中国大陆科学钻探工程主孔石榴石单辉橄榄岩的岩石化学研究

Geochemical study on garnet wehrlite from Chinese Continental Scientific Drilling(CCSD)borehole.

中国大陆科学钻探工程主孔石榴石橄榄岩产出于603.20～683.53m之间,主要为石榴石单辉橄榄岩,内部还见有巨晶状角闪石岩(曾经是岩脉)和榴辉岩与橄榄岩之间厘米级的方辉辉石岩.石榴石橄榄岩多数含有钛斜硅镁石.从其内部所夹的榴辉岩条带可知,橄榄岩发生过褶皱等构造变形作用.其围岩和内部夹层榴辉岩中的锆石中舍柯石英的事实可能表明岩体经历过超高压变质作用.石榴石橄榄岩的主要造岩矿物橄榄石的Fo端员低(0.82～0.86)、全岩岩石化学的低Mg#值(77.26～85.07)和高易熔元素以及低于典型阿尔卑斯型橄榄岩的(Mg+Ni)/(Fe+Mn)比值,意味着该石榴石橄榄岩体的原岩为岩浆固结成因.石榴石橄榄岩的稀土元素总量低,配分型式有LREE富集型((La/Lu)N比值为9～16.6)、LREE适度富集型((La/Lu)N比值为1.47～8.22)和LREE亏损型或平坦型((La/Lu)N比值为0.62、1.19～1.92)三种,并构成从亏损或平坦型到富集型的韵律,反映了结晶分异的特点.微量元素蛛网图上,石榴单辉橄榄岩微量元素的丰度总体上低于N-MOBB的丰度.Cu、Ni和MgO的判别图表明,石榴石橄榄岩的原岩岩浆的固结成岩时期主要为镁铁质矿物的分离结晶作用而硫化物不是重要的矿物相.Ti和P的剧烈变化表明了有磁铁矿和磷灰石的堆积作用.微量元素判别图表明岩体受到了明显的地壳物质混染作用.铂族元素丰度总量低,其丰度分布型式图表明原岩岩浆的部分熔融程度高,而且元素的判别图表明岩浆具有S不饱和特点.硫化物残留在地幔中而使得岩浆中PGE相对Cu、Ni亏损,在Ni-PGE-Cu图上呈总体上的凹形分布.同属PPGE的Pt比Pd所显示的更高的相容性也表明了岩浆中S的不饱和特点.而Ir的负异常可能说明原始岩浆在侵入前的分离结晶作用其间发生了Ir的结晶析出,或者是在地幔部分熔融时Ir残留在地幔残留体中.因此,主孔石榴石橄榄岩的原始岩浆是上地幔适当部分熔融程度的产物((Pd/Ir)N≥1),可能在源区有硫化物的残余而造成了PGE相对Cu、Ni亏损而在Ni-PGE-Cu图上呈总体上的凹形分布.后S不饱和的岩浆经历分离结晶作用而造成了Pt相时于Pd亏损的特点.

The garnet peridotite in CCSD borehole occurs between the depths of 603.20 similar to 683.53m, mainly composed of garnet wehrlite, with eclogite interlayers, magacrystal amphibolite (originally vein) and orthopyroxenite on the contact between peridotite and eclogite. Most of the garnet peridotite contains titanium clinohumite. The rocks were deformed and folded. Coesite in zircons from wall rocks and eclogite interlayers of peridotite demonstrates that the peridotite experienced ultrahigh pressure metamorphism. The low Fo in olivine (0.82 similar to 0.86), low Mg-# (77.26 similar to 85.07) of whole rock, high concentration of fusible compositions and much lower (Mg + Ni)/(Fe + Mn) ratio imply the protlolith of the peridotite was crystallized from a magma intrssive body. The garnet peridotite owns low total REE whose partition patterns can be subdivided into flat, enriched and enriched according to the ((La/Lu)(N) ratios of 9 similar to 16.6, 1.47 similar to 8.22 and 1.19 similar to 1.92. These three types of REE partition patterns can show rythms from the bottom to top of the cores, which might reflect the characteristics of magma crystallization. On the spider diargram, the curves of trace elements show chaotic form and lower concentrations than that of primitive mantle. On the plots of MgO vs Ni and Cu it is shown that the magma of protolith mainly crystallized mafic minerals rather than sulphide minerals. The drastic variation of concentrations of Ti and P in bulk rocks implies the cumulation of magnetite and apatite. The trace element discrimitive plots also demostrate there is crustal contamination in the pridotite. The low concentration of PGE in bulk rocks and slightly positive inclination distribution pattern implies that the magma of protolith was originated by high degree of partial melting. The discrimitive plots of PGE also show the insaturation of sulpher in the rocks which means PGE are relatively depleted in magma comparing to Cu and Ni on the plot of Ni-PGE-Cu distribution pattern. All belonging to PPGE, Pt shows relatively more incmpitable nature than Pd in the rocks, which shows S insaturation in the magma. The negative anomaly of Ir is possibly because the crystallization and segregation of Ir-bearing phases before magma intrusion or retention of Ir phase in the mantle during ralitivelly low degree of partial melting. Therefore, the magma of protolith of the garnet peridotite is the product of partial melting of upper mantle (resulting in (Pd/Ir)(N) >= 1), and the retention of sulphide phases in the magma origin source caused relative depletion of PGE to Cu and Ni on the plot of Ni-PGE-Cu distribution pattern. The S-insaturated magma experienced fractional crystallization after intrusion in the crust and caused relative depletion of Pt to Pd.