华北太行岩墙群斜锆石生长世代和U-Pb年龄及其对岩浆演化的启示

The generations and U-Pb dating of baddeleyites from the Taihang dyke swarm in North China and their implications for magmatic evolution

斜锆石U-Pb定年是基性岩墙时代确定的有效方法。前人对太行岩墙群高分异组岩墙（Ti O2通常〉1%,Mg O一般〈6%）进行了较多的定年,但缺少对低分异组岩墙（Ti O2约为1%,Mg O〉6%）的高精度测年。两者年龄是否一致是讨论两组岩墙是否来源于同一岩浆的基本前提。本研究对华北太行岩墙群丰镇地区低、高分异组代表性岩墙进行了斜锆石离子探针207Pb/206Pb定年。结果表明,低分异组车道沟岩墙207Pb/206Pb平均年龄为1768±4Ma（n=9,MSWD=2.1）;而高分异组酸刺湾岩墙斜锆石207Pb/206Pb年龄可以分为两组,加权平均值分别为1780±3Ma（n=8,MSWD=0.65）和1760±3Ma（n=5,MSWD=1.7）。斜锆石成分分析表明,酸刺湾岩墙两组年龄对应的矿物成分存在差异,如晚期斜锆石比早期更富Ti,Zr/Hf比值变化范围相对较小。据此认为,酸刺湾岩墙两组斜锆石可能结晶于岩浆不同阶段：早期（～1780Ma）的斜锆石可能结晶于成分更加原始的岩浆,很可能形成于岩浆房中;晚期（～1760Ma）的斜锆石可能结晶于成分更加演化的岩浆,可能是岩浆通道（岩墙）或岩浆房中。Zr/Hf值与单点年龄的负相关趋势可能记录了单斜辉石的结晶过程。已有的太行岩墙群U-Pb年代学结果集中于1785～1760Ma。我们的研究认为,太行岩墙群岩浆房存在了约20Myr,即岩浆房形成于～1785Ma,岩墙就位于1785～1760Ma。

It＇s an effective way to determine the ages of mafic dykes by the U-Pb isotopes of baddeleyites in them. Although a bunch of ages had been reported for the high-differentiated Taihang dykes（ generally Ti O2 1% and Mg O 6%）,there is no precise dating for the low-differentiated ones（ Ti O2～ 1% and Mg O 〉6%）. The age relationship of the two groups of dykes is important in constraining their petrogenetic correlations. In this study,we analyze baddeleyite207 Pb /206 Pb ages of the low-differentiated and highdifferentiated dykes from the Fengzhen area using SIMS method. The results show that the low-differentiated Chedaogou dyke crystallized at 1768 ± 4Ma（ n = 9,MSWD = 2. 1）; whereas the age of high-differentiated Suanciwan dyke has two distinct age groups,one at 1780 ± 3Ma（ n = 8,MSWD = 0. 65） and the other at 1760 ± 3Ma（ n = 5,MSWD = 1. 7）. Two groups of baddeleyites from the Suanciwan dyke have some differences in composition,e. g.,the baddeleyites of ～ 1760 Ma group are richer in Ti and have narrower Zr / Hf ratio range than the ～ 1780 Ma ones. This may indicate that the baddeleyites were crystallized at different magmatic stages： the older one（ ～ 1780Ma） probably crystallized in the magma chamber with more primitive compositions; whereas the younger one（ ～ 1760Ma） could be formed in the conduit with more evolved compositions during the ascent and differentiation of the dyke or also in magma chamber at later stages. The negative correlations between ages and Zr / Hf may be due to the fractional crystallization of clinopyroxene. All the reported ages of the Taihang dyke swarm are between 1785 Ma and 1760 Ma. Our study suggests that the magma of the Taihang dyke swarm may have existed for ～ 20 Myr,i. e.,it formed at ～ 1785 Ma,and emplaced during 1785 ～ 1760 Ma.