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通过对秦岭造山带东段北秦岭构造域的新元古代方城碱性岩体研究,对Rodinia超大陆形成后岩石圈拉张起始时间提供了重要制约.方城岩体岩石类型主要为角闪云霞正长岩、霓辉正长岩和碱性长石正长岩.岩石为中性(SiO2=54%～62%),富碱(K2O+Na2O=12%～15%)、铝(Al2O3=16.81%～23.26%)与大离子亲石元素,无Nb,Ta,Zr,Hf异常,轻稀土元素相对富集、重稀土元素分异较弱,呈较显著的负Eu异常(δEu=0.13～0.23),其εNd(t)值为?1.37～?3.90,Nd模式年龄为1364～1569Ma.正长岩岩浆形成的温度较高(915～1044℃).岩体形成于板内-非造山伸展构造环境,可能主要来源于上地幔的小比例部分熔融,在上升或侵位过程中受到少量地壳物质的混染.方城碱性正长岩的锆石LA-ICP-MSU-Pb年龄为(844.3±1.6)Ma(MSWD=0.86),表明其形成于新元古代早期,为秦岭造山带及华南地区已知形成时代最早的新元古代碱性岩,也暗示在844Ma左右秦岭造山带新元古代构造演化已由碰撞后伸展转换为板内非造山拉张阶段.因此,Rodinia拼合过程中沿扬子克拉通边缘形成的碰撞造山过程结束并转入拉张体制地球动力学背景的时限应不晚于844Ma.
Through the study of the Neoproterozoic Fangcheng alkaline rocks in the North Qinling tectonic domain in the eastern section of the Qinling orogenic belt, it provides important constraints on the onset of lithospheric pull-out after the formation of the Rodinia supercontinent. Nephotropite and Nephotropite feldspar, rocks are neutral (SiO2 = 54% ~ 62%), alkali-rich (K2O + Na2O = 12% ~ 15%), aluminum Al 2 O 3 = 16.81% ~ 23.26%) with large ion lithophile elements, without Nb, Ta, Zr and Hf anomalies, with relatively light LREE contents and heavy HREE content weaknesses. 0.13 ~ 0.23). The εNd (t) values ranged from 1.37 to 3.90 and the Nd mode ages ranged from 1364 to 1569Ma. The temperature of the syenite is higher (915 ~ 1044 ℃) The orogenic extensional tectonic setting may be mainly due to the small fractional partial melting of the upper mantle and the contamination by a small amount of crustal material during the ascending or emplacement process.Zircon LA-ICP-MSU-Pb ages (844.3 ± 1.6) Ma (MSWD = 0.86), indicating that it was formed in the early Neoproterozoic. It is the earliest Neoproterozoic alkaline rock of the Qinling orogenic belt and the southern region of China, and also implies that at around 844Ma The Neoproterozoic tectonic evolution of the orogenic orogenic belt has been transformed from post-collision extension to intraplate non-orogenic extension.Therefore, the collision orogeny formed along the edge of Yangtze craton during the Rodinia jointing process is ended and transferred to the tension system geodynamic background The deadline should be no later than 844Ma.