碎屑金红石:沉积物源的一种指针

近年来,碎屑金红石的研究已成为沉积物源区分析的一个新前沿。金红石的地球化学组成,尤其是Cr,Nb,Zr等微量元素的含量,对其母岩的形成条件和所经历的地质过程都具有重要的指示意义,同时,碎屑金红石在沉积、成岩过程中表现出极高的稳定性,因而是物源分析的理想指针矿物。首先介绍金红石的矿物学和地球化学基本性质,分析不同来源的金红石典型特征,重点阐述碎屑金红石在物源分析中运用的5个方面:①金红石重矿物比值;②金红石矿物化学成分Cr-Nb判别图解;③金红石Zr含量温度计;④金红石的U-Pb和(U-Th)/He定年;⑤金红石Lu-Hf同位素。综合上述5个方面的物源分析研究,可以获取金红石的母岩类型、形成温度及后期所经历的热演化史等信息。碎屑金红石的物源研究处于起步和探索阶段,仍存在一些亟需解决的问题。

Detrital Rutile： A Sediment Provenance Indicator

Rutile has a wide range of applications in earth sciences.In recent years,more and more attention has been paid to the investigation of rutile.Detrital rutile is one of the chemically and physically most stable heavy minerals in sedimentary cycle,and the trace elements in rutile,such as Cr,Nb,Zr,are widely used as monitors of formation conditions and processes of parent-rocks.Therefore,detrital rutile could be used as a reliable provenance indicator and may serve as a key heavy mineral in sediment provenance analysis,especially for the highly diagenetic sandstones.This paper aims to provide an overview of the applications of rutile in provenance analysis.After giving a summary of basic features of rutile and various rutile-bearing rocks,four aspects of the applications have been discussed: ① Heavy mineral ratios concerned rutile.For instance,ZTR(the ratio of zircon,tourmaline and rutile in the total nonopaque heavy minerals) and RuZi(the ratio of rutile and zircon) Index.ZTR Index could be used to evaluate the maturity of sandstones,while RuZi Index indicates the source lithology.② Modified Nb-Cr discrimination diagram for distinguishing detrital rutile from metamafic rocks or metapelitic rocks.The details are described as follows: The lower limit of Nb in metapelitic rutiles should be set at 600×10-6,and detrital rutiles with Cr 600×10-6 are interpreted to be derived from metapelitic rocks.③ Zr-in-rutile thermometer for calculating the formation temperature of detrital rutiles.Based on the comparison of various published Zr-in-rutile thermometers and provenance analysis case study,we propose the preference of the thermometer of Watson et al.④ U-Pb dating and(U-Th)/He thermochronology of rutile for understanding the cooling history of the parent-rocks.Rutile from high-grade metamorphic rocks can contain sufficient uranium to allow U-Pb geochronology,and the U-Pb age of each detrital rutile represents the time when the rutile reached the closure temperature during the cooling of parent-rock.⑤ Lu-Hf isotopes of rutile for tracing the history and evolution of the parent-rock in the crust-mantle system.In summary,the use of detrital rutile in provenance analysis is still in its preliminary stage,hence there are also some unresolved problems which could be the directions and goals of rutile provenance study in the future.