What are igneous andesites
and what has the "dark melt" in the Chicxulub crater structure with them common

Norbert Brügge. Germany
Dipl.-Geol.

Andesites are the volcanic equivalent of a dioritic magma with aphanitic to porphyritic texture. Andesite is an often dark-colored. fine-grained rock. which have 57 to 63 % SiO2 and <6 % Na2O+K2O as weight (illustrated in the TAS diagram). In a general sense, it is the intermediate type between basalt and granite. The mineral assemblage is typically dominated by plagioclase as well pyroxene (clinopyroxene and orthopyroxene) and/or amphibole (hornblende). Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.
Texturally, most andesites are porphyritic rocks with prominent phenocrysts. These phenocrysts are typically enclosed in the fine-grained groundmass.
Classification of andesites may be refined according to the most abundant phenocryst. The quartz-alkalifeldspar-plagioclase-foide abundances in andesite are illustrated in the Streckeisen QAPF diagram.


QAPF

                                                                                                                                                                                                 

A practical guide for the igneous rocks of the Carpathian-Pannonian region; Intermediate igneous rocks -- Diorite
Szabolcs Harangi et al. -- Eötvös Loránd University. Hungary
 http://elte.prompt.hu/sites/default/files/tananyagok/APracticalGuideForTheIgneousRocks/ch02s03.html

Links to some thin sections of typical plagioclase-phyric pyroxene andesites in this publication:
 

 

For comparison thin or polish sections of Chicxulub andesites ("dark melt")
 (plagioclase-phyric pyroxene andesite)


PEMEX Y6/N19


PEMEX Y6/N19


PEMEX Y6/N19 (Plagioclase)


PEMEX Y6/N17


PEMEX C1/N9


PEMEX C1/N10



 

 

Y6 N-17 Y6 N-19 C1 N-10 C1 "Andesite"
SiO2 62.30 61.90 63.20 60.50 54.80 57.40 61.20 58.50 61.70 58.30 57.60 64.60 59.71-58.45
Al2O3 14.60 13.10 12.60 13.60 15.30 16.41 14.70 15.50 13.70 13.70 15.50 14.90 15.84-13.77
MgO 2.90 3.20 3.10 3.20 3.15 2.96 2.74 2.75 2.55 2.92 3.05 2.75 5.42-4.06
CaO 8.80 10.40 10.20 10.50 11.90 10.18 9.30 10.55 10.01 12.10 11.41 5.50 11.24-9.35
Na2O 2.10 4.40 4.00 4.70 3.25 3.10 2.75 3.41 2.54 3.41 3.62 3.71 4.60-4.01
K2O 2.50 1.90 1.90 1.90 1.75 2.47 2.91 2.19 2.27 1.06 1.76 2.72 2.42-2.07
FeO 4.80 4.80 4.50 5.00 5.50 4.80 3.90 3.56 3.83 3.70 4.24 4.60 4.36-3.49
TiO2 0.50 0.40 0.40 0.40 0.56 0.54 0.40 0.40 0.36 0.39 0.43 0.53 0.13
MnO 0.10 0.10 0.10 0.10 0.13 0.12 0.09 0.11 0.06 0.11 0.11 0.09 0.14


Chicx-3A: "Glassy" andesitic melt  in "Suevite" (80R-2/706.4 mbsf)


Chicx-3A: Green schlieren (88R-3)


Chicx-3A: Brown schlieren (89R-3)


Chicx-3A: Green schlieren (92R-3/737.6 mbsf)


Chicx-3A: 93R-3


Chicx-3A: 93R-3: Fine-grained matrix made of plagioclase laths


Chicx-3A: 94R-3


Chicx-3A: 95R-3/747 mbsf  (+granitoid clast)


Chicx-3A: Andesite intrusion into Granite (265R-2/~1215 mbsf)


Chicx-3A: Schlieren andesite (268R-1/~1215 mbsf)


Chicx-3A: Clast rich andesite melt (268R-1/~1215 mbsf)


Chicx-3A: Large clast in black/brown andesite melt (280R-3/~1265 mbsf)