东南极米洛半岛尖晶石-堇青石麻粒岩变质作用与年代学研究

Metamorphism and geochronology of the spinel−cordierite granulite in the Mirror Peninsula,East Antarctica

东南极普里兹构造带被认为受到格林维尔期及泛非期两期构造热事件的影响,分别与罗迪尼亚和冈瓦纳超大陆的演化密切相关,但对两期构造热事件的性质还存在争议。为了进一步完善该构造带的演化历史,从矿物学、岩石学、相平衡模拟、锆石年代学等角度对普里兹构造带米洛半岛尖晶石-堇青石麻粒岩进行了研究。结果表明其主期矿物组合为尖晶石+堇青石+黑云母+矽线石+少量石榴子石+钾长石+钛铁矿,温压条件为870~910℃、0.64~0.69 GPa,晚期退变至810~820℃、0.49~0.53 GPa,并暗示了更高的峰期变质条件(T>910℃,P>0.69 GPa)。CL图像显示锆石具有明显的核-幔-边结构,LA-ICP-MS锆石U–Pb年代学分析显示核部年龄主要集中在613±7 Ma到877±9 Ma,最大值916±11 Ma,比典型的格林维尔期年龄年轻。锆石边部加权平均年龄为526±8 Ma,其Th/U比值范围较大(0.06~1.23),多数Th/U比值较高(>0.1),应代表峰后冷却结晶阶段。尖晶石-堇青石麻粒岩记录了中低压/高温—超高温的变质条件,结合区域已有资料,可能具有顺时针轨迹,其变质演化历史可能反映了碰撞造山之后的伸展阶段,推测与冈瓦纳超大陆的聚合过程有关。

[Objective]The Prydz Bay belt in East Antarctica recorded two significant tectono-thermal events,the Grenvillian event and the Pan-African event,which are considered to be closely related to the evolution of the Rodinia and Gondwana supercontinents.However,the geological history and the tectonic nature of the two events remain controversial.[Methods]Mineralogical and petrological analyses,phase equilibria modelling and zircon geochronology are combined to investigate the spinel−cordierite granulite from the Mirror Peninsula in order to better understand the tectono-thermal history of the Prydz Bay belt.[Results]The spinel−cordierite granulite contains different stages of mineral assemblages.The major stage of mineral assemblage involves cordierite,spinel,biotite,sillimanite,K-feldspar and minor garnet and ilmenite.The later stage of mineral assemblage is indicated by the emergence of magnetite as the increasing volumes of biotite and cordierite.Minor garnet and corundum are locally preserved,implying the mineral reaction‘g+cor→sp+sill’and more garnet and corundum in the peak stage.The garnet grains consist of 70%−72%almandine,20%−22%pyrope,~4%grossularite and~4%spessartine.The X_(Fe)(Fe^(2+)/(Fe^(2+)+Mg^(2+)))of representative garnet grains ranges from 0.77 to 0.80.The spinel exhibits an X_(Fe)range from 0.80 to 0.86.Different cordierite grains have similar compositions with Al of 3.89−3.93 a.p.f.u(atoms per formula unit)and X_(Fe)of 0.32−0.36.Biotite has high TiO_(2)(4.13%−5.23%)and Ti(0.23−0.30 a.p.f.u).Kfeldspar grains consist of 78%−85%orthoclase,15%−23%albite and~1%anorthite.Based on the mineral compositions and phase equilibrium modelling,the pressure−temperature(P−T)conditions of the major stage of mineral assemblage are constrained to 870−910℃and 0.64−0.69 GPa,followed by later retrogression to 810−820℃and 0.49−0.53 GPa.A peak stage with higher P−T conditions(T>910℃,P>0.69 GPa)can be inferred based on the relict peak minerals and characteristic mineral compositions(e.g.Ti in biotite).Zircon grains commonly show core-mantle-rim structures in cathodoluminescence(CL)images.The LA−ICP−MS zircon U−Pb dating analyses reveal a wide age range from 613±7 Ma to 877±9 Ma(except a maximum of 916±11 Ma)for the cores.The zircon bright rims yield a weighted mean age of 526±8 Ma with a wide range of Th/U(0.06−1.23),mostly higher than 0.1.[Conclusion]Based on the results,a few conclusions can be drawn:(1)The spinel−cordierite granulite recorded medium−low pressure/high-ultrahigh temperature metamorphism with a clockwise P−T evolution path and high dT/dP.(2)The results of zircon geochronological analysis show that zircon cores mainly record U−Pb ages in the range of 800~600 Ma,younger than typical ages of Grenvillian events,which may reflect younger inherited zircon cores or significant isotopic resetting.(3)The age of~530 Ma of zircon rims is interpreted to represent the post-peak cooling stage of the Pan-African tectono-thermal event.[Significance]This study examined the P−T conditions and the zircon ages of the spinel−cordierite granulite in the Mirror Peninsula.In combination with previous results,the P−T−t path constructed for the spinel−cordierite granulite provides new constraints on the evolution of the Prydz Bay belt during the Pan-African period.