بررسی پتروگرافی و ژئوشیمیایی (عناصر اصلی و فرعی) نهشتههای پالئوسن شمال بیرجند بهمنظور تعیین برخاستگاه، سنگ منشأ و شرایط هوازدگی دیرینه انجام شد. شواهد پتروگرافی ازجمله کوارتزهای مونوکریستالین نیمهگردشده با خاموشی مستقیم، فراوانی خردهسنگهای رسوبی و به مقدار کمتر خردهسنگهای دگرگونی و ولکانیکی و همچنین رسم نتایج تجزیهوتحلیل مودال ذرههای آواری روی نمودارهای QtFL و QmFlt نشان میدهد ماسهسنگهای پالئوسن مطالعهشده حاصل چرخۀ دوبارۀ رسوبات پساز کوهزایی هستند. بر اساس مطالعههای ژئوشیمیایی، موقعیت تکتونیکی ماسهسنگهای مطالعهشده احتمالاً حاشیه قارهای فعال است. اندیسهای هوازدگی شیمیایی CIA و PIA محاسبهشده برای ماسهسنگهای این برش نشاندهندۀ شرایط هوازدگی متوسط تا زیاد در ناحیۀ منشأ هستند. دادههای پتروگرافی، ژئوشیمیایی و نقشههای جغرافیایی دیرینۀ منطقه در زمان پالئوسن شرایط آبوهوایی نیمهخشک را در زمان نهشتهشدن این ماسهسنگها نشان میدهند.
عنوان مقاله [English]
Petrography and geochemistry of the Paleocene sandstones from north of Birjand: Implications for provenance, tectonic setting and paleoclimate conditions
The relation between the tectonic setting, provenance and composition of siliciclastic deposits has been investigated by many researchers. The geochemistry of major and trace elements of siliciclastic deposits provides information about provenance, the paleoweathering conditions and tectonic setting of sedimentary basins. In terms of structural division, the study area is located in the Sistan Suture Zone (SSZ). The Paleocene- Eocene siliciclastic strata in the east of Iran have significant development, diversity and thickness lithologically. The studied section is located in the 17 kilometers of north of Birjand and 3 kilometers of east of Fariznuk village the most important way to reach the study area is the main road of Birjand- Mashhad. The Fariznuk section with Paleocene age is 113 meters in thickness and forms from three lithostratigraphic units consisting lower conglomerate, middle sandstone and upper carbonate units. The purpose of this study is to investigate the petrographic and geochemical properties of Fariznuk section in the north of Birjand in order to determine the provenance, tectonic setting and paleoweathering conditions.
Materials and Methods
In this study, 90 rock samples were collected from the studied sequence. The study of 35 sandstone samples components composition (modal analysis) is performed by using the point counting method of Gazi- Dickinson. Folk (1980) compositional classification of sandstones is used for studied samples. Geochemical analyses (major and some trace elements) are carried out by XRF method for 15 fine grain sandstone samples.
Discussion of Results and Conclusions
In the provenance studies, that are performed by petrographic methods, by using the compositional and textural evidences, the characteristics of provenance of sediments can be investigated. Fine to coarse grain sandstones of the studied sequence have angular to sub rounded grains with poorly to well sorting. Based on mineralogical composition these samples are litharenite (Q52.9, F7.3, RF39.8). The plutonic monocrystalline quartz in the studied sandstone samples confirms the presence of intrusive igneous rocks in the source area. While the existence of straight to weakly undolatory extinction in monocrystalline quartz together with metamorphic rock fragments are indicative the presence of metamorphic rocks with above igneous rocks in the source area. Also, the presence of volcanic rock fragments such as peridotite rock fragments from the underlying ophiolite set and andesite rock fragments, plagioclase and zircon heavy minerals can be suggested an igneous (intrusive and volcanic) provenance for these sandstones. The high abundance of sedimentary rock fragments such as sandstone, siltstone, chert and carbonate in these samples indicate a sedimentary source rock with above igneous and metamorphic rocks. Regarding to the litharenite composition of studied sandstones and high abundance of sedimentary rock fragments, in particular chert rock fragments, it seems that the erosion of older sedimentary deposits has the most role in the formation of sandstones in this region. Drawing the point counting results of the Fariznuk Sandstones on the QmFLt and QtFL (Dickinson et al. 1983) diagrams indicate that the most of samples plot in the recycled orogen area. The quartzolithic composition of studied sandstones low amounts of feldspars and high abundance of sedimentary rock fragments relative to volcanic rock fragments, probably suggest the collisional tectonic areas that are documented by drawing the QpLvLs diagram (Dickinson et al. 1983) for these samples.
In addition to petrographic data and modal analysis, the major element geochemistry can be effective in the classification of sedimentary rocks as well as separation between mature and immature sediments. Based on major oxides geochemical analysis, Fariznuk sandstone samples are located in the litharenite area in the Pettijohn et al. (1987) diagram that is correlated with petrographic data. By using the geochemical data of the siliciclastic rocks also can be obtained many information about the source rock types. The plot of geochemical results of studied samples on the presence diagrams indicates that the provenance of studied siliciclastic rocks probably were quartzose sedimentary, felsic and intermediate igneous rocks that are matched with petrographic results. The results of the major oxides analysis of Fariznuk sandstone samples are drawn in different diagrams to determine the tectonic setting of these sediments. The studied sandstone samples are plotted in the active continental margin and island arc tectonic settings. The Sistan Suture Zone (SSZ) during Cretaceous to Early Paleocene, prior to sedimentation of the studied siliciclastic deposits, was tectonically active and some researchers documented the existence of magmatic arcs associated with subduction at this time. Therefore, the results of geochemical and petrographical data of the studied sandstone samples (active continental margin and island arcs) can be correlated with the Sistan Suture Zone tectono magmatic conditions at this time. According to geological map of this region, the probable source rocks of the studied siliciclastic deposits were ophiolite units, felsic and intermediate igneous rocks (granite, andesite and tuff) and cretaceous flysch rocks (shale, sandstone, limestone and conglomerate) with cretaceous age in the western and southwestern parts of the Fariznuk section.
The calculated chemical weathering index of Fariznuk sandstone samples (CIA, PIA) indicates medium to high paleoweathering conditions in the source area. According to petrographical and geochemical studies and its integration with world paleogeographic map during the Paleocene time, the climatic conditions