Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea–Verbano Zone
Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea–Verbano Zone
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DOI：
10.1016/j.chemgeo.2008.07.023
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发表时间：
2009-04
期刊：
影响因子：
3.9
通讯作者：
G. Luvizotto;T. Zack
中科院分区：
文献类型：
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作者：
G. Luvizotto;T. Zack
详细的结构观察和原位分析（EMP，西姆斯和LA-ICP-MS）被用来表征微量元素的行为，在包埋和退变质反应涉及金红石。Ivrea-Verbano带是一个典型的麻粒岩区域，Strona和d 'Ossola山谷的岩石是角闪岩向麻粒岩相转变的一个例子。虽然不同的岩石类型出现在该地区，详细的采样和岩相工作表明，金红石只出现在麻粒岩相副片麻岩。这些岩石显示了丰富的纹理，不仅允许调查的微量元素的行为，响应于金红石生长，但也峰后过程对金红石化学的影响。较低品位样品中金红石中的Nb浓度与较高品位样品相比显示出更大的范围（一个样品内从500 ppm至5000 ppm）。这种模式可以用黑云母分解形成的金红石的斜长来模拟。锆浓度在金红石的特点是由一个巨大的蔓延和双峰分布。最大锆浓度增加，根据该地区已知的一般变质梯度。温度（从锆在金红石测温法），虽然可行，是相当高的比以前的计算（从约增加。850至930 °C）。第二簇的锆浓度在金红石发生在相当恒定的浓度（约。1000 ppm），并解释为与高温下的强烈流体流入和/或缓慢冷却速率有利于峰后扩散重置有关。蚀变结构以复杂的微脉网为特征，是晚期流体流入的证据。流体对金红石产生了强烈的影响，较老的金红石颗粒的腐蚀和金红石细脉的形成证明了这一点。
Detailed textural observations and in situ analyses (EMP, SIMS and LA-ICP-MS) are used to characterize trace element behavior during prograde and retrograde metamorphic reactions involving rutile. The Ivrea–Verbano Zone is a classic granulite area and rocks from the Strona and d'Ossola valleys are an example of the amphibolite to granulite facies transition. Although different rock types occur in the area, detailed sampling and petrographic work show that rutile only occurs in granulite facies paragneisses. These rocks show a rich inventory of textures that allow not only for the investigation of trace element behavior in response to prograde rutile growth, but also for the effect of post-peak processes on rutile chemistry. Nb concentrations in rutile from lower grade samples show a larger spread (from 500 to 5000 ppm within one sample) when compared to those from higher grades. This pattern can be modeled using prograde rutile growth formed from biotite breakdown. Zr concentrations in rutile are characterized by an anomalously large spread and a bimodal distribution. Maximum Zr concentrations increase according to the general metamorphic gradient known for this area. Temperatures (from Zr-in-rutile thermometry), although feasible, are considerably higher than previous calculations (increasing from ca. 850 to 930 °C). A second cluster of Zr concentrations in rutile occurs at rather constant concentrations (ca. 1000 ppm) for all localities and is interpreted to be related to intense fluid influx at high-temperature and/or to post-peak diffusional resetting favored by slow cooling rates. Alteration textures, characterized by a complex network of micro-veins, are evidence for the late fluid influx. The fluid strongly affected the rutiles, which is evidenced by corrosion of older rutile grains and formation of rutile veinlets.