3）. The felsic minerals were changed into mascovite,paragonite and zoisite,with ccuring of some single crystal of sphene. According to C. I. P. W standard mineral values and the remnantstexture,the protolith is thought to be saturated gabbro-alkaline olivine gabbro. In addition,garnet is of uniform composition and free of inclusion,which formed after plagioclase and augite;omphaeite is ab-sent in this metamafic rocks;rutile have not been replaced completely by ilmenite and sphene;the region is within high-pressure（ HP） belt（ no ultrahigh-preesure（ UHP） belt）;and P-T conditions based on the reaction of garnet（ Gr1Alm2） ＋ Rutil = ilmenite ＋ anorthite ＋ quartzare are 520 - 600℃and 6 × 10^8- 11 × 10^8Pa（ Fig 8）. So the rocks should have undergone metamorphism of epidote amphibolite facies,not UHP eclogite facies. Dafu mountain deposit locates in the melange of paleotrench. which consists of metamafic,metaultramafic rocks,eclogite and marbles with different sizes. The deposit is hosted in the largest metamafic block（ Fig 3）. Thus it is quite possible to find more metamafic and eclogite type of rutile deposits there.