河南鱼池岭钼矿有关花岗岩锆石U-Pb年龄和Hf同位素研究及其对成矿时代的制约

摘要

The Yuchiling porphyry-type Mo deposit is located at the northwestern part of the Heyu granitic batholith along the southern margin of the North China Craton. Zircon LA-ICP-MS U-Pb dating yields an age of 135.2±2.4 Ma for the ore-bearing parent rock (granite porphyry), and of 138.2±2.3 Ma for the country rock (the unit. 3 intrusion of the Heyu granitic batholith), indicating that they were all the products of the mid-Early Cretaceous magmatism, and the formation of the Yuchiling Mo deposit should not be older than~135 Ma. The zircon Hf isotope compositions of the Yuchiling ore-bearing granitic porphyry and the granite country rocks vary greatly. The 176Hf/177Hf values of the former are from 0.281364 to 0.282420 with theεHf(t) values ranging from-9.6 to-46.9, and those of the latter are from 0.281774 to 0.282337 with theεHf(t) values ranging from-12.5 to-32.8, suggesting that these rocks have undergone a significant magmatic mixing during the crystallization. Also, the mature end-member is likely to be the Neoarchean-Paleoproterozoic Taihua Group basement and the immature end-member should be the Yanshanian upwelling and underplating mantle-derived materials or the lower crust reworking of the preexisting mantle-derived constitutes. The Yuchiling porphyry-type Mo deposit was formed when the Paleo-Pacific regime superposed, destroyed, and transformed the Paleo-Tethys tectonic systems comprehensively. The intense thinning of the crust induced the underplating or intraplating of the basaltic magma, leading to partial melting of ancient crustal rocks with higher Mo background, contaminating a small amount of mantle-derived constitutes or juvenile crust materials on different degrees. This Mo-rich magmatic system had experienced a high degree of differentiation and subsequently ascended to the shallow level of crust and emplaced as granitic porphyry. As the country rock is granite, the crannies of the porphyry exocontact are few. And for this reason molybdenum mineralization of the ore-bearing porphyry body have taken place in varying degrees and shown a characteristic of whole-rock mineralization.%鱼池岭斑岩型钼矿床位于华北克拉通南缘合峪花岗岩基西北部，对其赋矿母岩--花岗斑岩及其围岩--合峪花岗岩基第三单元侵入体进行锆石LA-ICP-MS U-Pb定年，获得年龄分别为（135.2±2.4）Ma和（138.2±2.3）Ma，均为早白垩世中期岩浆作用的产物，表明鱼池岭钼矿床的形成时代约为135 Ma。鱼池岭含矿花岗斑岩及其围岩花岗岩锆石Lu-Hf同位素示踪结果显示，这二类岩石Hf同位素组成变化极大，前者176Hf/177Hf比值变化范围在0.281364~0.282420，εHf（t）=-9.6~-46.9，而后者则在0.281774~0.282337之间分布，εHf（t）=-12.5~-32.8，表明它们在结晶过程中经历了较为显著的岩浆混合作用，其成熟度较高的端元很可能是新太古代-古元古代太华群基底，而成熟度较低的端元则为燕山期上涌并底侵于地壳底部的幔源物质或是之前遭受幔源物质改造的下地壳组分。鱼池岭斑岩型钼矿床在中生代古太平洋构造域全面叠置、破坏并改造古特提斯构造体系的背景下形成，强烈的地壳减薄作用诱发了基性玄武质岩浆底侵或内侵，使得该地区具高钼地球化学背景的古老地壳岩石发生部分熔融，并不同程度混染有少量幔源组分或新生地壳物质。这样一个富钼的岩浆体系经高程度分异演化后上升至较浅层次侵位，形成花岗斑岩体。由于围岩为花岗岩，斑岩体的外接触带裂隙较少，因而自身发生不同程度钼矿化，表现出全岩矿化的特点。