Petrogenesis of an ultrahigh‐pressure garnet‐bearing ultramafic body from Maowu, Dabie Mountains, east‐central China
Petrogenesis of an ultrahigh‐pressure garnet‐bearing ultramafic body from Maowu, Dabie Mountains, east‐central China
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DOI：
10.1046/j.1440-1738.1998.00188.x
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发表时间：
1998-04
期刊：
影响因子：
1.5
通讯作者：
J. Liou;R. Y. Zhang
中科院分区：
文献类型：
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作者：
J. Liou;R. Y. Zhang
大别超高压地体中含石榴石超镁铁质岩体(∼250×>；50m2)具有几个明显的岩石学特征：(1)大部分岩石为含石榴石超镁铁质岩层，包括斜方辉石岩和单斜辉石岩，还有少量方辉橄榄岩和富绿辉石的层，这些成分层厚度在5~1.6m之间；(Ii)金红石无处不在，在单斜辉石岩中含量最高(可达1-2Vol%)；(Iii)独居石以包裹体和基质相存在于硅酸盐中；(4)出溶片状磁铁矿赋存于橄榄石(FO93)中，独居石赋存于磷灰石中；(V)铬铁矿以细粒包裹体形式赋存于顽辉石和斜方辉石中；(Vi)粗粒石榴石中常见的包裹体有滑石、斜绿泥石和角闪石等水合相；以及(Vii)主要硅酸盐的镁/(镁+铁)值较高。大多数超镁铁质岩石富含稀土元素(REE)、磷、铬和二氧化钛，与西阿尔卑斯山、西片麻岩和波西米亚地块常见的镁铬或铁钛石榴石橄榄岩明显不同。片理较好的斜方辉石由Grt(Prp60-74)+En(En92-96；0.05-0.14wt%Al_2O_3)+Chl(2-3wt%Cr2O_3)+斜方镁矿+菱镁矿+铬铁矿组成。绿辉岩主要由拉长绿辉石(>；90vol%)和稀有石榴石(Prp45Alm41Gr13)、金红石、磷灰石和独居石组成。绿辉石(JD63-67Ac6-12-Aug20-31)具有明显的成分分带特征，包裹体中含有柯石英残留物和柯石英之后的石英假象。次要逆行相包括Px后的滑石/透闪石、GRT后的Chl、Cpx(Jd49Ac13Aug38)和绿辉石后的Ab。含石榴石的超镁铁质和绿辉岩在∼35~50kbar和750±50℃的超高压变质作用下，在极低的xCO2(<；0.001)条件下，在P<；15kbar和650°C发生了少量的角闪岩相叠加。原岩可能是在中朝克拉通和扬子克拉通碰撞期间，在三叠纪俯冲到地幔深度100多公里之前发生交代的元古界超镁铁质地壳堆积体。
The Maowu garnet‐bearing ultramafic body (∼ 250 × > 50 m2) in the Dabie ultrahigh‐pressure (UHP) terrane has several distinct petrologic characteristics: (i) most rocks are layers of garnet‐bearing ultramafics including orthopyroxenite and clinopyroxenite with minor harzburgite and omphacite‐rich layers; these compositional layers range in thickness from 5 cm to 1.6 m; (ii) rutile is ubiquitous and is most abundant in clinopyroxenite (up to 1–2 vol%); (iii) monazite is common as inclusions in silicates and as a matrix phase; (iv) exsolved plates of magnetite occur in olivine (Fo93), and monazite in apatite; (v) chromite occurs as fine‐grained inclusions in enstatite and clinohumite; (vi) hydrous phases including talc, clinochlore and amphibole are common as inclusions in coarse‐grained garnet; and (vii) major silicates are high in Mg/(Mg + Fe) values. Most of the ultramafic rocks are high in rare earth elements (REE), P, Cr and TiO2, and are significantly different from Mg–Cr or Fe–Ti garnet peridotites which are common in the western Alps, Western Gneiss Region, and the Bohemian Massif. Well‐foliated orthopyroxenite is composed of Grt (Prp60–74) + En (En92–96; 0.05–0.14 wt% Al2O3) + Chl (2–3 wt% Cr2O3) ± clinohumite ± magnesite ± Di (Di94–97) + chromite. Omphacitic rocks contain mainly elongated omphacite (> 90 vol%) with rare garnet (Prp45Alm41Gr13), rutile, apatite, and monazite. Omphacites (Jd63–67Ac6–12Aug20–31) display pronounced compositional zoning and contain inclusions of coesite relics and quartz pseudomorphs after coesite. Minor retrograde phases include talc/tremolite after Px, Chl after Grt, Cpx (Jd49Ac13Aug38) and Ab after omphacite. Garnet‐bearing ultramafic and omphacitic rocks have been subjected to UHP metamorphism at pressures of ∼ 35–50 kbar and 750 ± 50 °C under extremely low XCO2 conditions (< 0.001); a minor amphibolite facies overprint took place at P < 15 kbar and 650 °C. The protoliths may be Proterozoic ultramafic crustal cumulates that were subjected to metasomatism prior to Triassic subduction to mantle depths of more than 100 km during the collision of the Sino‐Korean and Yangtze Cratons.