Through a systematic study on trace elements and REE geochemistry of mudstonedeposited in the basin and lower slope environments during Upper Proterozoic to Triassic in theSouthwest Yangtze Massif, three gaxhemical abnormal horizons of which the gerehemicalcharacteristics are quite different from those of other horizons have been established for the firsttime. They are the Lower Cambrian, the Upper Devonian and the Upper Permian. As compared with the crustal evolution in this area, these three geochemical abnormal horizons are correspoding to the pullingapart perieds of gcotectonic cycles, which illustrates that the uncommon depositional sources Pouring into the basin from the earth’s interior may be one of themost important causes to originate the geochemical anomalies in these horizons. Thus it can berealized that the gaxhemistry of opst-Archean sedimentary rocks has a great deal to do with thecrustal evolution and it can be used as a tracer to analyze the crustal evolution. The elements inthis area are mainly concentrated in these geochemical abnormal horizons, and the degree of enrichment and deficiency of trace elements in other horizons is very limited. A series of researchon mineralization indicates that the main strata-bound ore deposits discovered in the SouthwestYangtze Massif occur in the Cambrian, Devonian and Permian-Triassic strata. The results ofisotope tracer research have also proved that most of the metallogenic elements in these ore dePosits came from the host strata, which illustrates that the geochemical abnormal horizons mayhave made great contributions to these ore-forming processes. Thus it can be concluded that itis only the particular horizons corresponding to the particular periods of earth’s evolution thatcan they be the significant source beds because only in these uncommon horizons there can behighly enriched metallogenic elements, which may be one of the most important reasons for explaining the time-bound nature of mineralization.
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