Discovery Of Khondalites In The Helan Mountain And Its Chronological Evidence

The North China Craton (NCC) is one of the world’s oldest Cratons. The basement of the NCC could be divided into the Eastern and Western Blocks and the Trans-North China Orogen; the Eastern and the Western Blocks were amalgamated along the Trans-North China Ogogen at1.9-1.8Ga. The Western Block formed by the amalgamation between the Ordos Block to the south and the Yinshan Block to the north during-2.0-1.9Ga along the NEE-trending Khondalite Belt. The Helan Mountain was located between the Ordos Block and the Yinshan Block and it was an important part of the west Khondalite Belt.This paper carries out studies about a geological profile across the Zhaochigou Formation-compex, and the petrogeochemistry, LA-ICP-MS U-Pb dating and Hf isotopic compositional analysis were done for detrital zircons in two matasedimentary samples that were taken from the Zhaochigou Formation-complex in the Helan Mountain. Based on these various works we concluded the following understandings.1.The Zhaochigou profile can be divided into three members from the low layer to the upper layer according to the geological profile data. The lower member (Pt1z1) mainly consists of metamorphosed pebbly feldspathic sandstone, quartzite and fine-grained metamorphic quartz greywacke; the middle member (Pt1z2) involved with leucoleptite sandwiched between dark gray leptynite; the upper member (Pt1z3) is composed of gneiss sandwiched between schist.2.The studies of petrogeochemistry suggested that the Zhaochigou Formation-complex holds the similar geochemical characteristics with the typical khondalites of NCC. The protolith of the Zhaochigou Formation-complex was a set of low sorting, low maturity clastic rocks which consist of greywacke, lithic sandstone and feldspathic sandstone. The protolith may be origin from the Late Paleoproterozoic magmatism which with the nature of continental island arc in the Khondalite Belt of NCC, and then deposited in the active continental margin with the nature of back-arc basin.3. The LA-ICP-MS U-Pb dating and Hf isotopic compositional analysis for the Zhaochigou Formation-complex showed that:(1)The Paleoproterozoic Zhaochigou Formation-complex in the middle section of the Helan Mountain is a part of the Khondalite Series in the western Khondalite Belt of the NCC.(2) The age of protolith of the Zhaochigou Formation-complex has been frozen at about1.99-1.96Ga which can be regarded as the age of the Khondalite Series.(3) Data from the metamorphic zircons in the Zhaochigou Formation-complex suggest that the Khondalite Series of the NCC have experienced two stages of metamorphism and the early stage was taken place at1.96-1.95Ga and the later stage was at about1.87Ga.(4) The eastern and western edges of the Khondalite Belt may share the same provenance. More ancient magmatic activity within the Khondalite Belt may have provided the source materials for the Khondalite Series and the Ordos block to the south could also be a potential source area.(5) A crustal accretion event occurred at2.5-2.3Ga in the Western Block of the NCC. The zircons formed at2.15-1.99Ga were originated from the reworking or transformation of juvenile crustal at2.5-2.3Ga.4. The LA-ICP-MS U-Pb dating and Hf isotopic compositional analysis for the Zhaochigou granites showed that:(1) the emplacement age of the Zhaochigou granites was1958±41Ma.(2) there were crustal accretion event in2393-2228Ma at study area.(3) the materials source of the Zhaochigou granites maybe diversity which could be consists of partial melting of old crusts and materials from mantle.