ABSTRACT

Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: I^st layer at 2.5–1 GPa is enriched mantle (Fe#Ol ~ 0.11 − 0.14) with Gar- pyroxenites and Sp peridotites; II^nd at 2.5–3.2 GPa – Gar-Sp (Fe#Ol 0.08 − 0.10) peridotite. III^d at 4.3–3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 − 0.095. IV^th at 3.2–5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 − 0.125); V^th at 5.8 − 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VI^th layer at 5.8–6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VII^th layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets.

Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Yb_n, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.

摘要

来自 Priazovie 泥盆纪金伯利岩管的地幔异种晶体的主要、次要和微量元素组成提供了对乌克兰东南部地盾下方地幔结构及其演化的深入了解。石榴石产生的低温条件由单矿物热压法确定。地幔岩石圈在 4.2 GPa 处急剧分裂，以高温 Cpx-Ilm-Phl 趋势、榴辉岩和火山岩地球化学变化为标志。检测到七层：我^ST在2.5-1 GPA层是富集地幔（FE＃01〜0.11 - 0.14）与Gar-辉石岩和Sp橄榄岩; II ^nd在 2.5–3.2 GPa – Gar-Sp (Fe#Ol 0.08 - 0.10) 橄榄岩。4.3-3.2 GPa 的III ^d由具有 Fe#Ol ~0.07 - 0.095 的太古代-元古代橄榄岩形成。第四^次在 3.2–5 GPa- 包含具有较高 Ca 的辉石 Gar、榴辉岩、Chr 和 Cpx (Fe#Ol ~0.10 − 0.125)；5.8 - 5 GPa 处的V ^th以亚钙石榴石、富铬铬铁矿和镁铬钛铁矿为标志；5.8-6.8 GPa 的^第VI层在岩石圈底部附近含有富铁的辉石、铁铝榴石和 Cr-Mg 钛铁矿；^第VII层 > 6.8 GPa 由“热”富铁石榴石组成。

石榴石在 LREE、LILE、Hf、Zr 中的含量随着压力的降低而增加。原始石榴石具有圆形 REE 图案；耗尽的在 Nd 处具有 S 型图案。从 6.5 到 3 GPa 的石榴石显示出增加的 La/Yb _n, Zr-Hf, LILE。橄榄岩单斜辉石具有倾斜的线性微量元素模式，从 La 到 Pr 圆形，具有高 LILE 和 HFSE 水平。富铁组（与榴辉岩反应）显示出钟形不规则图案，LILE 接近 LREE 水平。地幔上部 LILE (HFSE and) 富集的一个可能原因是太古代时期的俯冲交代（有成熟大陆沉积物的参与）被 1.8 Ga 和更早的羽流激活，这产生了广泛的集中熔体流动，云母重熔。角闪石交代岩在 5.8 到 2.5 GPa 的地幔岩性中提供连续的 REE 和 LILE 富集。