LOS VIDRIOS

The mafic Sierra Pinacate Volcanic Field with the rhyolite structures of Los Vidrios to the east with the Rio Sonoyta flowing to the south. The Tertiary rhyolite is substantially older than the Quaternary mafic eruptive events in the Sierra Pinacate. Landsat photo.

Coalesced rhyolite domes at Los Vidrios, east of the Sierra Pinacate Volcanic Field, northern Sonora. Rio Sonoyta wash in foreground

Located between UTM 352 and 354 East and 29 and 30 North of the Puerto Peñasco H12-1 Mexican Topographic Quad 1:250,000 northwest Sonora, Mexico. This is a probable middle Tertiary glass source located about 8 to 12 km south of the US/Mexican border along the Rio Sonoyta and on the eastern border of the predominately mafic Quaternary Sierra Pinacate Volcanic Field. The source is comprised of a northwest-southeast arcing series of rhyolite and rhyolite/obsidian domes. Nodules up to 7 cm in diameter are found in a rhyolite/tuff/obsidian conglomerate eroding into Rio Sonoita and embedded in perlite within the rhyolite domes. Rhyolite/obsidian units, some up to 30 m by 40 m, are common on many domes. One small dome between UTM 29-30 North and 352-353 East (on the 353 line), consists of an alternating series of rhyolite and obsidian strata with nodules embedded within the glass, similar to Slate Mountain. Densities within the devitrified glass matrix are up to 50 per m², perhaps five times that if pea size nodules are counted. Within the conglomerate densities reach 10 to 20 per 5 m². This dome complex has been named Lomas del Norte and considered and early eruptive event as part of the Sierra Pinacate volcanic field (Vidal-Solano 2008). The elemental composition acquired by Vidal-Solano and his team matches the XRF analysis by this lab (2008:695; and from this lab. The plateau date by ⁴⁰Ar/³⁹Ar for obsidian from Los Vidrios is 14.23±0.15 Ma and statistically contemporaneous with Los Sitios del Agua even though the elemental composition is very different (see below; Vidal-Solano 2008:697).

Secondary deposition consists of erosion through the Rio Sonoita river system south at least as far as the Sierra Blanca and perhaps 65 km to the Gulf of California. The thin cortex on the nodules varies from brown to gray and velvet-like in the conglomerate above the Rio Sonoita. The aphyric glass is very consistently black and very opaque even in the thinnest flakes. No banding or lighter shades were noticed in hundreds of nodules collected.

The knapping quality of this obsidian is good, but the glass is quite brittle and bipolar reduction sometimes resulted in 'exploding' nodules. Pressure flaking, however was quite successful when a flake was successfully removed and flakes were easily removed with minimum platform collapse. There are no known geological or archaeological references to this source other than a short mention in Lumholtz (1912:286).

In October 2008 while on the way to the Los Sitios del Agua obsidian source along the Rio Sonoyta (former AZ Unknown A), I visited the bajada below the above collection locality and collected about 50 samples. The analysis of a sample of these are included below with the date prefix 102508. The 1986 samples were analyzed on the old Spectrace 440 instrument. The analysis here on the new ThermoScientific Quant'X indicates that the elemental concentrations are essentially the same, and there is little inter-instrument variability (see tables below).

Raw elemental concentrations for Los Vidrios source standards and USGS RGM-1 rhyolite standard. All measurements in parts per million (ppm).

Sample	Ti	Mn	Fe	Zn¹	Rb	Sr	Y	Zr	Nb	Ba	Pb	Th
1986-1	725	216	13174		262	13	73	238	32
1986-2	709	198	11970		238	13	68	217	35
1986-3	833	203	13277		266	14	76	239	35
1986-4	893	232	13804		271	15	77	240	31
1986-5	837	223	13521		271	14	79	241	32
1986-6	798	198	13402		270	15	80	236	31
1986-A1	836	208	13104		258	13	73	236	29
1986-A2-1	675	179	11743		236	12	68	223	30
1986-A2-2	951	236	11495	84	228	13	62	212	28	32	24	23
1986-A2-3	905	252	10980	74	220	19	64	207	26	19	27	27
1986-A2-4	992	282	12611	90	242	17	70	225	33	5	28	25
1986-A3	879	224	13666		266	16	76	242	34
1986-A3	658	201	13195		264	13	75	241	35
102508-1-10	808	221	10194	75	207	16	63	197	26	40	16	19
102508-1-11	784	230	10456	83	231	12	66	216	30	32	28	25
102508-1-12	816	251	10974	85	235	11	65	221	25	32	27	29
102508-1-13	743	244	10274	77	226	10	69	215	31	41	27	28
102508-1-14	728	246	10157	79	227	13	64	214	30	42	26	27
102508-1-15	765	244	11118	86	239	12	68	218	30	35	26	34
102508-1-16	848	265	12009	94	253	12	68	223	29	24	32	34
102508-1-17	795	231	10567	80	232	15	66	221	31	34	28	32
102508-1-18	815	252	11845	93	248	13	72	229	32	39	29	33
102508-1-19	760	253	11395	88	244	13	61	222	35	41	28	27
102508-1-20	830	265	12077	92	254	12	73	231	31	41	31	28
-21	918	265	12937	95	255	16	71	231	34	46	29	42
-23	877	243	11671	87	232	14	66	212	30	12	26	36
RGM1-S4	1569	284	13146	46	145	108	25	216	14	828	26	20

¹ The 1986 analyses on the Spectrace 440 EDXRF did not acquire Zn or Ba. Three of the 1986 collected samples were analyzed in 2018.

Major oxides for one Los Vidrios source standard collected in October 2008 and analysis of RGM-1

Lumholtz, C. 1912, New Trails in Mexico: An Account of One Year's Exploration in North-Western Sonora, Mexico, and South-Western Arizona. Glorieta, New Mexico: Rio Grande Press.

Shackley, M.S. 2005, Obsidian: Geology and Archaeology in the North American Southwest. Tucson, University of Arizona Press.

Sample	SiO₂	Al₂O₃	CaO	Fe₂O₃	K₂O	MgO	MnO	Na₂O	TiO₂
Los Vidrios
102508-1-16	74.542	12.304	1.474	2.214	5.215	0.047	0.054	3.68	0.255
RGM1-S4	75.680	12.477	1.3024	1.806	4.550	<.001	0.0379	3.77	0.196