کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
5784239 1638629 2017 19 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Geochemical interpretation of the Precambrian basement and overlying Cambrian sandstone on Bornholm, Denmark: Implications for the weathering history
ترجمه فارسی عنوان
تفسیر ژئوشیمیایی زیرزمین پیشمبرین و ماسه سنگ کامبرین در بوردورلم، دانمارک: تاثیرات تاریخچه آب و هوایی
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
چکیده انگلیسی


- The genetic relationship between granitoids and sediments is clarified.
- The fresh equivalent of a weathered basement granite is defined.
- The weathering regime of Precambrian granites is illustrated.
- Mass transfer during the weathering of granite is determined quantitatively.
- The Precambrian climate is interpreted from the weathering processes.

A geochemical study of the Precambrian basement granites from the BorggÃ¥rd borehole on Bornholm, Denmark, suggests that the granites were moderately weathered (Chemical Index of Alteration-CIA = 66-71) during subaerial exposure in a humid climate. The microcline is well preserved, whereas plagioclase was thoroughly altered to clay minerals (Plagioclase Index of Alteration-PIA = 93-99) which is likely due to its original Ca-rich composition. The primary Fe-Ti accessory minerals were oxidized to hematite and anatase. Evidence from REE distribution patterns and immobile element ratios, e.g. Zr/Hf and Nb/Ta, between the weathered basement granite from the BorggÃ¥rd borehole and regional granitoids on Bornholm, constrains the Svaneke Granite as the original basement lithology. A tau (τ) mass transport model (assuming immobile Ti) was applied to quantify the mass transfer during weathering of the basement granite. The results show a depletion of major elements in the following order: Na > Ca > Mg > Si; Al and Ti are immobile and stay constant; K shows sample dependent enrichment or depletion; Fe is slightly enriched. The Cambrian sandstone overlying the basement in the BorggÃ¥rd borehole, assigned to the Gadeby Member of the Nexø Formation, is feldspathic litharenite-litharenite in composition. Provenance indicators including (Gd/Yb)N, Zr/Hf and Nb/Ta ratios and petrological features indicate that source material was derived from both weathered and fresh basement granite of intermediate composition. The Gadeby Member equivalents in Germany, the basal lower Cambrian Adlergrund Konglomerat Member (AKM) in the offshore G-14 well north of Rügen, and the approximately coeval Lubmin Sandstein Formation (LSF) from the Loissin-1 borehole, mainland Germany, must have been sourced from a basement with compositions comparable to the intermediate group of the regional granitoids on Bornholm. The source materials for the AKM (CIA = 71-72, PIA = 94-96), the Gadeby Member in the BorggÃ¥rd borehole (CIA = 52-69, PIA = 56-99) and an outcrop in Nexø, eastern Bornholm (CIA = 52-66, PIA = 61-96), have endured similar degrees of weak to moderate weathering but lost most of the plagioclase. The LSF has a comparable weathering history (CIA = 63-73), but the plagioclase is better preserved (PIA = 65-78). The significant variation of weathering rates of plagioclase and K-feldspar in the basement granite and the provenance of sandstone from the BorggÃ¥rd borehole are likely due to the different permeability developed within the internal crystal structures, a Ca- rich plagioclase original composition of the plagioclase, and the occurrence of weathering in a very humid climate. ​K metasomatism occurred in the basement granite and sandstone in both the BorggÃ¥rd and the G14-1 boreholes, mainly through the conversion of aluminous clay minerals (e.g. kaolinite) to illite, with transformation of plagioclase to K-feldspar occurring locally. This my have taken place during deep burial in the Caledonian foreland basin.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Lithos - Volumes 286–287, August 2017, Pages 369-387
نویسندگان
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