藏北晚第三纪富高场强元素中酸性火山岩对高原地壳深部热状态变化的指示及其意义

Comparing with the Paleogene volcanic rocks, the Neogene mid-acidic volcanic rocks have significantly high high fields strength elements (HFSE) contents in northern Tibet in our recently preliminary work. The HFSE content in mid-acidic magma can generally indicate the P-T conditions of their source. The obviously different HFSE contents of lavas between the Paleogene and Neogene imply a significant change of the thermal state beneath the deep crust in northern Tibet, suggesting the change of dynamic mechanism in this period. So it is very important that the further researching on the Neogene enriched-HFSE lavas probable reveal the formation of the hot geotherm in northern Tibet at present. This project plans to investigate the Neogene enriched-HFSE volcanic rocks in northern Tibet. Based on detailed field observation and petrology, geochronology and geochemistry, Sr-Nd-Pb-Hf-O isotopes of those Neogene enriched-HFSE lavas, as well as comparing with the Paleogene lavas in northern Tibet, it will bring to light the petrogenesis mechanism, tectonics events and their geodynamic conversion process in northern Tibet in Cenozoic. Combing with previous studies, we can effectively constrain the process of uplifting and the possible deep crust-flow mechanism in northern Tibet by the geochronology and geochemistry of volcanic rocks erupted in Paleogene and Neogene.

我们初步的研究结果显示，藏北同一地区发育的晚第三纪中酸性火山岩与早第三纪火山岩相比有明显高的高场强元素(HFSE)含量，而中酸性岩浆中HSFE的含量通常可以指示其形成源区的温压条件，因此藏北早、晚第三纪中酸性火山岩中HFSE含量的显著变化表明高原北部地壳深部热状态的明显改变，指示晚第三纪藏北地壳深部动力学机制的显著变化，故对藏北晚第三纪富HFSE中酸性火山岩的深入研究很可能对目前藏北地区高地热梯度的形成有重要启示。本项目拟以这些藏北晚第三纪富HFSE中酸性火山岩为研究对象，确定其的形成时代和时空分布特征，通过岩石学和地球化学以及同位素示踪其物质源区，并将其与研究区早第三纪火山岩作对比，揭示这些富HFSE中酸性火成岩的成因机制和导致这些岩浆产生的构造事件及其地球动力学转换过程，从而对青藏高原的抬升过程及其可能的地壳深部物质流动机制等提供有效的地球化学和时代约束。

青藏高原北部广泛发育有早第三纪和晚第三纪的火山岩，二者均有着高的Sr/Y和La/Yb比值，指示其与加厚的下地壳有关；然而富含高场强元素晚第三纪的火山岩却有比早第三纪更高的La/Yb比值和低的Mg#，指示其源于一个热且逐渐加厚的壳源。同时高原中北部处于同一高度的古新世盆地 (＞33Ma) 被25-20 Ma活动的昆仑断裂分割为南部的可可西里盆地和北部的柴达木盆地，上述盆地在~20 Ma以后并没有发生明显的构造运动和变形，然而目前可可西里盆地海拔高度(~5.0 km)明显高于柴达木盆地 (~2.8 km) 近~2 km以上；而且晚第三纪的岩浆作用主要分布在昆仑断裂带以南。上述研究区晚第三纪岩浆-沉积构造的演化特征指示青藏高原中北部在该时期主要是通过岩浆底侵到导致地壳加厚和高原的隆升。同时通过简单的模拟显示，晚第三纪昆仑断裂以南因岩浆底侵而导致的地壳加厚厚度约为~15km。结合前人对研究区早第三纪的研究成果，初步认为高原中部在早第三纪时期的地壳加厚以构造缩短为主，而在晚第三纪则以岩浆底侵为主，并且后者对高原抬升和地壳增厚的作用类似于前者构造挤压的作用。