Shulou(Shulou.com)11/24 Report--

The original title: "the pattern opens!" How to retrieve the memory of 8 million years? "

Do you know? As early as 8 million years ago, the Loess Plateau was a vast lake, but today we see the accumulation of loess with an area of 635000 square kilometers and a thickness of 50-80 meters, where it comes from and how it is transported, there is still no unified conclusion. Vicissitudes of life, the situation is changing, it may only take a moment for an event to break out, but what you see has gone through millions of years.

Before we begin today's story, let's introduce the protagonist-sediment: broadly speaking, the material formed by erosion, weathering and transport from the carrier, including the remains of animals and plants, is called sediment.

Today, the protagonist of our story: sediment, it wants to remember how it came to this lake basin. Let's help it find memories.

We may wonder whether they come from inland mountains or continental island arcs. Is the direction northeast or southwest? Is it just a short haul or a long journey? Is it the wind or the current that carries them? What is the original lithology? It is not easy to solve this series of problems. Here are some ways to solve them.

Rocks are made up of minerals, which contain various elements. Different rocks have different mineral types, contents and assemblages, as well as the contents of elements in different minerals, and the contents of minerals and elements will also be affected by different climatic environment (redox, acidity and alkalinity, temperature and humidity, etc.) and tectonic background. In addition, the carrying distance will affect the morphology and structure of the rock. Therefore, we can deduce the sedimentary environment, tectonic setting and parent rock types at that time by analyzing these minerals and element contents, and we can also preliminarily judge the transport distance by rock morphology.

Heavy minerals refer to terrigenous clastic minerals whose specific gravity is more than 2.9 (or 2.86). The content in sedimentary rocks is less than 1%, but it is resistant to erosion, strong stability and not easy to be destroyed. Recording parent rock information is a strong point.

The main results are as follows: 1. Electron probe can be used to analyze the content, chemical composition, type and optical properties of heavy minerals. According to their respective characteristics and specific element content, the source lithology and environment can be determined by typical chemical composition diagram or index.

● pyroxene: Ti- (Ca+ Na) determines whether the protolith is tholeiite or alkaline basalt, and (Ti+ Cr)-Ca distinguishes whether the source is an orogenic belt or a non-orogenic belt environment.

● ilmenite: MnO > 2.0%: felsic magmatic rock; MgO > 1.5%: basic-neutral magmatic rock; TiO2:40%-52%: magmatic rock, 50% Murray 60%: metamorphic rock

● magnetite: MgO, TiO2, V2O3, Al2O3 contents to distinguish igneous rock types

● monazite: lower La / Nd: granite and granitic pegmatite; higher La / Nd: alkaline magmatic rock and marble

2. By comparing apatite with tourmaline, we can know whether the deposition process is affected by the circulation of acidic groundwater.

Monazite / zircon ratio (MZi) can show the source area of deep buried sandstone.

Garnet / zircon ratio (GZi) is used to judge the stability of garnet in sequence.

3. There is a strict symbiotic relationship between mineral symbiotic assemblages, the rocks from different parent rocks have specific mineral assemblages, and the heavy mineral assemblages of sediments in different periods are also different. The parent rock type and provenance direction can be determined according to the mineral assemblage variation diagram on the plane.

Characteristics of heavy mineral assemblage

Source rock characteristics

Amphibole + andalusite

High pressure blueschist facies rocks

Omphacite + phengite

High pressure rocks, such as eclogites

Chrome spinel + magnesia olivine

Basic / ultrabasic rocks

Clastic rock analysis 1, sandstone QLF main diagram and three auxiliary diagrams, three basic provenance areas of continental block, magmatic arc and recycle orogenic belt can be distinguished from QFL diagram. On the auxiliary diagrams of QmFLt, QpLvLs and QmPK, the provenance information can be determined more accurately.

2. The conglomerate can distinguish the main lithology and transportation distance of the source rock by using the statistical data such as the content, particle size, percentage and grinding separation of different components in the gravel.

The fission track method is used to analyze the provenance region by using the radiation damage in the lattice caused by the fission of trace uranium impurities in apatite and zircon. the track is formed after a series of chemical treatment, and the distribution of the density and length of the track is observed and statistically analyzed to provide information related to the age and tectonic evolution of the provenance area.

Based on the data of drilling, logging and seismic in the basin, and through detailed stratigraphic correlation and division, the sedimentary method can infer the relative location of the provenance area by making a certain period of stratigraphic isopachy map, sedimentary facies spreading map and other related maps. combined with lithology, composition, sedimentary body shape, grain size, sedimentary structure (ripple, staggered layer, etc.), paleo-flow direction and plant microfossils and other data to make the provenance area more reliable.

Geochemical method 1. Geochemical method (major and trace elements)

Rocks from different environments contain different chemical components.

The discriminant diagrams of K2O, Na2O, SiO2, CaO, Al2O3, Fe2O3 and MgO are used to distinguish the source areas of passive continental margin, active continental margin, oceanic island arc and continental island arc.

Nd isotopic composition (reflecting average provenance age), Eu anomaly (reflecting magmatic differentiation in the crust), enrichment of large ion lithophile elements (i.e. LILE, reflecting provenance components), alkaline earth element depletion (reflecting heavy mineral enrichment), Zr and Hf enrichment (reflecting heavy mineral enrichment), Cr enrichment (reflecting ultramafic provenance).

Taylor et al have used REE, Th, Sc and high field strength elements to determine the source rocks, in which the ratio of compatible and incompatible elements can be used to distinguish feldspathic and mafic components. The rare earth model can be used to indicate the provenance, if the LREE / HREE ratio is low and there is no Eu anomaly, the provenance may be basic rock, while if the LREE / HREE ratio is high and there is Eu anomaly, the provenance is mostly siliceous rock. In addition, diagrams such as La-Th-Sc, Th-Co-Zr / 10, Th-Sc-Zr / 10 and La / Yi-Sc / Cr can be used to determine the tectonic environment of the provenance, that is, oceanic island arc, continental island arc, active continental margin and passive continental margin.

2. Isotope tracer dating

Age can be calculated by the law of radioisotope decay.

Sm-Nd isotopic data are used to infer the source of sedimentary rocks and to estimate the time when the continental crust separated from the mantle.

The commonly used methods are U-Pb, Sm-Nd, Ar, K-Ar, Rb-Sr and so on, which can reflect the tectonic background, properties and diversity of the provenance area.

Combined with these methods, we can know the source, age, transport distance, direction, parent rock type, sedimentary environment and tectonic setting of the sediment.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), written by Ruoyu, American Editor: Ke ran

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